28. Reproductive System

28 Reproductive System O U T L I N E 28.1 Comparison of the Female and Male Reproductive Systems 843 28.1a Perineum 843 28.2 Anatomy of the Female Reproductive System 28.2a 28.2b 28.2c 28.2d 28.2e 28.2f Ovaries 845 Uterine Tubes 852 Uterus 852 Vagina 855 External Genitalia 857 Mammary Glands 857 28.3 Anatomy of the Male Reproductive System 28.3a 28.3b 28.3c 28.3d 28.3e 28.3f 28.3g 844 Scrotum 861 Spermatic Cord 863 Testes 863 Ducts in the Male Reproductive System Accessory Glands 867 Semen 868 Penis 869 861 866 28.4 Aging and the Reproductive Systems 871 28.5 Development of the Reproductive Systems 872 28.5a Genetic Versus Phenotypic Sex 872 28.5b Formation of Indifferent Gonads and Genital Ducts 872 28.5c Internal Genitalia Development 874 28.5d External Genitalia Development 874 MODULE 14: REPRODUC TI V E SYSTEM mck78097_ch28_842-878.indd 842 2/14/11 4:43 PM Chapter Twenty-Eight he female and male reproductive systems provide the means for the sexual maturation of each individual and produce the special cells necessary to propagate the next generation. In this chapter, we first discuss the general similarities between the two reproductive systems and then focus on the specific structures and functions of each system. T Reproductive System 843 Table 28.1 Reproductive System Homologues Female Organ Male Organ Common Function Ovaries Testes Produce gametes and sex hormones Clitoris Glans of penis 28.1 Comparison of the Female and Male Reproductive Systems Contain autonomic nervous system axons that stimulate feelings of arousal and sexual climax Labia majora Scrotum Protect and cover some reproductive structures Learning Objectives: Vestibular glands Bulbourethral glands Secrete mucin for lubrication 1. Describe the similarities between the female and male reproductive systems. 2. Outline the events of puberty in females and males. 3. List the components of the perineum in females and males. Besides their obvious differences, the female and male reproductive systems share several general characteristics. For example, some mature reproductive system structures are derived from common developmental structures (primordia) and serve a common function in adults. Such structures are called homologues (hōm ́ō-log; homo = same or alike, logos = relation) (table 28.1). The structures listed in this table are described in detail later in this chapter. Both reproductive systems have primary sex organs called gonads (gō n ́ ad; gone = seed)—ovaries in females and testes in males. The gonads produce sex cells called gametes (gam ́ēt; husband or wife), which unite to form a new individual. Female gametes are called oocytes, whereas male gametes are called sperm. In addition, the gonads produce large amounts of sex hormones (estrogen and progesterone in the female and androgens in the male), which affect maturation, development, and changes in the activity of the reproductive system organs. Both reproductive systems have accessory reproductive organs, including ducts to carry gametes away from the gonads toward the site of fertilization (in females) or simply to the outside of the body (in males). Fertilization occurs when female and male gametes fuse. The sexual union between a female and a male is known as copulation (kop-ū-lā ś hu n̆ ; copulatio = a joining), coitus (kōi -́ tu s̆ ; to come together), or sexual intercourse. If fertilization occurs, then the support, protection, and nourishment of the developing human occurs within the female reproductive tract. Both the female and male reproductive systems are primarily nonfunctional and “dormant” until a time in adolescence known as puberty. At puberty (pū ́ b er-tē; puber = grown up), external sex characteristics become more prominent, such as breast enlargement in females, penis and scrotum enlargement in males, and pubic hair in both sexes. The mck78097_ch28_842-878.indd 843 reproductive organs become fully functional. Also, the gametes begin to mature, and the gonads start to secrete their sex hormones. Puberty is initiated when the hypothalamus significantly increases GnRH (gonadotropin-releasing hormone) secretion (see chapter 20). GnRH acts on specific cells in the anterior pituitary and stimulates them to release FSH (folliclestimulating hormone) and LH (luteinizing hormone). (Prior to puberty, FSH and LH are virtually nonexistent in boys and girls.) As levels of FSH and LH increase, the gonads produce significant levels of sex hormones and start the processes of gamete maturation and sexual maturation. Study Tip! A simplified flowchart of the endocrine pathway in puberty is as follows: GnRH (from hypothalamus) FSH and LH (from anterior pituitary) Sex hormone release and gamete maturation (in the gonads). Both reproductive systems produce gametes. However, the female reproductive tract typically releases a single gamete (secondary oocyte) monthly, while the male reproductive tract produces large numbers (100 million) of gametes (sperm) daily. These male gametes are stored within the male reproductive tract for a short time, and if they are not expelled from the body within that period, they are resorbed. 28.1a Perineum In both females and males, the perineum (per ́ i-nē ú m ̆ ) is a diamond-shaped area between the thighs that is circumscribed anteriorly by the pubic symphysis, laterally by the ischial tuberosities, 2/14/11 4:43 PM 844 Chapter Twenty-Eight Reproductive System Pubic symphysis Pubic symphysis Urogenital triangle Ischiocavernosus muscle Bulbospongiosus muscle Superficial transverse perineal muscle Ischial tuberosity Anus Anus Anal triangle External anal sphincter Coccyx Female Male Figure 28.1 Perineum. In both females and males, the perineum is the diamond-shaped area between the thighs extending from the pubis anteriorly to the coccyx posteriorly, and bordered laterally by the ischial tuberosities. An imaginary horizontal line extending from the ischial tuberosities subdivides the perineum into a urogenital triangle anteriorly and an anal triangle posteriorly. and posteriorly by the coccyx (figure 28.1). Two distinct triangle bases are formed by an imaginary horizontal line extending between the ischial tuberosities of the ossa coxae. Both triangles house specific structures in the floor of the trunk: ■ ■ The anterior triangle, called the urogenital triangle, contains the clitoris and the urethral and vaginal orifices in females and the base of the penis and the scrotum in males. Within the urogenital triangle are the muscles that surround the external genitalia, called the ischiocavernosus, bulbospongiosus, and superficial transverse perineal muscles. The posterior triangle, called the anal triangle, is the location of the anus in both sexes. Surrounding the anus is the external anal sphincter. Review table 11.12 and figure 11.15 as well when learning these structures. W H AT D I D Y O U L E A R N? 1 ● 2 ● What is puberty? Compare the structures in the female and male urogenital triangles. mck78097_ch28_842-878.indd 844 28.2 Anatomy of the Female Reproductive System Learning Objectives: 1. Describe the gross and microscopic anatomy of the ovaries. 2. Explain follicle development, the ovarian cycle, and the process of ovulation. 3. Detail the anatomy of the uterine tubes and their function. 4. Identify the regions of the uterus, and outline the uterine cycle. 5. Describe the anatomy of the vagina and the external genitalia. 6. Detail the gross and microscopic anatomy of the mammary glands. A sagittal section through the female pelvis illustrates the internal reproductive structures and their relationships to the urinary bladder and rectum (figure 28.2). As the peritoneum folds around the various pelvic organs, it produces two major dead-end recesses, or pouches. The anterior vesicouterine (ves ́i-kō-ū t́ er-in; vesica = bladder, utero = uterus) pouch forms the space between the uterus and the urinary bladder, and the posterior rectouterine (rek-tō-ū t́ er-in) pouch forms the space between the uterus and the rectum. 2/14/11 4:43 PM Chapter Twenty-Eight Reproductive System 845 Ureter Uterine tube Ovary Fimbriae of uterine tube Uterus Rectouterine pouch Vesicouterine pouch Urinary bladder Cervix of uterus Pubic symphysis Rectum Urethra Vagina Clitoris External urethral orifice Vaginal orifice Anus Labium minus Labium majus Figure 28.2 Sagittal Section of the Female Pelvic Region. A sagittal section of the female pelvis illustrates the position of the uterus with respect to the rectum and urinary bladder. The primary sex organs of the female are the ovaries. The accessory sex organs include the uterine tubes, uterus, vagina, clitoris, and mammary glands. 28.2a Ovaries The ovaries are paired, oval organs located within the pelvic cavity lateral to the uterus (figure 28.3). In an adult, the ovaries are mck78097_ch28_842-878.indd 845 slightly larger than an almond—about 2 to 3 cm (centimeters) long, 2 cm wide, and 1 to 1.5 cm thick. Their size usually varies during each menstrual cycle as well as during pregnancy. The ovaries are anchored within the pelvic cavity by specific cords and sheets of connective tissue. A double fold of peritoneum, called the mesovarium (mez ́ō-vā ŕ ē-u m ̆ ; mesos = middle, ovarium = ovary), attaches to each ovary at its hilum, which is the 2/14/11 4:43 PM 846 Chapter Twenty-Eight Reproductive System Ovarian artery Ovarian vein Suspensory ligament Mesosalpinx (part of broad ligament) Ovarian ligament Infundibulum Uterine tube Fimbriae Ovary Uterus Broad ligament Uterine artery Uterine vein Ureter (a) Posterior view Cervix Uterosacral ligament Vagina Uterine tube External os Tunica albuginea Cortex Medulla Mesosalpinx Fimbriae Ovaries Uterine tube Uterus Mesovarium Hilum Broad ligament Urinary bladder (b) Lateral sectional view Round ligament Figure 28.3 Internal Organs of the Female Reproductive System. (a) A posterior view shows the internal organs of the female reproductive system, which include the ovaries, uterine tubes, uterus, and vagina. (b) A lateral sectional view of the ovary shows the mesovarium in relation to the mesosalpinx of the broad ligament. (c) A cadaver photo provides a superior view of the female pelvis and reproductive organs. Mons pubis (c) Superior view mck78097_ch28_842-878.indd 846 2/14/11 4:43 PM Chapter Twenty-Eight anterior surface of the ovary where its blood vessels and nerves enter. The mesovarium secures each ovary to a broad ligament, which is a drape of peritoneum that hangs over the uterus. Each ovary is anchored to the lateral aspect of the uterus by an ovarian ligament; a suspensory ligament attaches to the lateral edge of each ovary and projects superolaterally to the pelvic wall. The ovarian blood vessels and nerves are housed within each suspensory ligament, and they join the ovary at its hilum. Smooth muscle fibers within both the mesovarium and the suspensory ligament contract at the time of ovulation to bring the ovaries into close proximity with the uterine tube openings. Each ovary is supplied by an ovarian artery and an ovarian vein. The ovarian arteries are direct branches off the aorta, immediately inferior to the renal vessels. The ovarian veins exit the ovary and drain into either the inferior vena cava or one of the renal veins. Traveling with the ovarian artery and vein are autonomic nerves. Sympathetic axons come from the T10 segments of the spinal cord, whereas parasympathetic axons come from CN X (vagus nerve). When an ovary is sectioned and viewed microscopically, many features are visible (figure 28.4). Surrounding the ovary is a thin, simple cuboidal epithelial layer called the germinal epithelium, so named because early anatomists erroneously thought it was the origin of the female germ (sex) cells. Deep to the germinal epithelium is a connective tissue capsule called the tunica albuginea (al-būjin ́ē-a ̆; albugo = white spot), which is homologous to the tunica albuginea of the testis. Deep to the tunica albuginea, the ovary can be partitioned into an outer cortex and an inner medulla. The cortex contains ovarian follicles (described next), while the medulla is composed of areolar connective tissue and contains branches of the ovarian blood vessels, lymph vessels, and nerves. Ovarian Follicles Within the cortex are thousands of ovarian follicles. Ovarian follicles consist of an oocyte surrounded by follicle cells which support the oocyte. There are several different types of ovarian follicles, each representing a different stage of development (figure 28.4): 1. A primordial follicle is the most primitive type of ovarian follicle. Each primordial follicle consists of a primary oocyte surrounded by a single layer of squamous follicle cells. A primary oocyte is an oocyte that is arrested in the first meiotic prophase. About 1.5 million of these types of follicles are present in the ovaries at birth. Reproductive System 847 2. A primary follicle forms from a maturing primordial follicle. Each primary follicle consists of a primary oocyte surrounded by one or more layers of cuboidal follicular cells, which are now called granulosa cells. Each primary follicle secretes estrogen as it continues to mature. The estrogen stimulates changes in the uterine lining. 3. A secondary follicle forms from a primary follicle. Each secondary follicle contains a primary oocyte, many layers of granulosa cells, and a fluid-filled space called an antrum. Within the antrum is a serous fluid that increases in volume as ovulation nears. Surrounding the primary oocyte are two protective structures, the zona pellucida and the corona radiata. The zona (zō n ́ ă; zone) pellucida (pe-lū ś ı̆d-ā; pellucidus = allowing the passage of light) is a translucent structure that contains glycoproteins. External to the zona pellucida is the corona (kō-rō n ́ ă; crown) radiata (rā-dē-ă t́ ă; radiating), which is the innermost layer of granulosa cells. 4. A vesicular follicle (also called a mature follicle or Graafian follicle) forms from a secondary follicle. A vesicular follicle contains a secondary oocyte (surrounded by a zona pellucida and the corona radiata), numerous layers of granulosa cells, and a large, fluid-filled, crescent-shaped antrum. A secondary oocyte has completed meiosis I and is arrested in the second meiotic metaphase. Vesicular follicles become large and can be distinguished by their overall size as well as by the size of the antrum. 