20.1 Reproduction and its types
Learning outcomes
After reading this section, you should be able to
a. Define reproduction and describe its types.
b. List the functions of the reproductive system.
c. Distinguish among the functions that occur in males, females, and both.
d. Give advantages of sexual and asexual reproduction.
e. Semen and its composition
Reproduction
Reproduction is the basic phenomenon of life. It is the basic characteristic of all living organisms through
which all living organism reproduce offspring of their own kind.
Need for reproduction
Reproduction is important for continuity of race of an organism.
Reproduction is important for the survival of all living things.
Reproduction guarantees transmission of one generation's genetic material into the next generation.
Types of reproduction
There are two types of reproduction.
a. Asexual reproduction
b. Sexual reproduction
a. Asexual reproduction
Multiplication of organisms without involving the fusion of nuclei of two different sex cells i.e. male gamete
and female gamete is called as asexual reproduction.
In asexual reproduction new organisms are produced from a single parent. This type of reproduction is more
common in plant kingdom and protozoan animals.
Asexual reproduction includes budding, binary fission, multiple fission, fragmentation, tissue culture, cutting,
grafting, apomixes and parthenogenesis.
b. Sexual reproduction
Sexual reproduction involves the fusion of two different sex cell, male gamete and female gamete. The union
of gametes is called as fertilization. And a diploid zygote is formed.
Types of sexual reproduction
On the basis of size of reproductive gametes sexual reproduction is of the following types.
a. Isogamous
In isogamous all the reproductive gametes are alike. The two opposite gametes fertilize to form zygote
e.g. chlamodomonas.
b. Anisogamous reproduction
In this type of reproduction the gametes are alike but they are unequal in size.
c. Oogamous reproduction
In oogamous reproduction eggs are few in number and are large and sessile, while male are large in
number and are small and motile.
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Advantages of asexual reproduction:
Asexual reproduction ensures propagation of derided organisms with desired characters.
Progeny is completely identically to their parents.
Asexual reproduction is faster enough and requires less amount of energy.
There is no need for a pollinator.
Advantages of sexual reproduction:
Sexual reproduction involves combination of genes that may make the organisms better suited to environment.
The new organism may have new characters absent in parents, thus generate variations.
Sexual reproduction Contribute to diversity in groups of organisms.
Functions of the Reproductive System
Reproduction is an essential characteristic of living organisms. Functional male and female reproductive
systems are necessary for humans to reproduce. The reproductive system performs the following functions:
a. Production of gametes
Specialized organs of the reproductive system produce gametes: sperm cells in males are produced in testes
and oocytes (eggs) in females are produced in ovaries.
b. Fertilization
The reproductive system enables fertilization of the oocyte by the sperm. The duct system in males nourishes
sperm cells until they are mature and are deposited in the female reproductive tract by the penis. The
female reproductive system receives the sperm cells from the male and transports them to the fallopian tube
where fertilization occurs.
c. Development and nourishment of a new individual
The female reproductive system enables the zygote to nurture into a new individual in the uterus until birth and
provides nourishment (milk) after birth.
d. Production of reproductive hormones
The reproductive system produces hormones that help in development of the gender-specific body form.
These hormones are also essential for the normal function of the reproductive system and for reproductive
behavior.
HUMAN REPRODUCTIVE SYSTEM
Gender is a common way to classify people. Just think of all the times you have had to check a box for male or
female while filling out a form. The reproductive system controls the development of the structural and
functional differences between males and females, and it has profound effects on human behavior.
Although the male and female reproductive systems are strikingly different from one another, still there are
numerous similarities among these. Many reproductive organs of males and females are derived from the same
embryological structures, and some of the same hormones function in both males and females, even though
these hormones act in very different ways.
The male and female reproductive system in humans is as follows.
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20.2 MALE REPRODUCTIVE SYSTEM
Learning outcomes
After reading this section, you should be able to
a. Describe the scrotum and its role in regulating the temperature of the testes.
b. Describe the structure of the penis, seminal vesicles, prostate gland, and bulbourethral glands and explain their functions.
c. List the ducts of the male reproductive system and explain their function.
d. Describe the structure of the testes, and function of the specialized cells of the testes.
e. Describe the scrotum and its role in regulating the temperature of the testes.
f. List down the function of glands associated with male reproductive system.
g. Explain the structure and function of male reproductive system.
Male reproductive system has four components.
a. A pair of testes, produce sperms
b. Ducts system (include epididymis and vas deferens), transport sperm from tester to external
c. Supporting structures (penis and scrotum); External genetalia (penis) transfer sperm to female vagina.
d. Accessory glands (include the seminal vesicles, the prostate gland, and the bulbourethral glands)
Testes; the male gonads
The testes and epididymides, in which the sperm cells develop, are located outside the body cavity in the
scrotum. The testes are small, ovoid organs, each about 4–5 cm long.