5. When a vesicular follicle ruptures and expels its oocyte (in a process called ovulation), the remnants of the follicle remaining in the ovary turn into a yellowish structure called the corpus luteum (loo-tē ́ŭm; luteus = saffron-yellow). The corpus luteum does not contain an oocyte. However, the corpus luteum secretes the sex hormones progesterone (prō-jes ́ter-ōn; pro = before; gestation) and estrogen (es ́trō-jen; oistrus = estrus, gen = producing). These hormones stimulate the continuing buildup of the uterine lining and prepare the uterus for possible implantation of a fertilized ovum. 6. When a corpus luteum regresses (breaks down), it turns into a white, connective tissue scar called the corpus albicans (al ́bi-kanz; white). Most corpus albicans structures are completely resorbed, and only a few may remain within an ovary. Table 28.2 summarizes the different structures that develop during a female’s monthly cycle. Table 28.2 Ovarian Follicles and Structures That Develop in the Ovary Ovarian Structure Type of Oocyte Anatomic Characteristics Time of First Appearance Primordial follicle Primary oocyte Single layer of flattened follicular cells surround an oocyte Fetal period Primary follicle Primary oocyte Single or multiple layers of cuboidal granulosa cells surround an oocyte Puberty Secondary follicle Primary oocyte Multiple layers of granulosa cells surround the oocyte and a small, fluid-filled antrum Puberty Vesicular follicle Secondary oocyte Many layers of granulosa cells surround the oocyte and a very large antrum Puberty Corpus luteum No oocyte Yellowish, collapsed folds of granulosa cells Puberty Corpus albicans No oocyte Whitish connective tissue scar, remnant of a degenerated corpus luteum Puberty mck78097_ch28_842-878.indd 847 2/14/11 4:43 PM 848 Chapter Twenty-Eight Reproductive System Zona pellucida Primordial follicles Primary oocyte Follicle cells Primary Granulosa oocyte cells Primary follicle Primary Corona oocyte radiata Granulosa Antrum cells Secondary follicle LM 500x LM 500x (b) Primordial follicles LM 50x (c) Primary follicle (d) Secondary follicle Zona pellucida Granulosa cells Secondary follicle Primary oocyte Antrum Suspensory ligament of ovary Primary follicles Medulla Primordial follicles Tunica albuginea Vesicular follicle Antrum Germinal epithelium Secondary oocyte Zona pellucida Ovarian ligament Corona radiata Zona pellucida Ovulated secondary oocyte Corpus albicans (a) Cross section of ovary Corpus albicans Corpus luteum Developing corpus luteum Corona Zona Secondary Antrum radiata pellucida oocyte Corpus luteum LM 80x (g) Corpus albicans Cortex LM 25x (f) Corpus luteum LM 100x (e) Vesicular follicle Figure 28.4 Stages of Follicle Development Within an Ovary. The ovary produces and releases both female gametes (secondary oocytes) and sex hormones. (a) A coronal view of the ovary contents depicts the different stages of follicle maturation, ovulation, and corpus luteum development and degeneration. Note that all of the follicles and structures shown in this image would appear at different times during the ovarian cycle—they do not occur simultaneously. Further, the follicles do not migrate through the ovary; rather, all follicles are shown together merely for comparative purposes. Histologic sections identify (b) primordial follicles, (c) a primary follicle, (d) a secondary follicle, and (e) part of a vesicular follicle. After ovulation, the remnant of the vesicular follicle forms (f) the corpus luteum, which then degenerates into (g) the corpus albicans. mck78097_ch28_842-878.indd 848 2/14/11 4:43 PM Chapter Twenty-Eight Oogenesis is the maturation of a primary oocyte to a secondary oocyte and is illustrated in figure 28.5. To distinguish a primary oocyte from a secondary oocyte, remember that: ■ A primary oocyte is arrested in prophase I (the term “primary” also means “one”). ■ A secondary oocyte is arrested in metaphase II (the term “secondary” means “two”). Before Birth The process of oogenesis begins in a female fetus before birth. At this time, the ovary contains primordial germ cells called oogonia (ō-ō-gō ń ē-a ̆; sing., oogonium; oon = egg), which are diploid cells, meaning they have 23 pairs of chromosomes. During the fetal period, the oogonia start the process of meiosis, but they are stopped at prophase I. At this point, the cells are called primary oocytes. At birth, the ovaries of a female child are estimated to contain approximately 1.5 million primordial follicles within its cortex. The primary oocytes in the primordial follicles remain arrested in prophase I until after puberty. Additionally, remember that the only ovarian follicle containing a secondary oocyte is a vesicular follicle—all other ovarian follicles have primary oocytes only. Oogonia are diploid cells (containing 23 pairs of chromosomes; or 46 total) that are the origin of oocytes. Mitotic divisions of oogonia produce primary oocytes, which are diploid cells. 849 Oogenesis and the Ovarian Cycle Study Tip! Embryonic and fetal period: Reproductive System Oogenesis (Development of Oocytes) 46 Follicle Development Oogonium Mitosis Primary oocytes start the process of meiosis but are arrested in the first meiotic prophase. 46 Primary oocyte (arrested in prophase I) 46 Primary oocyte (remains arrested in prophase I) Primordial follicle Childhood: Ovary is inactive. It houses primordial follicles. Monthly, from puberty to menopause: Primary follicle Approximately 20 primordial follicles mature into primary follicles every month. A few of these primary follicles mature into secondary follicles. Secondary oocyte (arrests in metaphase II) Only one or two of the secondary follicles mature into a vesicular follicle, where the primary oocyte completes the first meiotic division to produce a polar body and a secondary oocyte. The secondary oocyte is a haploid (containing 23 chromosomes only) cell that is arrested in the second meiotic metaphase. If the secondary oocyte is fertilized, it completes the second meiotic division and becomes an ovum. If the secondary oocyte is not fertilized, it degenerates. 23 First polar body (degenerates) Secondary follicle Vesicular follicle Ovulation Sperm Meiosis II completed (only if fertilization occurs) Ovulated secondary oocyte 23 Second polar body (degenerates) 23 Ovum Figure 28.5 Oogenesis. Oogenesis begins in a female fetus, when primary oocytes develop in primordial follicles. The ovary and these follicles remain inactive during childhood. At puberty, a select number of primordial follicles each month undergoes maturation and produces a female gamete (secondary oocyte). mck78097_ch28_842-878.indd 849 2/14/11 4:44 PM 850 Chapter Twenty-Eight Reproductive System Childhood During childhood, a female’s ovaries are inactive, and no follicles develop. In fact, the main event that occurs during childhood is atresia (a -̆ trē ź ē-a ̆; a = not, tresis = a hole), in which some primordial follicles degenerate. By the time a female child reaches puberty, only about 400,000 primordial follicles remain in the ovaries. From Puberty to Menopause When a female child reaches puberty, the hypothalamus increases its release of GnRH (gonadotropinreleasing hormone), which stimulates the anterior pituitary to release FSH (follicle-stimulating hormone) and LH (luteinizing hormone). The levels of FSH and LH vary in a cyclical pattern and produce a monthly sequence of events in follicle development called the ovarian cycle. The three phases of the ovarian cycle are the follicular phase, ovulation, and the luteal phase (figure 28.6). The follicular phase occurs during days 1–13 of an approximate 28-day ovarian cycle. At the beginning of the follicular phase, FSH and LH stimulate about 20 primordial follicles to mature into primary follicles. It is unclear why some of the primordial follicles in the ovary are stimulated to mature into primary follicles, while the remainder remains unaffected by the FSH and LH secretion. As the follicles develop, their granulosa cells release the hormone inhibin, which helps inhibit FSH production, thus preventing excessive ovarian follicle development and allowing the current primary follicles to mature. Shortly thereafter, a few of these primary follicles mature and become secondary follicles. The primary follicles that do not mature undergo atresia. Late in the follicular phase, one secondary follicle in an ovary matures into a vesicular follicle. Under the influence of LH, the volume of fluid increases within the antrum, and the oocyte is forced toward one side of the follicle, where it is surrounded by a cluster of granulosa cells termed the cumulus (kū m ́ ū-lu s̆ ; heap) oophorus (ō-of ́ōr-u s̆ ; phorus = bearing). The innermost layer of these cells is the corona radiata. As the secondary follicle matures into a vesicular follicle, its primary oocyte finishes meiosis I, and two cells form (see figure 28.5). One of these cells receives a minimal amount of cytoplasm and forms a polar body, which is a nonfunctional cell that later deteriorates. The other cell receives the bulk of the cytoplasm and becomes the secondary oocyte, which continues to develop and reaches metaphase II of meiosis before it is arrested again. This secondary oocyte does not complete meiosis unless it is fertilized by a sperm. If the oocyte is never fertilized, it breaks down and degenerates about 24 hours later. Ovulation (ov ū́ -lā ś hu n̆ ) occurs on day 14 of a 28-day ovarian cycle and is defined as the release of the secondary oocyte from a vesicular follicle (figure 28.6). Typically, only one ovary ovulates each month—that is, the left ovary ovulates one month, and the right ovary ovulates the next. Ovulation is induced only when there is a peak in LH secretion. As the time of ovulation mck78097_ch28_842-878.indd 850 approaches, the granulosa cells in the vesicular follicle increase their rate of fluid secretion, forming a larger antrum and causing further swelling within the follicle. The edge of the follicle that continues to expand at the ovarian surface becomes quite thin and eventually ruptures, expelling the secondary oocyte. The luteal phase occurs during days 15–28 of the ovarian cycle, when the remaining granulosa cells in the ruptured vesicular follicle turn into a corpus luteum. The corpus luteum secretes progesterone and estrogen that stabilize and build up the uterine lining, and prepare for possible implantation of a fertilized ovum. The corpus luteum has a life span of about 10–13 days if the secondary oocyte is not fertilized. After this time, the corpus luteum regresses and becomes a corpus albicans. As the corpus luteum degenerates, its levels of secreted progesterone and estrogen drop, causing the uterine lining to be shed as menstruation (men-strooā ś hu ̆n), also called menses or a period. This event marks the end of the luteal phase. A female’s first menstrual cycle, called menarche (me-nar ́kē; men = month, arche = beginning), is the culmination of female puberty and typically occurs around age 11–12. If the secondary oocyte is fertilized and if it successfully implants in the uterine lining, this fertilized structure (now a preembryo) begins its own development (as discussed in chapter 3). The pre-embryo starts secreting human chorionic gonadotropin (hCG), a hormone that enters the mother’s bloodstream and acts on the corpus luteum. Essentially, hCG lets the corpus luteum know that implantation has occurred and that the corpus luteum should continue producing progesterone, which will build and stabilize the uterine lining. After 3 months, the placenta of the developing fetus starts producing its own progesterone and estrogen, so by the end of the third month, the corpus luteum has usually regressed and formed a corpus albicans. After Menopause The time when a woman is nearing menopause is called perimenopause. During perimenopause, estrogen levels begin to drop, and a woman may experience irregular periods, skip some periods, or have very light periods. When a woman has stopped having monthly menstrual cycles for 1 year and is not pregnant, she is said to be in menopause (men ́ō-pawz; pauses = cessation). The age at onset of menopause varies considerably, but typically is between 45 and 55 years. Menopause is reached when there are no longer any ovarian follicles or the follicles that remain stop maturing. As a result, significant amounts of estrogen and progesterone are no longer secreted. Thus, a woman’s endometrial lining does not grow, and she no longer has a menstrual period. W H AT D O Y O U T H I N K ? 1 ● If a woman has one ovary surgically removed, can she still become pregnant? Why or why not? 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 851 Ovarian cycle Primary follicle Days 1 Secondary follicle 3 5 Vesicular follicle 7 9 Ovulation 11 Follicular phase 13 15 Corpus luteum forms 17 19 21 23 Regression Corpus albicans 25 27 1 Luteal phase Ovulation Gonadotropin levels FSH LH Days 1 3 5 7 9 11 13 15 17 19 21 23 25 27 1 Ovulation Ovarian hormone levels Estrogen Progesterone Days 1 3 5 7 9 11 13 15 17 19 21 23 25 27 1 5 7 9 11 13 15 17 19 21 23 25 27 1 Uterine cycle Menstrual flow Functional layer Basal layer Days 1 3 Menstrual phase Proliferative phase Secretory phase Figure 28.6 Hormonal Changes in the Female Reproductive System. Cyclic changes in gonadotropins affect ovarian hormone production. FSH causes development of estrogen-producing ovarian follicles during the follicular phase of the ovarian cycle. Estrogen stimulates the proliferative phase in the uterine cycle. Estrogen levels spike as ovulation approaches. High levels of LH promote ovulation at the midpoint of the ovarian cycle. The corpus luteum becomes functional after ovulation, and it produces both progesterone and estrogen to promote uterine lining development. If fertilization does not occur, the corpus luteum degenerates, and menstrual flow begins at the start of the next uterine cycle. mck78097_ch28_842-878.indd 851 2/14/11 4:44 PM 852 Chapter Twenty-Eight Reproductive System 28.2b Uterine Tubes CLINICAL VIEW The uterine tubes, also called the fallopian (fa-lō ṕ ē-an) tubes or oviducts (ō v́ i-du ̆kt; duco = to lead), extend laterally from both sides of the uterus toward the ovaries (figure 28.7). Fertilization of the secondary oocyte occurs in the lateral part of these tubes, and the pre-embryo begins to develop as it travels toward the uterus. Usually it takes the pre-embryo about 3 to 4 days to reach the lumen of the uterus. The uterine tubes are small in diameter, and reach their maximum length of between 10 and 12 centimeters after puberty. These tubes are covered and suspended by the mesosalpinx (mez ́ō-sal ṕ inks; salpinx = trumpet), a specific superior part of the broad ligament of the uterus (see figure 28.3a). Each uterine tube is composed of contiguous segments that are distinguishable in both gross examination and histologic sections: The infundibulum (in-fu n̆ -dib ū́ -lu m ̆ ; funnel) is the free, funnel-shaped, lateral margin of the uterine tube. Its numerous individual fingerlike folds are called fimbriae (fim ́ brē-ē; fringes). The fimbriae of the infundibulum are not attached to the ovary but enclose it at the time of ovulation. The ampulla (am-pul ́ la ̆; two-handled bottle) is the expanded region medial to the infundibulum. Fertilization of a secondary oocyte typically occurs there. The isthmus (is m ́ us) extends medially from the ampulla toward the lateral wall of the uterus. It forms about onethird of the length of the uterine tube. The uterine part (intramural part or interstitial segment) extends medially from the isthmus and is continuous with the wall of the uterus. ■ ■ ■ ■ Study Tip! One way to remember the segments of the uterine tubes is as follows: ■ The infundibulum (the only segment with an “F” in it) has the fimbriae. ■ The ampulla is the arm of the uterine tube. ■ The isthmus is the longest. ■ The uterine part is in the uterus. The wall of the uterine tube consists of a mucosa, a muscularis, and a serosa. The mucosa is formed from a ciliated columnar epithelium and a layer of areolar connective tissue. The mucosa is thrown into linear folds, which reduce the size of the uterine tube lumen. After ovulation, the cilia on the apical surface of the epithelial cells of both the infundibulum and the ampulla begin to beat in the direction of the uterus. This beating causes a slight current in the fluid within the uterine tube lumen, drawing the ovulated oocyte into the uterine tube and moving it toward the uterus. The muscularis is composed of an inner circular layer and an outer longitudinal layer of smooth muscle cells. The muscular layer increases in relative thickness as the uterine tube approaches the lateral wall of the uterus. Some peristaltic contractions in the mck78097_ch28_842-878.indd 852 Tubal Pregnancy In a tubal pregnancy (or ectopic pregnancy), the fertilized oocyte implants in the uterine tube, rather than traveling to the uterus for implantation. The main danger in a tubal pregnancy is that the uterine tube is unable to expand as the embryo grows. Thus, the embryo can remain viable no later than week 8, at which time it has become too large for the confines of the uterine tube. The woman experiences severe cramping, and the uterine tube may rupture if the embryo is not surgically removed. If the uterine tube ruptures, a massive hemorrhage into the abdominopelvic cavity can endanger the life of the mother. Unfortunately, there currently is no way to treat a tubal pregnancy that can spare the developing embryo. muscularis help propel the secondary oocyte, or pre-embryo if fertilization has occurred, through the uterine tube toward the uterus. The serosa is the external serous membrane covering the uterine tube. W H AT D I D Y O U L E A R N? 3 ● 4 ● 5 ● 6 ● What are the types of ovarian follicles? What is ovulation? What is menarche, and when does it occur? What type of epithelium lines the uterine tubes, and what is its function? 28.2c Uterus The uterus (ū t́ er-u s̆ ; womb) is a pear-shaped, thick-walled muscular organ within the pelvic cavity. It has a lumen (internal space) that connects to the uterine tubes superolaterally and to the vagina inferiorly (figure 28.7a). Normally, the uterus is angled anterosuperiorly across the superior surface of the urinary bladder, a position referred to as anteverted (an-te-vert é d; ante = before, versio = a turning). If the uterus is positioned posterosuperiorly (so that it is projecting toward the rectum), this position is called retroverted (re t́ rō-ver-ted). In older women, the uterus may shift from anteverted to retroverted. The uterus serves many functions. Following fertilization, the pre-embryo makes contact with the uterine lining and implants in the inner uterine wall. The uterus then supports, protects, and nourishes the developing embryo/fetus by forming a vascular connection that later develops into the placenta. The uterus ejects the fetus at birth after maternal oxytocin levels increase to initiate the uterine contractions of labor. If a secondary oocyte is not fertilized, the muscular wall of the uterus contracts and sheds its inner lining as menses. The uterus is partitioned into the following regions: ■ The fundus (fu n̆ ́du s̆ ) is the broad, curved superior region extending between the lateral attachments of the uterine tubes. 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 853 Muscularis Mucosa Lumen of uterine tube Simple ciliated columnar epithelium LM 35x (b) Uterine tube Ovarian blood vessels Suspensory ligament of ovary LM 400x Uterine tube Ovarian ligament Fundus of uterus Uterine tube Uterine part Isthmus Lumen of uterus Ampulla Infundibulum Fimbriae Mesosalpinx Ovary Body of uterus Endometrium Myometrium Perimetrium Broad ligament Wall of uterus Round ligament Isthmus Uterine blood vessels Internal os Cervical canal Ureter Cervix External os Lumen of uterus Epithelium Transverse cervical ligament Functional layer Vagina (a) Posterior view Uterine glands Endometrium Basal layer Figure 28.7 Uterine Tubes and the Uterus. The uterine tubes are paired passageways that capture the ovulated secondary oocyte, provide the site for fertilization, and transport the oocyte to the uterus. (a) The relationship between a uterine tube and the uterus is shown in a posterior view (left) and a partially cut-away diagram (right). (b) A photomicrograph shows a cross section of the uterine tube. (c) A photomicrograph shows the layers of the endometrium and part of the myometrium in the wall of the uterus. mck78097_ch28_842-878.indd 853 Myometrium LM 45x (c) Uterus 2/14/11 4:44 PM 854 Chapter Twenty-Eight Reproductive System CLINICAL VIEW Cervical Cancer Cervical cancer is one of the most common malignancies of the female reproductive system. It is estimated that over 12,000 new cases of invasive cervical cancer and four times that many noninvasive cervical cancer cases are diagnosed each year. Approximately 4000 women die from cervical cancer annually. Risk factors for cervical cancer include increased age, HIV infection, and low socioeconomic status. However, the most important risk factor is previous human papillomavirus (HPV) infection. Some classes of HPV are considered “high risk” because they are frequently associated with genital and anal cancers in men and women and are sexually transmitted. The Papanicolaou (Pap) smear has become a very effective method of detecting cervical cancer in its early and curable stage. The test is done in the doctor’s office as follows: 1. The health-care professional inserts a metal or plastic instrument called a speculum (spek ū́ -lŭm; mirror) into the vagina to keep the vagina open in order to examine the cervix. 2. Epithelial cells are scraped from the edge of the cervix and placed (smeared) on a microscope slide. 3. The slide is sent to a lab, where a cytologist stains the cells and examines them under the microscope, noting any abnormal cellular development (termed dysplasia). If dysplastic cells are detected, the health-care professional will likely request a follow-up Pap smear and possibly even a biopsy. Sometimes, dysplastic cells are a result of irritation, infection, or some undetermined cause, and are not cancerous. But if advanced dysplasia is seen, most physicians immediately recommend a biopsy of the cervix, and may even insist on HPV testing. If cervical cancer is present, surgery is indicated. To treat a cancer that is localized, a portion of the cervix may be removed, a procedure known as a cone biopsy. In the case of invasive cancer, removal of the entire uterus, called a hysterectomy (his-ter-ek t́ ō-mē ; hystera = womb), is indicated. Researchers recently have developed vaccines (Gardasil and Cervarix) for the most common types of HPV that cause cervical cancer. The vaccines are targeted for women and girls between the ages of 9 and 26 years. Normal cells Epithelial cells LM 160x LM 140x Normal Pap smear. ■ ■ ■ The major part of the uterus is its middle region, called the body, which is composed of a thick wall of smooth muscle. A narrow, constricted inferior region of the body that is superior to the cervix is called the isthmus. The cervix is the narrow inferior portion of the uterus that projects into the vagina. Within the cervix is a narrow channel called the cervical canal, which connects to the vagina inferiorly. The superior opening of this canal is the internal os (os = mouth). The inferior opening of the cervix into the lumen of the vagina is the external os. The external os is covered with nonkeratinized stratified squamous epithelium. The cervix contains mucin-secreting glands that help form a thick mucus plug at the external os. This mucus plug is suspected to be a physical barrier that prevents pathogens from invading the uterus from the vagina. The mucus plug thins considerably around the time of ovulation, so sperm may more easily enter the uterus. mck78097_ch28_842-878.indd 854 Dysplastic cells Abnormal Pap smear. Support of the Uterus Several structures support the uterus. The muscles of the pelvic floor (pelvic diaphragm and urogenital diaphragm) (see figure 11.15) hold the uterus and vagina in place and help resist intraabdominal pressure exerted inferiorly on the pelvis. The round ligaments (figure 28.7) of the uterus extend from the lateral sides of the uterus, through the inguinal canal and attach to the labia majora. These ligaments help keep the uterus in an anteverted position. The transverse cervical ligaments (or cardinal ligaments) run from the sides of the cervix and superior vagina laterally to the pelvic wall. They help restrict inferior movements of the uterus. The uterosacral ligaments (or sacrocervical ligaments; not shown in figure 28.7) connect the inferior portion of the uterus posteriorly to the sacrum. Many of these ligaments travel between the folds of the broad ligament. Weakness in either the pelvic floor muscles or these ligaments can lead to prolapse (prō-laps ;́ prolapsus = a failing) of the uterus, in which the uterus starts to protrude through the vagina. 2/14/11 4:44 PM Chapter Twenty-Eight Despite its name, the broad ligament is not a strong support for the uterus, but rather a peritoneal drape over the uterus. Blood Supply Each internal iliac artery extends a branch called a uterine artery through the broad ligament to the lateral wall of the uterus (see figure 28.7). Numerous smaller branches from the uterine artery then penetrate the muscular wall of the uterus and further diverge into arcuate arteries. Thereafter, each arcuate artery gives rise to smaller vessels, called radial arteries, which extend into the innermost layer (endometrium) of the uterus. Here they branch into spiral arteries, which swirl throughout the endometrium, extending between and throughout the uterine glands (described below) toward the mucosal surface. Wall of the Uterus The uterine wall is composed of three concentric tunics: the perimetrium, myometrium, and endometrium (figure 28.7). The outer tunic of most of the uterus is a serosa called the perimetrium (per-imē t́ rē-u ̆m; metra = uterus). The perimetrium is continuous with the broad ligament. The myometrium (mı̄ ́ō-mē t́ rē-u ̆m; mys = muscle) is the thick, middle tunic of the uterine wall formed from three intertwining layers of smooth muscle. In the nonpregnant uterus, the muscle cells are less than 0.25 millimeter in length. During the course of a pregnancy, smooth muscle cells increase both in size (hypertrophy; hı̄-per t́ rō-fē) and in number (hyperplasia; hi-perplā ź hē-a )̆ . Some cells may exceed 5 millimeter in length by the end of gestation. The innermost tunic of the uterus, called the endometrium (en ́dō-mē t́ rē-u ̆m), is an intricate mucosa composed of a simple columnar epithelium and an underlying lamina propria. The lamina propria is filled with compound tubular glands (also called uterine glands), which enlarge during the uterine cycle. Two distinct layers form the endometrium. The deeper layer is the basal layer, also called the stratum basalis (bā-sā ́ lis). The basal layer is immediately adjacent to the myometrium, and is a permanent layer that undergoes few changes during each uterine cycle. The more superficial of the two endometrial layers is the functional layer, or Reproductive System 855 stratum functionalis (fŭnk-shŭn-ăl ́ is). Beginning at puberty, the functional layer grows from the basal layer under the influence of estrogen and progesterone secreted from the ovarian follicles. If fertilization and implantation do not occur, this lining is shed as menses. Uterine (Menstrual) Cycle and Menstruation The cyclical changes in the endometrial lining occur under the influence of estrogen and progesterone secreted by the ovary. The uterine cycle (or menstrual cycle) consists of three distinct phases of endometrium development: the menstrual phase, proliferative phase, and secretory phase (see figure 28.6, bottom). The menstrual (menś troo-ăl; menstrualis = monthly) phase occurs approximately during days 1–5 of the cycle. This phase is marked by sloughing of the functional layer and lasts through the period of menstrual bleeding. The proliferative (prō-lif ́er-ă-tiv; proles = offspring, fero = to bear) phase follows, spanning approximately days 6–14. The initial development of the functional layer of the endometrium overlaps the time of follicle growth and estrogen secretion by the ovary. The last phase is the secretory (se-krēt ́e-̆ rē, sē ́kre-̆ tōr-ē) phase, which occurs between approximately days 15–28. During the secretory phase, increased progesterone secretion from the corpus luteum results in increased vascularization and development of uterine glands. If the secondary oocyte is not fertilized, the corpus luteum degenerates, and the progesterone level drops dramatically. Without progesterone, the functional layer lining sloughs off, and the next uterine cycle begins with the menstrual phase. Table 28.3 compares the uterine cycle with the ovarian cycle discussed previously, and table 28.4 summarizes the hormones that influence the ovarian and uterine cycles. The day ranges listed on figure 28.6 and table 28.3 assume that the woman has a 28-day cycle, meaning she ovulates at day 14 and has a menstrual period every 28 days. If a woman has a longer cycle (say, she menstruates about every 35 days), her menstrual phase and/or her proliferative phase is longer than average. Typically, a woman ovulates 14 days before menstruation, so the secretory phase day ranges do not vary as much. W H AT D O Y O U T H I N K ? 2 ● CLINICAL VIEW Endometriosis Endometriosis (en ́ dō-mē -trē -ō ś is) occurs when part of the endometrium is displaced onto the external surface of organs within the abdominopelvic cavity. Scientists think that during the regular uterine (menstrual) cycle of some women, a small amount of endometrium may be expelled from the uterine tubes and become implanted on the surface of the ovaries, uterine tubes, urinary bladder, and intestines. If this displaced endometrium remains viable, it responds to hormone stimulation during each menstrual growth phase. Unfortunately, at the end of the monthly cycle, this displaced endometrium cannot slough and be expelled. Thus, the ensuing hemorrhage and breakdown of the displaced endometrium cause considerable pain and eventually scarring that often leads to deformities of the uterine tubes. Treatments include the use of hormones designed to retard the growth and cycling of the displaced endometriotic tissue, as well as surgical removal of the ectopic endometrium. mck78097_ch28_842-878.indd 855 What factors could influence the length and timing of a woman’s monthly uterine cycle? 28.2d Vagina The vagina (va -̆ jı̄ n ́ a )̆ is a thick-walled, fibromuscular tube that forms the inferiormost region of the female reproductive tract and measures about 10 centimeters in length in an adult female (see figure 28.2). The vagina connects the uterus with the outside of Table 28.3 Comparison of Ovarian and Uterine Cycle Phases Day1 Ovarian Cycle Phase 1–5 Uterine Cycle Phase Menstrual phase Follicular phase 6–13 Proliferative phase 14 Ovulation 15–28 Luteal phase Secretory phase 1 This table assumes a 28-day cycle between menstrual periods. If a woman has a longer or shorter cycle, the day ranges for the follicular, menstrual, and proliferative phases will vary. 2/14/11 4:44 PM 856 Chapter Twenty-Eight Reproductive System Table 28.4 Influence of Hormones on the Ovarian and Uterine Cycles Hormone Primary Source of Hormone Effects Gonadotropin-releasing hormone (GnRH) Hypothalamus Stimulates anterior pituitary to produce and secrete FSH and LH Follicle-stimulating hormone (FSH) Anterior pituitary Stimulates development and maturation of ovarian follicles Luteinizing hormone (LH) Anterior pituitary Stimulates ovulation (when there is a peak in LH) Estrogen Ovarian follicles (before ovulation), corpus luteum (after ovulation), or placenta (during pregnancy) Initiates and maintains growth of the functional layer of the endometrium Progesterone Corpus luteum or placenta (during pregnancy) Primary hormone responsible for functional layer growth after ovulation; causes increase in blood vessel distribution, uterine gland size, and nutrient production Inhibin Ovarian follicles Inhibits FSH secretion, so as to prevent excessive follicular development the body anteroventrally, and thus functions as the birth canal. The vagina is also the copulatory organ of the female, as it receives the penis during intercourse, and it serves as the passageway for menstruation. The vaginal wall is heavily invested with both blood vessels and lymphatic vessels. Arterial supply comes from the vaginal arteries, and venous drainage is via vaginal veins. The lumen of the vagina is flattened anteroposteriorly. The vagina’s relatively thin, distensible wall consists of three tunics: an inner mucosa, a middle muscularis, and an outer adventitia. The mucosa consists of a nonkeratinized stratified squamous epithelium and a highly vascularized lamina propria (figure 28.8). Figure 28.8 Histology of the Vagina. The epithelial lining of the vagina in a mature female is a stratified squamous epithelium. Nonkeratinized stratified squamous epithelium Mucosa Lamina propria Muscularis LM 50x mck78097_ch28_842-878.indd 856 2/14/11 4:44 PM Chapter Twenty-Eight Superificial cells of the vaginal epithelium contribute to the acidic environment that helps prevent bacterial and other pathogenic invasion. During each menstrual phase, large numbers of lymphocytes and granulocytes invade the lamina propria to help prevent infection during menstruation. The inferior region of the vaginal mucosa contains numerous transverse folds, or rugae. Near the external opening of the vagina, called the vaginal orifice, these mucosal folds project into the lumen to form a vascularized, membranous barrier called the hymen (hı̄ m ́ en; membrane). The hymen typically is perforated during the first instance of sexual intercourse, but also may be perforated by tampon use, medical exams, or very strenuous physical activity. The muscularis of the vagina has both outer and inner layers of smooth muscle. The outer layer is composed of bundles of longitudinal smooth muscle cells that are continuous with corresponding muscle cells in the myometrium. The smooth muscle cells of the inner circular layer are interwoven with the outer longitudinal muscle fibers at the point where the two muscle layers meet. Near the vaginal orifice are some skeletal muscle fibers of the muscularis layer that cause partial narrowing of the vaginal orifice. The adventitia contains some inner elastic fibers and an outer layer of areolar connective tissue. 