Fine structure of testes
Each testis is divided into 250 to 300 cone-shaped lobules. Each lobule contains one to four tightly coiled
seminiferous tubules in which sperm cells develop. Loose connective tissue surrounding the seminiferous
tubules contains clusters of endocrine cells called interstitial cells, or Leydig cells, which secrete testosterone.
Function of testes
Testis functions as both exocrine gland and endocrine gland. Their major exocrine secretion is sperm cells, and
their major endocrine secretion is the hormone testosterone. About 30 millions of sperms are produced each
day.
Leyden cells and testosterone
Leyden cells are present in the loose connective tissues surrounding the seminiferous tubules these cells
produce male sex hormone; testosterone.
A series of ducts (Duct system)
The accessory ducts include the vasa efferentia, the epididymis, the ductus deferens, the ejaculatory duct, and
the urethra.
a. Vasa efferentia
Vasa efferentia collect sperms from inside the testes and transfer them to the epididymis. There are about 10 to
20 vasa efferentia.
b. Epididymis
Epididymis is a convoluted, comma-shaped structure on the posterior side of the testis, the sperms after
formation flow from seminiferous tubules to the epididymis where they are stored. The final maturation of the
sperm cells occurs within the epididymis. It takes 12–16 days for sperm to travel through the epididymis and
appear in the ejaculate.
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The sperm cells then leave the epididymis, passing through the ductus deferens, ejaculatory duct, and
urethra to the exterior of the body.
c. Vas deferens and Ejaculatory duct
The ductus deferens or vas deferens emerges from the tail of the epididymis. The end of the ductus deferens
enlarges to form an ampulla. Adjacent to the ampulla of each ductus deferens is a sac-shaped gland called the
seminal vesicle.
A short duct from the seminal vesicle joins the ampulla of the ductus deferens to form the ejaculatory duct.
Each ejaculatory duct is approximately 2.5 cm long. These ducts projects into the prostate gland and ends by
opening into the urethra.
Urethra
Urethra is the terminal portion of the male duct system. The male urethra is about 20 cm long and extends
from the urinary bladder to the distal end of the penis. The urethra is a passageway for both urine and
male reproductive fluids. Here the glandular secretions added to sperms forming semen.
Supporting structures (Penis and Scrotum)
Penis
The penis is the male organ of copulation, through which sperm cells are transferred from the male to the
female. The penis contains three columns of erectile tissue Engorgement of this erectile tissue with blood
causes the penis to enlarge and become firm, a process called erection.
The skin of the penis, especially the glans penis, is well supplied with sensory receptors. A loose fold
of skin called the prepuce, or foreskin, covers the glans penis. In many cultures, the prepuce is surgically
removed shortly after birth, a procedure called circumcision.
Scrotum
The scrotum is a saclike structure that contains the testes. It is divided into two internal compartments by
an incomplete septa of connective tissue.
Sperms are sensitive to temperature and their production required about 35 0 C (2 degree less
temperature than the body temperature), scrotum helps in regulating the temperature.
In cold temperatures, the muscles of scrotum contracts, causing the skin of the scrotum to become firm
and wrinkled and reducing its overall size. This contraction pulls the testes nearer the body, which helps keep
the testes warm.
When temperature increases due to a warmer environment or as a result of exercise or fever, the
muscles relax, and the skin of the scrotum becomes loose and thin, allowing the testes to descend away from
the body and keep cool. The response of the dartos and cremaster muscles is important because sperm cells are
very temperature-sensitive and do not develop normally if the testes become too warm or too cool.
Accessory glands
Three exocrine glands secrete material into the ducts of the male reproductive tract. These glands are the
seminal vesicles, the prostate gland, and the bulbourethral glands.
Seminal Vesicles
The seminal vesicles are sac-shaped glands located next to the ampullae of the ducta deferentia. Each gland is
about 5 cm long. The seminal vesicles have a capsule containing fibrous connective tissue and smooth muscle
cells.
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Chemical composition of seminal secretions
The thick secretions of the seminal vesicles contain large amounts of fructose, citric acid, and other nutrients
that nourish the sperm cells. The secretions also contain fibrinogen and a coagulating enzyme called
vesiculase, which causes weak coagulation reaction of the semen immediately after ejaculation. In addition,
seminal vesicle secretions contain prostaglandins that can cause uterine contractions to help transport sperm
cells through the female reproductive tract.
Prostate Gland
The prostate gland, is a walnut shaped gland, it is about approximately 4 cm long and 2 cm wide. The Prostate
encircles the urethra just below the bladder. The thin, milky secretions of the prostate have a basic pH and
contains citrate as a nutrient source and several enzymes especially hyaluronidase. This prostatic secretions
help neutralize the acidic urethra and the vagina.