28.2e External Genitalia The external sex organs of the female are termed the external genitalia or vulva (vu ̆l v́ a ̆; a covering) (figure 28.9). The mons (monz; mountain) pubis is an expanse of skin and subcutaneous connective tissue immediately anterior to the pubic symphysis. The mons pubis is covered with pubic hair in postpubescent females. The labia majora (lā ́ bē-a ̆ ma -̆ jŏr á ̆; sing., labium majus; labium = lip, majus = larger) are paired, thickened folds of skin and connective tissue. The labia majora are homologous to the scrotum of the male. In adulthood, their outer surface is covered with coarse pubic hair; they contain numerous sweat and sebaceous glands. The labia minora (mı̄-nŏr á ̆; sing., labium minus; minus = smaller) are paired folds immediately internal to the labia majora. They are devoid of hair and contain a highly vascular layer of areolar connective tissue. Sebaceous glands are located in these folds, as are numerous melanocytes, resulting in enhanced pigmentation of the folds. The space between the labia minora is called the vestibule. Within the vestibule are the urethral opening and the vaginal orifice. On either side of the vaginal orifice is an erectile body called the bulb of the vestibule (see figure 27.10), which becomes erect and increases in sensitivity during sexual intercourse. A pair of greater vestibular glands (previously called glands of Bartholin) are housed within the posterolateral walls of the vestibule. These mck78097_ch28_842-878.indd 857 Reproductive System 857 Mons pubis Glans of clitoris Prepuce Labia minora Urethral opening Vestibule Vaginal orifice Openings for greater vestibular glands Labia majora Anus Figure 28.9 Female External Genitalia. Inferior view of the external genitalia, illustrating the urethral opening and the vaginal orifice, which are within the vestibule and bounded by the labia minora. are tubuloacinar glands that secrete mucin, which forms mucus to act as a lubricant for the vagina. Secretion increases during sexual intercourse, when additional lubrication is needed. These secretory structures are homologous to the male bulbourethral glands. The clitoris (klit ́ō-ris) is a small erectile body, usually less than 2 centimeters in length, located at the anterior regions of the labia minora. It is homologous to the penis of the male. Two small erectile bodies called corpora cavernosa form the body of the clitoris. Extending from each of these bodies posteriorly are elongated masses, each called the crus (kroos) of the clitoris, which attach to the pubic arch. Capping the body of the clitoris is the glans (glanz; acorn). The many specialized sensory nerve receptors housed in the clitoris provide pleasure to the female during sexual intercourse. The prepuce (prē ṕ oos; foreskin) is an external fold of the labia minora that forms a hoodlike covering over the clitoris. 28.2f Mammary Glands Each mammary gland, or breast, is located within the anterior thoracic wall and is composed of a compound tubuloalveolar exocrine 2/14/11 4:44 PM 858 Chapter Twenty-Eight CLINICAL VIEW: Reproductive System In Depth Contraception Methods The term contraception (kon-tră-sep ś hun) refers to birth control, or the prevention of pregnancy. A wide range of birth control methods are available, and they have varying degrees of effectiveness. Abstinence (ab ś ti-nens; abstineo = to hold back) means refraining from sexual intercourse. Abstinence (when practiced correctly) is the only 100% proven way not to become pregnant. Natural family planning (also known as the rhythm method) requires avoiding sexual intercourse during the time when a woman is ovulating. Because sperm can live for several days in the female reproductive tract, it is best to avoid intercourse both a few days prior and a few days after ovulating. The rhythm method requires that a woman know when she is ovulating, which may be difficult to determine consistently. As a result, this method has a high failure rate (25%). (a) Condoms Lactation (nursing a baby) can prevent ovulation and menstruation for several to many months after childbirth if a woman nurses her child constantly (i.e., much more than five times a day!). The frequent lactation sends signals to the hypothalamus to prevent FSH and LH from being secreted, thus preventing ovulation. Many U.S. women do not nurse a child constantly, so lactation is not a reliable birth control method for them. If a woman is lactating, she should always use another form of birth control as well, because she will not know when her ovulation cycle begins again. Barrier methods of birth control use a physical barrier to prevent sperm from reaching the uterine tubes. Barrier methods include the following: ■ Condoms, when used properly, collect the sperm and prevent them from entering the female reproductive tract. They are also the only birth control method that helps protect against sexually transmitted viruses and diseases, such as human (b) Spermicidal foams (c) Diaphragm Each ductus deferens is tied off and cut Uterine tubes are tied off and cut (d) Oral contraceptive (e) Intrauterine device (IUD) (f) Tubal ligation (g) Vasectomy Contraception includes barrier, chemical, and surgical methods. gland (figure 28.10). The gland’s complex secretory product (called breast milk) contains proteins, fats, and a sugar to provide nutrition to infants. The nipple (nip ́l; beak) is a cylindrical projection on the center of the breast. It contains multiple tiny openings of the excretory ducts that transport breast milk. The areola (a -̆ rē ́ō-la ̆; small area) is the pigmented, rosy or brownish ring of skin around the nipple. Its surface often appears uneven and grainy due to the numerous sebaceous glands, called areolar glands, immediately internal to the surface. The color of the areola may vary, depending upon whether or not a woman has given birth. In a nulliparous mck78097_ch28_842-878.indd 858 (nu ̆l-ip ́a -̆ rus̆ ; nullus = none, pario = to bear) woman (a woman who has never given birth), the areola is rosy or light brown in color. In a parous (par ú s̆ ) woman (a woman who has given birth), the areola may change to a darker rose or brown color. Internally, the breasts are supported by fibrous connective bands called suspensory ligaments. These thin bands extend from the skin and attach to the deep fascia overlying the pectoralis major muscle. Thus, the breast and the pectoralis major muscle are structurally linked. The mammary glands are subdivided into lobes, which are further subdivided into smaller compartments called lobules. Lobules 2/14/11 4:44 PM Chapter Twenty-Eight ■ ■ papillomavirus, herpes, and HIV. Condoms for males fit snugly on the erect penis, while vaginal condoms are placed in the vagina prior to sexual intercourse. The typical use failure rate for condoms is about 14–15%. Spermicidal foams and gels are chemical barrier methods that kill sperm before they travel to the uterine tubes. They are inserted into the vagina and/or placed on the penis prior to sexual intercourse. Foams and gels are not the most effective method of birth control (alone the failure rate is typically 26%); rather, they should be used in conjunction with a physical barrier method. Diaphragms and cervical caps are circular, rubbery structures that are inserted into the vagina and placed over the cervix prior to sexual intercourse. Spermicidal gel is used around the edges to help prevent sperm from entering the cervix. Some women find it difficult to correctly place the diaphragm or cervical cap, and incorrect placement can result in pregnancy. The failure rate for these products is high, typically 20% for diaphragms and up to 40% for cervical caps. Intrauterine devices (IUDs) are T-shaped, flexible plastic structures inserted into the uterus by a health-care provider. Once in place, the IUD prevents fertilization from occurring. The IUD may contain copper, a synthetic progestin, or levonorgestrel. IUDs containing copper are effective for up to 10 years; those with progestin (e.g., Progestasert) must be replaced every year; and those containing levonorgestrel (e.g., Mirena) are effective for 5 years. Although few women in North America use IUDs, their failure rate typically is low (2%). Chemical methods of birth control are very effective if used properly. They include the following: ■ Oral contraceptives, commonly called birth control pills, come in packets varying from 21 to 91 days. These pills contain low levels of estrogen and/or progestins. Some packets include a week of non-hormone-containing pills, during which circulating levels of estrogen and progestins drop, and menstruation occurs. (Note: Progesterone is one type of progestin.) The low levels of estrogen and progestins prevent the LH “spike” needed for ovulation. Thus, oral contraceptives prevent ovulation. Typically, menstrual flow is much lighter when a woman takes oral contraceptives because the circulating levels of hormones were low to begin with, so the uterine lining does not build up much. Oral contraceptives require a woman to take a pill a day, at about the same time each day. If she misses one or more days of pills, ovulation may occur. Typical use failure rate depends upon the product and is between 0.1% and 7%. contain secretory units termed alveoli that produce milk in the lactating female. Alveoli become more numerous and larger during pregnancy. Tiny ducts drain milk from the alveoli and lobules. The tiny ducts of the lobules merge and form 10 to 20 larger channels called lactiferous (lak-tif ́er-ŭs; lact = milk, fero = to bear) ducts. A lactiferous duct drains breast milk from a single lobe. As each lactiferous duct approaches the nipple, its lumen expands to form a lactiferous sinus, a space where milk is stored prior to release from the nipple. Breast milk is released by a process called lactation (laktā ś hu n̆ ; lactatio = to suckle), which occurs in response to a complex sequence of internal and external stimuli. Normally, a mck78097_ch28_842-878.indd 859 ■ ■ ■ ■ ■ Reproductive System 859 Estrogen/progestin (prō-jes ́tin; pro = before, gestation) patches are alternatives to the daily oral contraceptive. A patch placed on the body delivers a regular amount of estrogen/progestin through the skin (transdermally). The patch is replaced each week. The typical use failure rate is 1%. Implanted/injected progestins help prevent pregnancy by preventing ovulation and thickening the mucus around the cervix (thus creating a slight physical barrier to the sperm). Medroxyprogesterone (Depo-Provera) is an injectable contraceptive given once every 3 months, while etonogestrel (Implanon) is an implantable contraceptive that lasts for up to 3 years. The drawback is that ovulation may not occur for many months after stopping their use. The typical use failure rate for these products is 0.3%. A vaginal ring is a flexible polymer ring that contains etonogestrel and estradiol (NuvaRing) and is placed in the vagina. The hormones are slowly absorbed directly by the reproductive organs. One ring lasts 3 weeks; it is removed for a week and menstruation occurs. Typical use failure rate is 8%. Morning-after pills containing levonorgestrel (e.g., Preven or Plan B One-Step) must be taken within 48 to 72 hours after having unprotected intercourse and is most effective when taken within 24 hours. These pills work by inhibiting ovulation, altering the menstrual cycle to delay ovulation, or irritating the uterine lining to prevent implantation. Mifepristone (Mifiprex in the United States; RU-486 in Europe) was approved in 2000 by the U.S. Food and Drug Administration for use during the first 7 weeks of pregnancy. Mifepristone blocks progesterone receptors, so progesterone cannot attach to these receptors and thereby maintain a pregnancy. When taken with a prostaglandin drug, mifepristone induces a miscarriage. Mifepristone’s existence is very politically charged, with both sides of the abortion debate arguing for or against its use. The surgical methods of contraception are tubal ligation for females and vasectomy (va-sek t́ ō-mē ) for males. In a tubal ligation, both uterine tubes are cut, and the ends are clipped, tied, or cauterized shut. Thus, tubal ligation prevents both sperm from reaching the oocyte and the oocyte from reaching the uterus. A vasectomy is an outpatient procedure whereby each ductus deferens is cut and the ends tied, clipped, or cauterized shut. Sperm cannot leave the testis and thus are not ejaculated. Both surgeries are very effective birth control methods, but they are meant to be permanent and irreversible, so they are not considered options for people who wish to have more children. woman starts to produce breast milk when she has recently given birth. When a woman is pregnant, the levels of estrogen, progesterone, and prolactin rise dramatically. Recall from chapter 20 that the hormone prolactin is produced in the anterior pituitary and is responsible for milk production. Thus, when the amount of prolactin increases, the mammary gland grows and forms more expanded and numerous alveoli. While prolactin stimulates production of breast milk, the hormone oxytocin is responsible for milk ejection. Recall from chapter 20 that oxytocin is secreted by the hypothalamus and stored in the posterior lobe of the pituitary, and is also responsible for uterine contractions 2/14/11 4:44 PM 860 Chapter Twenty-Eight Reproductive System Suspensory ligaments Adipose tissue Suspensory ligaments Intercostal muscles Pectoralis minor Lactiferous sinus Pectoralis major Lobe Lobe Areolar gland Lactiferous sinus Alveoli Lactiferous ducts Nipple Areola Deep fascia Nipple Alveoli Rib Lobule Lobule Lactiferous ducts (a) Anteromedial view (b) Sagittal view Figure 28.10 Mammary Glands. The mammary glands are composed of glandular tissue and a variable amount of fat. (a) An anterior view is partially cut away to reveal internal structures. (b) A diagrammatic sagittal section of a mammary gland shows the distribution of alveoli within lobules and the extension of ducts to the nipple. CLINICAL VIEW Breast Cancer Breast cancer affects approximately 1 in every 8 women in the United States, and it also occurs in males, although infrequently. The incidence of breast cancer is rare before age 20. Then it rises steadily to peak at about the age of menopause. Some well-documented risk factors for breast cancer include: maternal relatives with breast cancer, longer reproductive span (early menarche coupled with delayed menopause), obesity, nulliparity (never having been pregnant), late age at first pregnancy, and the presence of mutations in specific breast cancer genes (BRCA1 and BRCA2). Except for the genetic influence, all of the risk factors are related to increased exposure to estrogen over a long period of time. be examined to see if the malignancy has spread. If it has, treatment depends upon the stage of the malignancy, but usually includes surgery and/or chemotherapy. Patients often take drugs that block the effect of the estrogen receptor (e.g., tamoxifen [Nolvadex] and raloxifene [Evista]) for years after the surgery. Breast cancers arise from the duct epithelium, not the actual milkproducing cells. Monthly self-examination has proved to be one of the most important means of early detection of breast malignancies. Mammography, which is an x-ray of the breast that can detect small areas of increased tissue density, can identify many small malignancies that are not yet palpable in a self-examination. Recommendations vary, but most physicians agree that women over the age of 40 should have a mammogram done every 1 to 2 years. Women with a family history of breast cancer should consider regular mammography before the age of 40. Because the lymph drainage from the breast goes predominately to the axilla, the axillary lymph nodes on the side with the cancer must mck78097_ch28_842-878.indd 860 Tumor Mammogram showing a malignant tumor. 2/14/11 4:44 PM Chapter Twenty-Eight during labor. In response to a stimulus, such as a baby crying or sucking the nipple, oxytocin is released, and milk is ejected from the nipple. As milk drains from the alveoli, increased levels of prolactin are released so the breast can produce more milk. Once a baby stops nursing, the levels of oxytocin drop, and milk ejection ceases. During the first few weeks of breast-feeding, the mother may experience uterine contractions called afterpains, which are caused by the increased levels of oxytocin in her bloodstream. These uterine contractions help shrink the uterus to its prepregnancy size. Afterpains typically become less noticeable and cease a few weeks after birth, by which time the uterus has shrunk considerably. ● 7 8 ● 9 ● Reproductive System 861 3. Outline the male reproductive duct system, and explain the function of each component. 4. Describe the anatomy and function of the male accessory reproductive organs. 5. List the components of the penis. In the male, the primary sex organs are the testes (tes t́ ēz; sing., testis). The accessory reproductive organs include a complex set of ducts and tubules leading from the testes to the penis, a group of male accessory glands, and the penis, which is the organ of copulation (figure 28.11). W H AT D I D Y O U L E A R N? 28.3a Scrotum Identify and describe the ligaments that support the uterus and hold it in place. The ideal temperature for producing and storing sperm is about 3° Celsius lower than internal body temperature. The scrotum (skrō t́ u m ̆ ), which is a skin-covered sac between the thighs, provides the cooler environment needed for normal sperm development and maturation (figure 28.12). The scrotum is homologous to the labia majora in the female. Externally, the scrotum contains a distinct, ridgelike seam at its midline, called the raphe (rā ́ fē; rhaphe = seam). The raphe persists in an anterior direction along the inferior surface of the penis and in a posterior direction to the anus. The wall of the scrotum is composed of an external layer of skin, a thin layer of superficial fascia immediately internal to the skin, and a layer of smooth muscle, the dartos (dar t́ os; skinned) muscle, immediately internal to the fascia. What name is given to the innermost tunic of the uterus? What are the two distinct layers that form this tunic? What mammary gland structures produce and drain milk from each lobe? 28.3 Anatomy of the Male Reproductive System Learning Objectives: 1. Describe the gross and microscopic anatomy of the testes. 2. Explain both spermatogenesis and spermiogenesis. Ureters Urinary bladder Pubic symphysis Ampulla of ductus deferens Ductus deferens Seminal vesicle Ejaculatory duct Urogenital diaphragm Prostate gland Bulbourethral gland Urethra Anus Penis Epididymis Glans Testis Scrotum Figure 28.11 Male Pelvic Region. A diagrammatic sagittal section shows the locations and relationships of the male pelvic structures. mck78097_ch28_842-878.indd 861 2/14/11 4:44 PM 862 Chapter Twenty-Eight Reproductive System Testicular artery Testicular vein Penis Ureter Inguinal ligament Urinary bladder Superficial inguinal ring Ductus deferens Structures within spermatic cord Pampiniform plexus Spermatic cord Testicular artery External spermatic fascia Cremaster muscle within cremasteric fascia Testicular nerve Epididymis Internal spermatic fascia Testis Layers of spermatic cord wall Dartos muscle Raphe Scrotum Penis Superficial inguinal ring Inguinal ligament Ductus deferens Structures within spermatic cord Spermatic cord Testicular nerve Cremaster muscle within cremasteric fascia Testicular artery and pampiniform plexus Epididymis Testis Testis Raphe Figure 28.12 Scrotum and Testes. A diagram and a cadaver photo show the scrotum, a skin-covered sac that houses the testes, in anterior view. A multilayered spermatic cord houses the blood vessels, nerves, and a sperm-carrying duct (ductus deferens) for each testis. mck78097_ch28_842-878.indd 862 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 863 Spermatic cord Blood vessels and nerves Ductus deferens Head of epididymis Duct of epididymis Seminiferous tubule Interstitial cells Seminiferous tubule Efferent ductule Tubule lumen Mediastinum testis (housing rete testis) Body of epididymis Septum Lobule Sustentacular cells Sperm cells Visceral layer of tunica vaginalis Spermatids Parietal layer of tunica vaginalis Spermatogonia Tunica albuginea LM 250x (b) Seminiferous tubule, cross section Tail of epididymis (a) Testis Figure 28.13 Testes and Seminiferous Tubules. (a) The gross anatomy of a testis is shown diagrammatically in a cut-away, partial sagittal section. (b) A photomicrograph reveals a cross section of a seminiferous tubule in the testis. When the testes are exposed to elevated temperatures, the dartos muscle relaxes, which unwrinkles the skin of the scrotum and allows the testes to move further away from the body. This inferior movement away from the body cools the testes. At the same time, another muscle (the cremaster muscle) also relaxes to allow the testes to move inferiorly away from the body. The opposite occurs if the testes are exposed to cold. In this case, the dartos and cremaster muscles contract, pulling the testes and scrotum closer to the body to conserve heat. 28.3b Spermatic Cord The blood vessels and nerves to the testis travel from within the abdomen to the scrotum in a multilayered structure called the spermatic cord (figure 28.12; see figure 28.13a). The spermatic cord originates in the inguinal canal, a tubelike passageway through the inferior abdominal wall. The spermatic cord wall consists of three layers: ■ ■ An internal spermatic fascia is formed from fascia deep to the abdominal muscles. The cremaster (krē-mas t́ er; a suspender) muscle and cremasteric fascia are formed from muscle fiber extensions of the internal oblique muscle and its aponeurosis, respectively. mck78097_ch28_842-878.indd 863 ■ An external spermatic fascia is formed from the aponeurosis of the external oblique muscle. Within the spermatic cord is a singular testicular artery that is a direct branch from the abdominal aorta. The testicular artery is surrounded by a plexus of veins called the pampiniform (pampin ́ i-form; pampinus = tendril, forma = form) plexus. This venous plexus is a means to provide thermoregulation by pre-cooling arterial blood prior to reaching the testes. Autonomic nerves travel with these vessels and innervate the testis. 28.3c Testes In the adult human male, each testis is an oval organ housed within the scrotum (figure 28.12). Each weighs approximately 10–12 grams, and displays average dimensions of 4 centimeters (cm) in length, 2 cm in width, and 2.5 cm in anteroposterior diameter. The testes produce sperm and androgens (male sex hormones). Each testis is covered both anteriorly and laterally by a serous membrane, the tunica vaginalis (va ̆j-in-a ̆l ı̆́ s; ensheathing). This membrane is derived from the peritoneum of the abdominal cavity. The tunica vaginalis has an outer parietal layer and an inner visceral layer that are separated by serous fluid. A thick, whitish, fibrous capsule called the tunica albuginea covers the testis and lies immediately deep to the visceral layer of the tunica vaginalis 2/14/11 4:44 PM 864 Chapter Twenty-Eight Reproductive System (figure 28.13a). At the posterior margin of the testis, the tunica albuginea thickens and projects into the interior of the organ as the mediastinum testis. Blood vessels, a system of ducts, lymph vessels, and some nerve fibers enter or leave each testis through the mediastinum testis. The tunica albuginea projects internally into the testis and forms delicate connective tissue septa, which subdivide the internal space into about 250 separate lobules. Each lobule contains up to four extremely convoluted, thin and elongated seminiferous (sem ́i-nif é r-u ̆s; semen = seed, fero = to carry) tubules. The seminiferous tubules contain two types of cells: (1) a group of nondividing support cells called the sustentacular (su ̆s-ten-tak ́ū-la ̆r; sustento = to hold upright) (Sertoli or nurse) cells, and (2) a population of dividing germ cells that continuously produce sperm beginning at puberty. The sustentacular cells assist with sperm development. These cells provide a protective environment for the developing sperm, and their cytoplasm helps nourish the developing sperm (figure 28.13b). In addition, the sustentacular cells will release the hormone inhibin when sperm count is high. Inhibin inhibits FSH secretion, and thus regulates sperm production. (Conversely, when sperm count declines, inhibin secretion decreases.) The sustentacular cells are secured together by tight junctions, which form a blood-testis barrier that is similar to the blood-brain barrier. The blood-testis barrier helps protect developing sperm from materials in the bloodstream. It also protects the sperm from the body’s leukocytes, which may perceive the sperm as foreign since they have different chromosome numbers and arrangements from the male’s other body cells. The spaces surrounding the seminiferous tubules are called interstitial spaces. Within these spaces reside the interstitial cells (or Leydig cells). Luteinizing hormone stimulates the interstitial cells to produce hormones called androgens (an ́drō-jen; andros = male human). There are several types of androgens, the most common one being testosterone. Although the adrenal cortex secretes a small amount of androgens, the vast majority of androgen release is via the interstitial cells in the testis, beginning at puberty. These hormones cause males to develop the classic characteristics of axillary and pubic hair, deeper voice, and sperm production. lie near the base of the seminiferous tubule, surrounded by the cytoplasm of sustentacular cells. To produce sperm, spermatogonia first divide by mitosis. One of the cells produced is a new spermatogonium (a new germ cell), to ensure that the numbers of spermatogonia never become depleted, and the other cell is a “committed cell” called primary spermatocyte. Primary spermatocytes are diploid and an exact copy of spermatogonia. It is the primary spermatocytes that undergo meiosis. When a primary spermatocyte undergoes meiosis I, the two cells produced are called secondary spermatocytes. Secondary spermatocytes are haploid, meaning they have 23 chromosomes only. These cells remain surrounded by the sustentacular cells, but move relatively closer to the lumen of the seminiferous tubule (as opposed to the base of the seminiferous tubule). Secondary spermatocytes complete meiosis (go through meiosis II) and form spermatids (sper m ́ a -̆ tid). A spermatid is a haploid cell and is surrounded by the sustentacular cell, very near to the lumen of the seminiferous tubule. The spermatids still have a circular appearance, rather than the sleek shape of mature sperm. During the final stage of spermatogenesis, a process called spermiogenesis, the newly formed spermatids differentiate to anatomically mature spermatozoa (sing., spermatozoon; sper m ́ a -̆ to-zo ó n) or sperm (figure 28.14b). During spermiogenesis, the spermatid sheds its excess cytoplasm, and the nucleus elongates. A structure called the acrosome (ak ŕ ō-sōm; akros = tip, soma = body) cap forms over the nucleus. This acrosome cap contains digestive enzymes that help penetrate the secondary oocyte for fertilization. As the spermatid elongates, a tail (flagellum) forms from the organized microtubules. The tail attaches to a midpiece (neck) region containing mitochondria and a centriole. These mitochondria provide the energy to move the tail. Although the sperm look mature, they do not yet have all of the characteristics needed to successfully travel through the female reproductive tract and fertilize an oocyte. The sperm must leave the seminiferous tubule through a network of ducts (described next) and reside in the epididymis for a period of time to become fully motile. Table 28.5 summarizes the stages of spermatogenesis. W H AT D O Y O U T H I N K ? 3 ● If a male’s testes were removed, would he still be able to produce androgens? Development of Sperm: Spermatogenesis and Spermiogenesis Spermatogenesis (sper m ́ a -̆ tō-jen ́e -̆ sis; genesis = origin) is the process of sperm development that occurs within the seminiferous tubule of the testis. Spermatogenesis does not begin until puberty, when significant levels of FSH and LH stimulate the testis to begin gamete development. The process of spermatogenesis is shown in figure 28.14a. All sperm develop from primordial germ (stem) cells called spermatogonia (sper m ́ a -̆ tō-gō ń ē-a ̆; sing., spermatogonium; sperma = seed, gone = generation). Spermatogonia are diploid cells (meaning they have 23 pairs of chromosomes for a total of 46). These cells mck78097_ch28_842-878.indd 864 Study Tip! Use these hints to help identify the various testis cells under the microscope: 1. Spermatogonia are closest to the seminiferous tubule base. As spermatogenesis occurs and the cells mature, more mature cells are found closer to the lumen of the seminiferous tubule. Note how close the spermatozoa are to the lumen. 2. Sometimes it is difficult to distinguish an entire sustentacular cell because its cytoplasm is pale and surrounds the developing sperm. You can identify sustentacular cells by their nucleus, which is oval or flattened and usually has a prominent nucleolus. 3. The interstitial cells are not located within the seminiferous tubule but external to it, usually in clumps of three or more cells. 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 865 Interstitial cells Interstitial space 46 Spermatogonium 1 1 Germ cells that are the origin of sperm cells are diploid cells (containing 46 chromosomes, or 23 pairs) called spermatogonia. Mitotic divisions of these cells produce a new germ cell and a committed cell. The committed cell is a primary spermatocyte. Mitotic division 46 Sustentacular cell 2 Secondary spermatocyte Wall of seminiferous tubule 23 2 The first meiotic division begins in the diploid primary First meiotic division 23 spermatocytes. The haploid cells (containing 23 chromosomes only) produced by the first meiotic division are called secondary spermatocytes. 23 Second meiotic division 3 Spermatid Primary spermatocyte 46 23 23 3 The second meiotic division originates with the secondary spermatocytes and produces spermatids. 23 Tight junctions 23 4 23 23 23 4 The process of spermiogenesis begins with spermatids and results in morphological changes needed to form sperm that will be motile. Spermatids becoming sperm 23 23 23 23 Lumen of seminiferous tubule Sperm cells (a) Spermatogenesis Acrosome cap Acrosome cap Developing acrosome cap Developing acrosome cap Spermatid nucleus Head Acrosome cap Nucleus Nucleus Midpiece Excess cytoplasm Mitochondria Mitochondria Spermatid nucleus Spermatid nucleus Developing flagellum (b) Spermiogenesis Mitochondria Tail (flagellum) Microtubules Developing flagellum Sperm Figure 28.14 Spermatogenesis and Spermiogenesis. (a) The processes of spermatogenesis and spermiogenesis take place within the wall of the seminiferous tubule. (b) Structural changes occur during spermiogenesis as a sperm forms from a spermatid. mck78097_ch28_842-878.indd 865 2/14/11 4:44 PM 866 Chapter Twenty-Eight Reproductive System Table 28.5 Stages of Spermatogenesis Cell Type Number of Chromosomes Haploid or Diploid Action Spermatogonium 23 pairs (46) Diploid Divides by mitosis to produce a new spermatogonium and a primary spermatocyte Primary spermatocyte 23 pairs (46) Diploid Completes meiosis I to produce secondary spermatocytes Secondary spermatocyte 23 Haploid Completes meiosis II to produce spermatids Spermatid 23 Haploid Undergoes spermiogenesis, where most of its cytoplasm is shed, and a midpiece, tail, and head form Spermatozoon (sperm) 23 Haploid Leaves seminiferous tubule and matures in epididymis W H AT D I D Y O U L E A R N? 10 ● 11 ● 12 ● What is the scrotum? It is homologous to what structure in the female? What is the function of the interstitial cells, and where are they located within the testis? Describe the process of spermatogenesis, and mention when it first occurs. 