Bulbourethral Glands
The bulbourethral glands, also called Cowper glands, are a pair of small glands located near the membranous
part of the urethra. In young males, each gland is about the size of a pea, but they decrease in size with age and
are almost impossible to observe in older men.
role of bulbourethral glands
The bulbourethral glands and urethral mucous glands produce a mucous secretion just before ejaculation. This
mucus lubricates the urethra, neutralizes the contents of the normally acidic spongy urethra, provides a small
amount of lubrication during intercourse, and helps reduce vaginal acidity.
F IG: 20.2 MAIN PARTS OF MALE REPRODUCTIVE SYSTEM
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Semen
Semen is a white, sticky mixture of sperm and secretions of accessory glands. The liquid substance in the
semen provides nutrients and protection to sperms and acts as a transport medium for sperms.
As a whole, semen is slightly alkaline (pH 7.2—7.6). It helps neutralize the acid environment of the male's
urethra and the female's vagina. The amount of semen propelled out of the male duct system during ejaculation
about 2-5 ml, but there are between 20 and 150 million sperm per milliliter.
Secretions of accessory glands
The seminal vesicles produce about 60% of the fluid, the prostate gland contributes about 30%, the testes
contribute 5%, and the bulbourethral glands contribute 5%.
Emission and Ejaculation
Emission is the discharge of all these secretions from the ducta deferentia into the urethra and ejaculation is the
forceful expulsion of semen from the urethra caused by contraction of the urethra, the skeletal muscles in the
pelvic floor, and the muscles at the base of the penis.
Hormones in semen
Prostaglandins in semen decrease the viscosity of mucus guarding the entry (cervix) of the uterus and stimulate
reverse peristalsis in the uterus, facilitating sperm movement through the female reproductive tract.
20.3 Sperms and their production (Spermatogenesis)
Learning outcomes
After reading this section, you should be able to
a. Describe the process of spermatogenesis.
b. Describe the structure of human sperm.
c. Where, specifically, are sperm cells produced in the testis?
d. Describe the role of meiosis in sperm cell formation.
e. List the parts of a mature sperm. What are their functions?
f. Explain the events of spermiogenesis.
g. What is a spermatid? How it transforms into a
spermatozoon?
h. Describe oogenesis.
i. Describe the role of hormones in spermatogenesis.
j. Analyze control of male reproductive system.
Structure of a human sperm
The sperm, or spermatozoon (animal seed), is a very small haploid cell. It has a head, a neck, a midpiece, and a
tail or flagellum.
a. Head
The head contains the nucleus having haploid set of chromosome. Just anterior to the nucleus is a vesicle
called the acrosome, which contains enzymes called hyaluronidase which is necessary for the sperm cell to
penetrate the female oocyte. Acrosome is produced by the Golgi apparatus.
b. Neck
The neck of sperm is very short and contains a pair of centrioles. The microtubules of one of the centriole
elongate and run the entire length of the tail. It forms the axial filament of the tail.
c. Middle piece
The middle piece contains many mitochondria arranged spirally around the axial filament. The process begins
around the age of 14 years in males, and continues throughout life. Every day, a healthy adult male makes
about 400 million sperm
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d. Tail or flagellum
The flagellum is similar to a cilium, and microtubules within the flagellum move, propelling the sperm cell
forward. The midpiece contains large numbers of mitochondria, which produce the ATP necessary for
microtubule movement.
Spermatogenesis
It is the process of sperm formation in males which involves a precise sequence of events. This process takes
place in somniferous tubules. Spermatogenesis starts 12–14 years of age, when the number and size of
interstitial cells increase and a lumen develops in each seminiferous tubule. It takes approximately 74 days for
sperm cells to be produced. For about 50 of those days, the sperm cells are in the seminiferous tubules.
Spermatogonia are the cells that give rise to sperm cells. These are the outermost cells which make the
epithelial wall of the seminiferous tubules.
Main events in spermatogenesis
1. The Spermatogonia divide by mitosis. One daughter cell remains a spermatogonium that can divide
again by mitosis producing additional Spermatogonia. The other daughter cell becomes a primary
spermatocyte.
2. The primary spermatocyte divides by meiosis (meiotic division I) to form secondary spermatocytes.
3. The secondary spermatocytes divide by meiosis (meiotic division II) to form spermatids.
4. The spermatids differentiate to form sperm cells (Spermiogenesis).
Spermatid
Each spermatid is around, non-motile haploid cell with one of each of the homologous pairs of chromosomes.
Therefore, each sperm cell contains 22 autosomes and either an X or a Y chromosome. Each spermatid
undergoes the last phase of spermatogenesis, called spermiogenesis to form a mature sperm cell, or
spermatozoon
Spermiogenesis
During the processes of spermiogenesis, each spermatid develops a head, a midpiece, and a tail, or flagellum.