28.3d Ducts in the Male Reproductive System The left and right testes each have their own set of ducts. These ducts store and transport sperm as they mature and pass out of the male body (figure 28.15). Ducts Within the Testis The rete (rē t́ ē; net) testis is a meshwork of interconnected channels in the mediastinum testis that receive sperm from the seminiferous tubules. The rete testis is lined by simple cuboidal epithelium with short microvilli covering its luminal surface. The channels of the rete testis merge to form the efferent ductules (see figure 28.13). Approximately 12–15 efferent ductules (duk t́ ool) connect the rete testis to the epididymis. They are lined with both ciliated columnar epithelia that gently propel the sperm toward the epididymis and nonciliated columnar epithelia that absorb excess fluid secreted by the seminiferous tubules. The efferent ductules drain into the epididymis. Epididymis The epididymis (ep-i-did ́ i-mis; pl., epididymides; epi = upon, didymis = twin) is a comma-shaped structure composed of an internal duct and an external covering of connective tissue. Its head lies on the superior surface of the testis, while the body and tail are on the posterior surface of the testis (see figure 28.13a). Internally, the epididymis contains a long, convoluted duct of the epididymis, which is approximately 4 to 5 meters in length and lined with pseudostratified columnar epithelium that contains stereocilia (long microvilli) (figure 28.15c). The epididymis stores sperm until they are fully mature and capable of being motile. Just as a newborn has the anatomic characteristics of an adult, but cannot move as an adult, the sperm mck78097_ch28_842-878.indd 866 that first enter the epididymis look like mature sperm but can’t move like mature sperm. If they are expelled too soon, they lack the ability to be motile, which is necessary to travel through the female reproductive tract and fertilize a secondary oocyte. If sperm are not ejaculated, the older sperm degenerate and are resorbed by cells lining the duct of the epididymis. Ductus Deferens When sperm leave the epididymis, they enter the ductus deferens (de ̆f é r-ens; carry away), also called the vas deferens. The ductus deferens is a thick-walled tube that travels within the spermatic cord, through the inguinal canal, and then within the pelvic cavity before it nears the prostate gland (figure 28.15a). The wall of the ductus deferens is composed of an inner mucosa (lined with pseudostratified ciliated columnar epithelium), a middle muscularis, and an outer adventitia (figure 28.15b). The muscularis contains three layers of smooth muscle: an inner longitudinal, middle circular, and outer longitudinal layer. Contraction of the muscularis is necessary to move sperm through the ductus deferens, since sperm do not exhibit motility until they are ejaculated from the penis. When the ductus deferens travels through the inguinal canal and enters the pelvic cavity, it separates from the other spermatic cord components and extends posteriorly along the superolateral surface of the bladder. It then travels inferiorly and terminates close to the region where the bladder and prostate gland meet. As the ductus deferens approaches the superoposterior edge of the prostate gland, it enlarges and forms the ampulla of the ductus deferens (figure 28.15a). The ampulla of the ductus deferens unites with the proximal region of the seminal vesicle to form the terminal portion of the reproductive duct system, called the ejaculatory duct. Ejaculatory Duct Each ejaculatory duct is between 1 and 2 centimeters long. The epithelium of the ejaculatory duct is a pseudostratified ciliated columnar epithelium. The ejaculatory duct conducts sperm (from the ductus deferens) and a component of seminal fluid (from the seminal vesicle) toward the urethra. Each ejaculatory duct opens into the prostatic urethra. 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System Urinary bladder 867 Adventitia Ureter Mucosa Ampulla Seminal vesicle Ejaculatory duct Prostate gland Prostatic urethra Bulbourethral gland LM 32x Muscularis Membranous urethra Mucosa (with pseudostratified ciliated columnar epithelium) Bulb Urogenital diaphragm Crus (b) Ductus deferens Ductus deferens LM 500x Corpus cavernosum Epididymis Section of duct of epididymis Testis Sperm in lumen of duct of epididymis Penis Corpus spongiosum Spongy urethra Glans (a) Posterior view LM 50x (c) Epididymis Figure 28.15 Duct System in the Male Reproductive Tract. (a) A posterior view depicts the structural components of the male reproductive ducts and accessory glands. (b) Micrographs show a cross section through the ductus deferens. (c) A micrograph shows a cross section through the epididymis. Urethra The urethra transports semen from both ejaculatory ducts to the outside of the body. Recall from chapter 27 that the urethra is subdivided into a prostatic (pros-tat ́ ik) urethra that extends through the prostate gland (see figure 28.15), a membranous urethra that travels through the urogenital diaphragm, and a spongy urethra that extends through the penis. 28.3e Accessory Glands Recall from earlier in this chapter that the vagina has a highly acidic environment to prevent bacterial growth. Sperm cannot survive in this type of environment, so a slightly alkaline (pH 7–8) secretion called seminal (sem ́ i-nal) fluid is needed to neutralize the acidity of the vagina. In addition, as the sperm travel through the female reproductive tract (a process that can take hours to several days), they are nourished by nutrients within the seminal mck78097_ch28_842-878.indd 867 fluid. The components of seminal fluid are produced by accessory glands: the seminal vesicles, the prostate gland, and the bulbourethral glands. Seminal Vesicles The paired seminal vesicles are located on the posterior surface of the urinary bladder lateral to the ampulla of the ductus deferens (figure 28.15a). Each seminal vesicle is an elongated, hollow organ approximately 5–8 centimeters long. The wall of each vesicle contains mucosal folds of pseudostratified columnar epithelium (figure 28.16a). It is the medial (proximal) portion of the seminal vesicle that merges with a ductus deferens to form the ejaculatory duct. The seminal vesicles secrete a viscous, whitish-yellow fluid containing fructose, prostaglandins, and bicarbonate. The fructose is a sugar that nourishes the sperm as they travel through the 2/14/11 4:44 PM 868 Chapter Twenty-Eight Reproductive System Seminal vesicle Prostate gland Mucosal folds in seminal vesicle Tubuloalveolar glands in prostate gland LM 25x LM 80x (a) Seminal vesicle (b) Prostate gland Figure 28.16 Seminal Vesicles and Prostate Gland. A drawing depicts the relative locations of the seminal vesicles and the prostate gland. Micrographs show sections through (a) a seminal vesicle and (b) the prostate gland. female reproductive tract. Prostaglandins are hormonelike substances that promote the widening and slight dilation of the external os of the cervix, which facilitates sperm entry into the uterus. Bicarbonate buffers the existing fluid. Prostate Gland The prostate (pros t́ āt; one who stands before) gland is a compact, encapsulated organ that weighs about 20 grams and is shaped like a walnut, measuring approximately 2 cm by 3 cm by 4 cm. It is located immediately inferior to the bladder. The prostate gland includes submucosal glands that produce mucin and more than 30 tubuloalveolar glands that open directly through numerous ducts into the prostatic urethra (figure 28.16b). Together, these glands contribute a component to the seminal fluid. The prostate gland secretes a slightly milky fluid that is weakly acidic and rich in citric acid, seminalplasmin, and prostate-specific antigen (PSA). The citric acid is a nutrient for sperm health, the seminalplasmin is an antibiotic that combats urinary tract infections in the male, and the PSA acts as an enzyme to help liquify semen following ejaculation. (Note that the slightly acidic secretion of the prostate does not cause the seminal fluid to be acidic, and thus the seminal fluid still functions to neutralize the acidity of the vagina.) 28.15). Each gland has a short duct that projects into the bulb (base) of the penis and enters the spongy urethra. Bulbourethral glands are tubuloalveolar glands that have a simple columnar and pseudostratified columnar epithelium. Their secretory product is a clear, viscous mucin that forms mucus. As a component of the seminal fluid, this mucus lubricates and buffers the urethra prior to ejaculation. 28.3f Semen Seminal fluid from the accessory glands combines with sperm from the testes to make up semen (sē m ́ en; seed). When released during intercourse, semen is called the ejaculate (ē-jak ū́ -lāt), and it normally measures about 3 to 5 milliliters in volume and contains approximately 200 to 500 million spermatozoa. In a sexually active male, the average transit time of human spermatozoa—from their release into the lumen of the seminiferous tubules, passage through the duct system, and appearance in the ejaculate—is about 2 weeks. Since semen is composed primarily of seminal fluid, a male who is very active sexually may have a reduced sperm count because there are fewer sperm to be released from the epididymis; however, the total semen volume remains close to normal for that individual. Bulbourethral Glands Paired, pea-shaped bulbourethral (bu ̆l ́ bō -ū-rē t́ hra ̆l) glands (or Cowper glands) are located within the urogenital diaphragm on each side of the membranous urethra (see figures 28.11 and mck78097_ch28_842-878.indd 868 W H AT D O Y O U T H I N K ? 4 ● If a male has a vasectomy, is he still able to produce sperm? If so, what happens to those sperm? How is the composition of semen changed in an individual who has had a vasectomy? 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 869 CLINICAL VIEW Benign Prostatic Hyperplasia and Prostate Cancer Benign prostatic hyperplasia (BPH) is a noncancerous enlargement of the prostate gland. BPH is a common disorder in older men; in fact, its incidence is greater than 90% for men over 80 years of age. Hormonal changes in aging males are the cause of the enlargement. In BPH, large, discrete nodules form within the prostate and compress the prostatic urethra. Thus, the patient has difficulty starting and stopping a stream of urine, and often complains of nocturia (excessive urinating at night), polyuria (more-frequent urination), and dysuria (painful urination). Some drug regimens help inhibit hormones that cause prostate enlargement, but when medications are no longer effective, surgical removal of the prostatic enlargement is indicated. The most commonly performed surgical procedure is called a TURP (transurethral resection of the prostate), in which an instrument called a resectoscope (rē-sek t́ ōskōp) is inserted into the urethra to cut away the problematic enlargement. Prostate cancer is one of the most common malignancies among men over 50, and the risk of developing it increases with age. Prostate cancer forms hard, solid nodules, most often in the posterior part of the prostate gland. Early stages of the cancer are generally asymptomatic, 28.3g Penis The penis (pē ń is; tail) and the scrotum form the external genitalia in males (figure 28.17a). Internally, the attached portion of the penis is the root, which is dilated internal to the body surface, forming both the bulb and the crura of the penis. The bulb attaches the penis to the bulbospongiosus muscle in the urogenital triangle, and the crura attach the penis to the pubic arch. The body, or shaft, of the penis is the elongated, movable portion. The tip of the penis is called the glans, and it contains the external urethral orifice. The skin of the penis is thin and elastic. At the distal end of the penis, the skin is attached to the raised edge of the glans and forms a circular fold called the prepuce (foreskin) (see Clinical View, p. 870). Within the shaft of the penis are three cylindrical erectile bodies (figure 28.17b). The paired corpora cavernosa (kav é r-nōsa ̆; sing., corpus cavernosum; caverna = grotto) are located dorsolaterally. Ventral to them along the midline is the single corpus spongiosum (spu n̆ ́ jē-ō-su m ̆ ), which contains the spongy urethra. Each corpus cavernosum terminates in the shaft of the penis, while the corpus spongiosum continues within the glans. The erectile bodies are ensheathed by the tunica albuginea, which also provides an attachment to the skin over the shaft of the penis. The erectile bodies are composed of a complex network of venous spaces surrounding a central artery. During sexual excitement, blood enters the erectile bodies via the central artery and fills in the venous spaces. As these venous spaces become engorged with blood, the erectile bodies become rigid, a process called erection (ē-rek ś hu ̆n; erecto = to set up). The rigid erectile bodies compress the veins that drain blood away from the venous spaces. Thus, the spaces fill with blood, but the blood cannot leave the erectile bodies until the sexual excitement ceases. Parasympathetic innervation (via the pelvic splanchnic nerves) is responsible for increased blood flow and thus the erection of the penis. mck78097_ch28_842-878.indd 869 but as it progresses, urinary symptoms may develop. Untreated prostate cancer can metastasize to other body organs. Early diagnosis and treatment of prostate cancer are vital for cure and long-term survival. A very effective screening tool is a digital rectal exam, whereby a physician inserts a finger into the rectum and palpates adjacent structures (including the prostate gland). In addition, most physicals for men over the age of 50 now include a test for prostatespecific antigen (PSA) in the blood. The PSA level in a healthy man is typically less than 4 ng/mL. An elevated PSA level can indicate either benign prostatic hyperplasia or prostate cancer. A needle biopsy of the prostate tissue can confirm the diagnosis of cancer. Several treatment options are available, depending on the stage of the cancer. For earlier stages of the disease, radiation therapy may be beneficial—either traditional external-beam radiation or interstitial radiotherapy, in which radioactive palladium or iodine “seeds” are permanently implanted in the prostate. For patients with a more aggressive cancer, the entire prostate and some surrounding structures are surgically removed, a procedure called a radical prostatectomy. No matter what the form of treatment, the physician continues to measure PSA levels in the patient’s blood to make sure all the cancerous structures have been removed and to check for recurrence. Ejaculation (ē-jak-ū-lā ś hu n̆ ; eiaculatus = to shoot out) is the process by which semen is expelled from the penis with the help of rhythmic contractions of the smooth muscle in the wall of the urethra. Sympathetic innervation (from the lumbar splanchnic nerves) is responsible for ejaculation. Although in most body systems sympathetic and parasympathetic innervation tend to perform opposite functions, the male reproductive system is an exception. Here, parasympathetic innervation is necessary to achieve an erection, while sympathetic innervation promotes ejaculation. Relaxation of autonomic activity after sexual excitement reduces blood flow to the erectile bodies and shunts most of the blood to other veins, thereby returning the penis to its flaccid condition. Study Tip! One way to remember the autonomic innervation for the penis is this phrase: “Point and Shoot!” The p in point (erection) also stands for parasympathetic innervation, while the s in shoot (ejaculation) stands for sympathetic innervation. W H AT D I D Y O U L E A R N? 13 ● 14 ● 15 ● What two structures unite to form the ejaculatory duct? What is the composition of semen, and what organs contribute to semen? Specifically, how do both parasympathetic and sympathetic innervation work on penile function during sexual arousal? 2/14/11 4:44 PM 870 Chapter Twenty-Eight Reproductive System CLINICAL VIEW Circumcision Pubic symphysis Membranous urethra Bulb of penis Right crus of penis Body of penis Root of penis Corpora cavernosa Corpus spongiosum Glans Scrotum External urethral orifice (a) Anterolateral view Dorsal vein (blue), artery (red), and nerve (yellow) Deep dorsal vein Corpus cavernosum Tunica albuginea Central artery Venous spaces Corpus spongiosum Deep fascia Superficial fascia Skin Circumcision (ser-kŭm-sizh ŭ́ n; circum = around, caedo = to cut) is the surgical removal of the prepuce (foreskin) of the penis. The drawings here compare a circumcised penis (a) with an uncircumcised penis (b). Most circumcisions are performed during the first few days or weeks of a male infant’s life, although some adult males do undergo the procedure. Circumcision is much more common in the United States than in other countries, and is the subject of considerable debate. Circumcision has several benefits: Circumcised males appear less likely to develop urinary tract infections because the bacteria that cause these infections tend to stick to the foreskin. (However, if a child is taught early about keeping the penis clean, these risks drop dramatically.) Circumcision may also protect against penile inflammation (because the glans of a circumcised penis can be kept clean more easily) and penile cancer. Finally, some research has suggested that circumcised males have a reduced risk of acquiring and passing on sexually transmitted diseases (STDs), including HIV. In 2006, the National Institutes of Health (NIH) announced that large, controlled clinical trials in Africa had demonstrated circumcision to be effective in preventing infection with and transmission of HIV. Circumcised men were approximately 50% less likely to become infected than uncircumcised men. The drawbacks to circumcision include the following: Infants are sometimes circumcised without anesthesia, subjecting them to pain and elevated stress levels. Circumcision also carries a risk of complications, including infection, excessive bleeding, and in rare cases, subsequent surgery. Finally, some individuals have suggested that circumcision may affect sensation during sexual intercourse, although this hypothesis has not been systematically tested or proven. Spongy urethra (b) Cross section Figure 28.17 Anatomy of the Penis. (a) An anterolateral view of the circumcised penis. (b) A diagrammatic, transverse section shows the arrangement of the erectile bodies. Prepuce (a) Circumcised penis mck78097_ch28_842-878.indd 870 (b) Uncircumcised penis 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 871 CLINICAL VIEW Sexually Transmitted Diseases Sexually transmitted diseases (STDs), also known as venereal diseases, are a group of infectious diseases that are usually transmitted via sexual contact. The incidence of STDs has been rising in recent years because individuals are having sexual intercourse at younger ages, and may have multiple sexual partners in their lifetime. Many times, the symptoms of STDs are not immediately noticeable, so infected individuals may spread the disease to someone else without realizing it. Mothers also may transmit STDs to their newborns, either directly across the placenta or at the time of delivery. Condoms, when properly used, have been shown to help prevent the spread of STDs, but they are not 100% effective. Individuals with multiple sex partners should consider being tested routinely for some of the more common STDs, because they may unknowingly be spreading one or more of these conditions. STDs are a leading cause of pelvic inflammatory disease in women, in which the pelvic organs (uterus, uterine tubes, and ovaries) become infected. Should bacteria from an STD infect the uterus and uterine tubes, scarring is likely to follow, leading to blockage of the tubes and infertility. We’ve already discussed two types of STDs, human papillomavirus (see Clinical View, “Cervical Cancer,” earlier in this chapter) and AIDS (in chapter 24). We now explore other common STDs. Chlamydia (kla-mid ḗ -ă) is the most frequently reported bacterial STD in the United States. The responsible agent is Chlamydia trachomatis. Most infected people are asymptomatic, while the rest develop symptoms within 1 to 3 weeks after exposure. These symptoms include abnormal vaginal discharge, painful urination (in both males and females), and low back pain. Chlamydia is treated with antibiotics. 28.4 Aging and the Reproductive Systems Learning Objective: 1. Outline the age-related changes that occur in the female and male reproductive systems. Our reproductive systems are basically nonfunctional for several years following birth. When we reach puberty, hormonal changes in the hypothalamus and anterior pituitary stimulate the gonads to begin producing sex hormones. Thereafter, changes occur in many body structures, the reproductive organs mature, and the gonads begin to produce gametes. The time of onset of puberty varies among individuals, but it clearly occurs at a younger average age in females and males today than it did 40 or 50 years ago. After reaching sexual maturity, the female and male reproductive systems exhibit marked differences in their response to aging. Gametes typically stop maturing in females by their 40s or 50s, and menopause occurs. A reduction in hormone production that accompanies menopause causes some atrophy of the reproductive organs and the breasts. The vaginal wall thickness decreases, as do glandular secretions for maintaining a moist, mck78097_ch28_842-878.indd 871 Genital herpes (her ṕ ē z; herpo = to creep) is caused by herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2). Infected individuals undergo cyclic outbreaks of blister formation in the genital and anal regions; the blisters are filled with fluid containing millions of infectious viruses. The blisters then break and turn into tender sores that remain for 2–4 weeks. Typically, future cycles of blistering are less severe and shorter in duration than the initial episode. There is no cure for herpes, but antiviral medications can lessen the severity and length of an outbreak. Gonorrhea (gon-ō-rē ắ ) is caused by the bacterium Neisseria gonorrhoeae, and is spread either by sexual contact or from mother to newborn at the time of delivery. Symptoms include painful urination and/or a yellowish discharge from the penis or vagina. Gonorrhea is treated with antibiotics, although in recent years many gonorrhea strains have become resistant to some antibiotics. If untreated, women may develop pelvic inflammatory disease, and men may develop epididymitis, a painful condition of the epididymis that can lead to infertility. If a newborn acquires the disease, then blindness, joint problems, and/or a life-threatening blood infection may result. Syphilis (sif ́ i-lis) is caused by the corkscrew-shaped bacterium Treponema pallidum. The bacterium is spread sexually via contact with a syphilitic sore (called a chancre), or a newborn may acquire it in utero. Babies can acquire congenital syphilis from their mothers and are often stillborn, but if they live, they have a high incidence of skeletal malformities and neurologic problems. Syphilis can be treated with antibiotics. A person can become reinfected with the disease if reexposed to the syphilitic sores. lubricated lining. The uterus shrinks and atrophies, becoming much smaller than it was before puberty. The lack of significant amounts of estrogen and progesterone in a menopausal woman also affects other organs and body systems. Women may experience “hot flashes,” in which their bodies perceive periodic elevations in body temperature, and they may develop thinning scalp hair and/or an increase in facial hair. Menopausal women are at greater risk for osteoporosis (thinning, brittle bones) and heart disease due to the drop in estrogen and progesterone levels. Menopausal hormone therapy (MHT), in the form of estrogen and progesterone supplements primarily, can be prescribed to peri- and postmenopausal women to help diminish these symptoms and risks. However, the risks associated with MHT (e.g., increased risk of breast cancer, stroke, heart attack, and blood clots) may outweigh the benefits, so physicians assess each individual for her suitability for MHT. Studies regarding MHT continue. In contrast, males do not experience the relatively abrupt change in reproductive system function that females do. A slight decrease in the size of the testes parallels a reduction in the size of the seminiferous tubules and the number of interstitial cells. As a consequence of the reduced number of interstitial cells, decreased testosterone levels in males in their 50s signal a change called the male climacteric (klı̄-mak t́ er-ik, klı̄-mak-ter ́ ik). However, males 2/14/11 4:44 PM 872 Chapter Twenty-Eight Reproductive System generally do not stop producing gametes as females do following menopause. Most males experience prostate enlargement (either benign or cancerous) as they age. This prostate enlargement can interfere with sexual and urinary functions. Also associated with aging are erectile dysfunction and impotence, which refer to the inability to achieve or maintain an erection. Besides aging, other risk factors for this condition include heart disease, diabetes, smoking, and prior prostate surgery. Many drugs have entered the market (e.g., sildenafil [Viagra]) that treat erectile dysfunction by prolonging vasodilation of the penile arteries and thus inhibiting relaxation of the erectile bodies. W H AT D I D Y O U L E A R N? 16 ● What are some female body changes that accompany menopause? 28.5 Development of the Reproductive Systems Learning Objectives: 1. Describe the development of the female and male reproductive systems. 2. Detail the common embryonic structures and the hormones that influence their development. The female and male reproductive structures originate from the same basic primordia, which differentiate into female or male structures, depending upon the signals the primordia receive. To better explain this process, we must first distinguish between the genetic and phenotypic sex of an individual. In contrast, phenotypic (fē ń ō-tip ́ik, fen ́ō-) sex refers to the appearance of an individual’s internal and external genitalia. A person with ovaries and female external genitalia (labia) is a phenotypic female, whereas a person with testes and male external genitalia (penis, scrotum) is a phenotypic male. Phenotypic sex starts to become apparent no earlier than the seventh week of development. How does the primordial tissue know whether to develop into female reproductive organs or male reproductive organs? In males, the sex-determining region Y (SRY) gene is located within the larger testis-determining factor (TDF) region on the Y chromosome. If the Y chromosome is present, the SRY gene produces proteins to stimulate the production of other hormones (e.g., testosterone and other androgens) that initiate male phenotypic development. If a Y chromosome is absent (e.g., the individual is a genetic female), or if the Y chromosome is either lacking or has an abnormal SRY gene, a female phenotypic sex results. Although undoubtedly more complicated, the female phenotypic sex may be thought of as the organism’s default pattern. This pattern is not changed unless SRY and its proteins are present. 28.5b Formation of Indifferent Gonads and Genital Ducts Early in the fifth week of embryonic development, paired genital ridges (or gonadal ridges) form from intermediate mesoderm. The genital ridges will form the gonads. These longitudinal ridges are medial to the developing kidneys at about the level of the tenth thoracic vertebra (figure 28.18, top). Between weeks 5 and 6, primordial germ cells migrate from the yolk sac to the genital ridges. These germ cells will form the future gametes (either sperm or oocytes). Shortly thereafter, two sets of duct systems are formed: ■ 28.5a Genetic Versus Phenotypic Sex Genetic sex (or genotypic sex) refers to the sex of an individual based on her or his sex chromosomes. An individual with two X chromosomes is a genetic female, while an individual with one X and one Y chromosome is a genetic male. Genetic sex is determined at fertilization. ■ The mesonephric (mez-ō-nef ŕ ik) ducts (or Wolffian ducts) form most of the male duct system. Recall that the mesonephric ducts also connect the mesonephros (intermediate kidney) to the developing urinary bladder. The paramesonephric ducts (Müllerian ducts) form most of the female duct system, including the uterine tubes, uterus, and superior part of the vagina. These ducts appear lateral to the mesonephric ducts. CLINICAL VIEW True Hermaphroditism and Pseudohermaphroditism The term hermaphrodite (her-maf ŕ ō-dı̄t) is derived from the Greek name Hermaphroditus, the mythological son of the Greek god Hermes and the goddess Aphrodite. In general, a hermaphrodite is an individual with both male and female sex characteristics. True hermaphroditism refers to an individual with both ovarian and testicular structures and ambiguous (or female) external genitalia. The person may be a genetic male (XY) or a genetic female (XX). True hermaphroditism is very rare, and typically the ovarian and testicular structures are not functional. Pseudohermaphroditism (soo ́ dō-her-maf ŕ ō-dı̄-tizm; pseudes = false) refers to an individual whose genetic sex and phenotypic sex do not match. A male pseudohermaphrodite is a genetic male (XY) whose external genitalia resemble those of a female (female phenotypic sex). These individuals usually have testes, but the structures that form the scrotum do not fuse completely, so the structure looks more like labia mck78097_ch28_842-878.indd 872 majora. Male pseudohermaphroditism usually results from a reduction in male hormones (e.g., testosterone) during development; thus, the sex-determining region Y (SRY) gene on the Y chromosome is present, but its proteins are insufficient in the absence of testosterone to masculinize the external genitalia. A female pseudohermaphrodite is a genetic female (XX) with external genitalia that resemble those of a male (male phenotypic sex). Although the ovaries and internal genitalia (e.g., uterine tubes and uterus) are female, the external genitalia (clitoris and labia) resemble male external sex organs. The clitoris enlarges to look like a small penis, and/or the two labia may become partially fused to resemble a scrotum. Female pseudohermaphroditism may result if the female fetus is exposed to excessive androgens (e.g., if the pregnant mother was given certain medications to help prevent miscarriage). More commonly, female pseudohermaphroditism is caused by congenital adrenal hyperplasia, in which the fetus’s adrenal glands produce excessive amounts of androgens. 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 873 Sexually Indifferent Stage Mesonephros Mesonephric duct Genital ridge Paramesonephric duct Kidney Cloaca Weeks 5–6 Female Male Testes Ovaries Efferent ductules Paramesonephric duct forming the uterine tube Epididymis Mesonephric duct (degenerating) Paramesonephric duct (degenerating) Mesonephric duct forming the ductus deferens Fused paramesonephric ducts forming the uterus Urinary bladder (moved aside) Urinary bladder Seminal vesicle Urogenital sinus forming the urethra Urogenital sinus forming the urethra and inferior vagina Weeks 10–12 Weeks 10–12 Uterine tube Urinary bladder Ovary Seminal vesicle Uterus Prostate gland Bulbourethral gland Ductus deferens Urinary bladder (moved aside) Vagina Urethra Epididymis Efferent ductules Testis Urethra Hymen At birth At birth Figure 28.18 Embryonic Development of the Female and Male Reproductive Tracts. Through the first 6 weeks of development, the embryo is termed “sexually indifferent.” Thereafter, genetic expression determines sex differentiation. mck78097_ch28_842-878.indd 873 2/14/11 4:44 PM 874 Chapter Twenty-Eight Reproductive System All human embryos develop both duct systems, but only one of the duct systems remains in the fetus. If the embryo is female, the paramesonephric ducts develop, and the mesonephric ducts degenerate. If the embryo is male, the mesonephric ducts grow and differentiate into male reproductive structures, while the paramesonephric ducts degenerate. 28.5c Internal Genitalia Development The development of the female internal reproductive structures is traced in figure 28.18, left. Because no SRY proteins are produced in the female, the mesonephric ducts degenerate. Between weeks 8 and 20, the paramesonephric ducts develop and differentiate. The caudal (inferior) ends of the paramesonephric ducts fuse, forming the uterus and the superior part of the vagina. The cranial (superior) parts of the paramesonephric ducts remain separate and form two uterine tubes. The remaining inferior part of the vagina is formed from the urogenital sinus (which also forms the urinary bladder and urethra). By about week 7 of development, the SRY gene on the Y chromosome begins influencing the indifferent gonad to become a testis, which then forms sustentacular cells and interstitial cells. Once the sustentacular cells form, they begin secreting anti-Müllerian hormone (AMH) (also known as Müllerian inhibiting substance), which inhibits the development of the paramesonephric ducts (see figure 28.18, right). These paramesonephric ducts degenerate, and between weeks 8 and 12, the mesonephric ducts form the male duct system—efferent ductules, epididymides, ductus deferens, seminal vesicles, and ejaculatory ducts. The prostate and bulbourethral glands do not form from the mesonephric ducts. Instead, they begin to form as endodermal “buds” or outgrowths of the developing urethra between weeks 10 and 13. As the prostate gland and bulbourethral glands develop, they incorporate mesoderm into their structures as well. Finally, note that the indifferent gonad originates near the level of thoracic vertebra T10. Throughout prenatal development, the developing testis descends from the abdominal region toward the developing scrotum. A thin band of connective tissue called the gubernaculum (goo ́ ber-nek ū́ -lu m ̆ ; helm) attaches to the testis and pulls it from the abdomen, through the developing inguinal canal, to its placement in the scrotum. As the embryo grows (but the gubernaculum remains the same length), the testis is passively pulled into the scrotum. This process is slow, beginning in the third month and not completed until the ninth month. mck78097_ch28_842-878.indd 874 It is common for premature male babies to have undescended testes because they were born before the testes had fully descended into the scrotum. Their testes usually descend shortly after birth. 28.5d External Genitalia Development As with the internal genitalia, female and male external genitalia develop from the same primordial structures (figure 28.19). By the sixth week, the following external structures are seen: ■ ■ ■ The urogenital folds (or urethral folds) are paired, elevated structures on either side of the urogenital membrane, a thin partition separating the urogenital sinus from the outside of the body (see chapter 27). The genital tubercle is a rounded structure anterior to the urogenital folds. The labioscrotal swellings (or genital swellings) are paired elevated structures lateral to the urethral folds. The external genitalia appear very similar between females and males until about week 12 of development, and they do not become clearly differentiated until about week 20. In the absence of testosterone, female external genitalia develop. The genital tubercle becomes the clitoris. The urogenital folds do not fuse, but become the labia minora. Finally, the labioscrotal folds also remain unfused and become the labia majora. In the male, production and circulation of testosterone cause the primitive external structures to differentiate. The genital tubercle enlarges and elongates, forming the glans of the penis and part of the dorsal side of the penis. The urogenital folds grow and fuse around the developing urethra and form the ventral side of the penis. Finally, the labioscrotal swellings fuse at the midline, forming the scrotum. Study Tip! You may be aware that the sex of an unborn child can typically be determined with an ultrasound sometime between weeks 18 and 22. You now know why the physician waits until this time—it is when the external genitalia first become clearly distinguishable. W H AT D I D Y O U L E A R N? 17 ● What is the difference between genetic and phenotypic sex? 2/14/11 4:44 PM Chapter Twenty-Eight Reproductive System 875 (a) Sexually indifferent stage Genital tubercle Urogenital fold Figure 28.19 Development of External Genitalia. (a) At 6 weeks of development, the external genitalia are undifferentiated. (b) By 12 weeks, the urogenital folds begin to fuse in the male and remain open in the female. (c) By 20 weeks, external genitalia are well differentiated. Labioscrotal swelling Week 6 Female Male Developing glans of penis Developing clitoris Labia minora Labia majora Anus (b) Week 12: Urogenital folds begin to fuse in the male Urethral orifice Glans of penis Glans of clitoris Urethral orifice Vaginal orifice Scrotum Anus (c) Week 20: External genitalia well differentiated mck78097_ch28_842-878.indd 875 2/14/11 4:44 PM 876 Chapter Twenty-Eight Reproductive System Clinical Terms castration (kas-trā ́shŭn; castro = to deprive of generative power) Removal of the testes or ovaries. cryptorchidism (krip-tōr ́ki-dizm; krypto = hidden, orchis = testis) A testis that has not descended completely into the scrotum. dysmenorrhea (dis-men-ōr-rē ́ă; dys = bad, men = month, rhoia = flow) Difficult and painful menstruation. salpingitis Inflammation of the uterine tubes. Chapter Summary 28.1 Comparison of the Female and Male Reproductive Systems 843 28.2 Anatomy of the Female Reproductive System 844 ■ Both reproductive systems have gonads that produce gametes and sex hormones, and a duct system to transport the gametes. 28.1a Perineum 843 ■ In both females and males, the perineum is a diamond-shaped area between the thighs that contains the urogenital and anal triangles. ■ Female internal reproductive organs include paired ovaries and uterine tubes, a uterus, and a vagina. 28.2a Ovaries 845 ■ The cortex of the ovary houses ovarian follicles that consist of an oocyte surrounded by follicle cells. ■ Changing levels of FSH and LH cause a primordial follicle to mature into a primary follicle. A secondary follicle matures from a primary follicle, and a vesicular follicle matures from a secondary follicle. ■ A peak in LH causes the secondary oocyte to be released from the vesicular follicle at ovulation; remaining follicular cells become the hormone-producing corpus luteum. ■ The ovarian cycle consists of the follicular phase, ovulation, and the luteal phase. 28.2b Uterine Tubes 852 ■ The uterine tubes are the site of fertilization. They have an infundibulum, ampulla, isthmus, and uterine part. ■ The uterine tube wall is composed of an inner mucosa (ciliated columnar epithelium), middle muscularis (two smooth muscle layers), and an external serosa. 28.2c Uterus 852 ■ The uterus is a thick-walled muscular organ that functions as the site of pre-embryo implantation, supports and nourishes the embryo/fetus, ejects the fetus at birth, and is the site of menstruation. ■ The uterine wall consists of an inner mucosa, the endometrium; a thick-walled middle muscular layer, the myometrium; and an outer serosa, the perimetrium. ■ The endometrium has a functional layer that is sloughed off as menses and a deeper basal layer that regenerates a new functional layer during the next uterine cycle. ■ Three distinct phases of endometrium development occur during the uterine cycle: menstrual phase, proliferative phase, and secretory phase. 28.2d Vagina 855 ■ The vagina is a fibromuscular tube that serves as the birth canal for the fetus, a receptacle for the penis during intercourse, and the passageway for menstrual discharge. 28.2e External Genitalia 857 ■ The external female sex organs, collectively called the vulva, include the mons pubis, labia majora, labia minora, and the clitoris. 28.2f Mammary Glands 857 28.3 Anatomy of the Male Reproductive System 861 ■ The paired mammary glands produce breast milk. ■ Prolactin is responsible for milk production; oxytocin is responsible for milk ejection. ■ The primary male reproductive system organs are the testes; accessory sex organs include ducts, male accessory glands, and the penis. 28.3a Scrotum 861 ■ The scrotum houses the testes outside the male body, where the lower temperature is needed to form functional sperm. 28.3b Spermatic Cord 863 ■ The spermatic cord transmits blood vessels and nerves from the abdominal cavity to the testis. 28.3c Testes 863 mck78097_ch28_842-878.indd 876 ■ The testis contains up to four seminiferous tubules. Between the seminiferous tubules are interstitial cells, which produce androgens. ■ Seminiferous tubules contain sustentacular cells and developing sperm cells. ■ Sustentacular cells nourish developing sperm cells. ■ Spermatogenesis is the meiotic process that forms haploid spermatids. ■ Spermiogenesis is the process by which spermatids differentiate into sperm. 2/14/11 4:44 PM Chapter Twenty-Eight 28.3 Anatomy of the Male Reproductive System (continued) 861 Reproductive System 877 28.3d Ducts in the Male Reproductive System 866 ■ Ducts store and transport sperm. They include the rete testis, the efferent ductules, the epididymis, the ductus deferens, and the ejaculatory duct. ■ The male urethra carries urine or semen at any one time. 28.3e Accessory Glands 867 ■ Accessory glands produce seminal fluid, a nutrient-rich, alkaline fluid that supports sperm. Accessory glands include seminal vesicles, the prostate gland, and the bulbourethral glands. 28.3f Semen 868 ■ Semen is a mixture of seminal fluid and sperm. 28.3g Penis 869 ■ The penis is the copulatory organ. ■ The body of the penis contains three parallel erectile bodies and the urethra. 28.4 Aging and the Reproductive Systems 871 ■ Females undergo a change in reproductive structure and fertility called menopause. Males undergo a male climacteric in which the production of testosterone is measurably reduced. 28.5 Development of the Reproductive Systems 872 ■ Both male and female reproductive structures originate from the same basic primordia. Gene expression determines how they differentiate. 28.5a Genetic Versus Phenotypic Sex 872 ■ Genetic sex is based on chromosome type; phenotypic sex refers to the appearance of the internal and external genitalia. 28.5b Formation of Indifferent Gonads and Genital Ducts 872 ■ Early in development, genital ridges form from intermediate mesoderm. Primordial germ cells migrate from the yolk sac to the genital ridges and form the future gametes. 28.5c Internal Genitalia Development 874 ■ In the absence of sex-determining region Y (SRY) gene, the female reproductive pattern develops. The male reproductive pattern develops as a result of the SRY gene and its proteins. 28.5d External Genitalia Development 874 ■ The external genitalia appear very similar until about week 12; external genitalia become fully differentiated about week 20. Challenge Yourself Matching Multiple Choice Match each numbered item with the most closely related lettered item. Select the best answer from the four choices provided. ______ 1. vagina a. houses the testes ______ 2. uterus b. produces follicles and sex hormones ______ 3. clitoris ______ 4. testes ______ 5. ovary ______ 6. prostate gland ______ 7. scrotum ______ 8. uterine tube ______ 9. penis ______ 10. semen c. secretion is milky; contains citric acid d. contains three erectile bodies e. normal site for implantation of a pre-embryo f. fertilization normally occurs here g. composed of both sperm and seminal fluid h. birth canal i. produces spermatozoa j. contains two erectile bodies mck78097_ch28_842-878.indd 877 ______ 1. In a. b. c. d. the male, what cells produce androgens? spermatogonia interstitial cells sustentacular cells All of these are correct. ______ 2. All of the following organs produce a component of seminal fluid except the a. bulbourethral glands. b. testes. c. seminal vesicles. d. prostate gland. ______ 3. Spermatogonia divide by mitosis to form a new spermatogonium and a. a sperm. b. spermatids. c. a primary spermatocyte. d. zygotes. ______ 4. Sperm are stored in the ______, where they remain until they are fully mature and capable of motility. a. epididymis c. ductus deferens b. seminiferous tubule d. rete testis 2/14/11 4:44 PM 878 Chapter Twenty-Eight Reproductive System ______ 5. The female homologue to the penis is the a. labia majora. b. labia minora. c. clitoris. d. vagina. ______ 6. Ovulation occurs due to a dramatic “peak” in which hormone? a. progesterone b. LH c. FSH d. prolactin ______ 7. Which statement is true about the uterus? a. The basal layer of the endometrium is shed each month during menses. b. The myometrium is composed of several layers of skeletal muscle. c. The cervix projects into the vagina. d. The round ligament is peritoneum that drapes over the uterus. ______ 8. Which structure contains a primary oocyte, several layers of granulosa cells, and an antrum? a. primordial follicle b. primary follicle c. secondary follicle d. vesicular follicle ______ 9. The most anteriorly placed structure in the female perineum is the a. vaginal orifice. b. cervix. c. labia minora. d. mons pubis. ______ 10. The paramesonephric ducts in the embryo form which of the following? a. uterine tubes and uterus b. ovary c. ductus deferens d. seminal vesicle Content Review 1. What are some similarities between the male and female reproductive systems? What are the anatomic homologues between these systems? 2. What hormones are associated with the female reproductive system, and what is the function of each hormone? 3. Identify the regions of the uterine tube. 4. List the uterine wall layers, and describe the basic anatomy of each layer. 5. Compare and contrast the ovarian cycle phases and the uterine cycle phases. When do they occur? What specific events are associated with each phase? 6. Describe these parts of the mammary gland: nipple, areola, lobe, lobule, and alveoli. 7. What is the function of sustentacular cells in the production of spermatozoa? 8. Describe the process of spermatogenesis, including which cells are diploid and which are haploid. 9. Compare the secretions of the seminal vesicles, the prostate gland, and the bulbourethral glands. 10. What changes occur in the penis to allow a male to attain an erection? Developing Critical Reasoning 1. Caitlyn had unprotected sex with her fiancé approximately 2 weeks after her last period, and is worried that she might have become pregnant. She asks her physician if there are times during her monthly menstrual cycle when she might be more likely to become pregnant. She also asks how birth control pills prevent a woman from becoming pregnant. What will the physician tell Caitlyn? 2. If parents wish to know the sex of their unborn baby, they usually have to wait until weeks 18–22 of development before a sonogram determining the sex can be performed. Based on your knowledge of reproductive system development, explain why the sex of the unborn baby can’t be determined easily before this time. Answers to “What Do You Think?” 1. A woman can become pregnant as long as she has one remaining functioning ovary. 2. Stress, age, medications, and body weight all can affect a woman’s monthly uterine (menstrual) cycle. Stress and excessively lean body mass can lead to amenorrhea (absence of periods). 3. If a male’s testes were removed, the adrenal glands could still produce a small amount of androgens. However, since the testes produce the overwhelming majority of androgens, the small amount produced by the adrenal glands would have little effect on the male. 4. If a male has a vasectomy, sperm still form in the seminiferous tubule and then mature in the epididymis. However, since the sperm are not ejaculated, they die, and their components are broken down and resorbed in the epididymis. An individual who has had a vasectomy ejaculates seminal fluid only, not semen (which contains sperm). www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more. mck78097_ch28_842-878.indd 878 2/14/11 4:44 PM

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