The nucleus of the sperm is located in the head.
F IG: 20.3 SECTIONS THROUGH TESTIS SHOWING THE SITE OF SPERM PRODUCTION
Hormonal control of spermatogenesis
Process of spermatogenesis is controlled by hormonal secretions from hypothalamus and pituitary gland. The
hypothalamus releases gonadotropin- releasing hormone (GnRH), which controls the release of two
gonadotropins from anterior pituitary.
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These two gonadotropins are luteinizing hormone (LH) and follicle-stimulating hormone (FSH). They are
named for their functions in females, but they also have important functions in males. LH in males is
sometimes called interstitial cell– stimulating hormone (ICSH)
Role of FSH
FSH binds primarily to sustentacular cells (sertoli cells) in the seminiferous tubules and promotes sperm cell
development from spermatid.
Role of LH and production of testosterone
LH binds to the interstitial cells in the testes and causes them to increase their rate of testosterone synthesis
and secretion. Testosterone causes the growth and development of germinal epithelium to form sperms.
Production of Inhibin
Inhibin hormone is produced by the sertoli cells and serves to control the spermatogenesis at normal rate.
When the sperm count is high, inhibin release increases and it inhibits anterior pituitary release of FSH and
hypothalamic release of GnRH. When sperm count falls, inhibin secretion declines steeply.
20.4 FEMALE REPRODUCTIVE SYSTEM
Learning outcomes
After reading this section, you should be able to
a. Name the organs of the female reproductive system and describe their functions.
b. Describe the anatomy and histology of the ovaries.
c. Discuss the development of the oocyte and the follicle and describe ovulation and fertilization.
d. Describe the structure of the uterine tubes, uterus, vagina, external genitalia, and mammary glands.
e. Discuss the structure and function of female reproductive system.
f. Describe the three layers lining the uterus. What is the role of myometrium?
g. Where the fertilization of an oocyte does takes place?
h. Name the parts of the uterus.
The female reproductive organs are the ovaries, the uterine tubes (oviduct), the uterus, the vagina, the external
genital organs, and the mammary glands.
Ovaries
Ovaries are female gonads which produce ova and release hormones. The two almond shaped ovaries are
small organs about 2–3.5 cm long and 1–1.5 cm wide lying on each side of the uterus.
Ovarian histology
Simple cuboidal cells called the ovarian epithelium, or germinal epithelium covers the surface of the ovary.
Immediately below the epithelium is a capsule of dense fibrous connective tissue, the tunica albuginea. The
outer part denser, of the ovary is called cortex is, and the looser, inner part of the ovary is called medulla. The
connective tissue of the ovary is called the stroma. Numerous ovarian follicles are distributed throughout the
stroma of the cortex. Each of which contains an immature egg, called oocyte.
Number and maturation of follicles
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Although at birth the ovaries together contain
about 1–2 million follicles, only about 500
follicles fully mature between puberty and
menopause. During a typical 4-week menstrual
cycle, one follicle matures and expels its egg, a
process called ovulation. Prior to ovulation,
cells of the follicle produce the primary female
sex hormone, estradiol (a type of estrogen).
After ovulation, the residual follicular tissue
grows within the ovary, forming a mass called
the corpus luteum (“yellow body”). The corpus
luteum secretes additional estradiol, as well as
progesterone, a hormone that helps maintain
the uterine lining during pregnancy. If the egg
cell is not fertilized, the corpus luteum
degenerates, and a new follicle matures during
the next cycle.
Uterine tubes
A uterine tube, also called a fallopian tube or oviduct, is associated with each ovary and extends from the area
of the ovary to the uterus. They receive the ovulated oocyte and are the site where fertilization generally
occurs. Each oviduct is about 10 cm long and contains sheets of smooth muscle both ciliated and non-ciliated
cells. The oocyte is carried toward the uterus by a combination of muscular peristalsis and the beating of the
cilia. Non-ciliated cells produce a secretion that keeps the oocyte (and sperm, if present) moist and nourished.
Uterus
Uterus is a hollow, thick-walled, muscular organ that functions to receive, retain, and nourish a fertilized
ovum. It is located in the pelvis, anterior to the rectum and posterior to the bladder.
Morphology
The uterus is the size and shape is about a medium-sized pear—about 7.5 cm long and 5 cm wide. The larger,
rounded part is called the fundus, and the narrower part is the cervix. The main part of the uterus, the body, is
between the fundus and the cervix.
Histology
The uterine wall is composed of three layers: the perimetrium, the myometrium, and the endometrium.
a. Perimetrium
The perimetrium, or serous layer, is the peritoneum that covers the uterus.
b. Myometrium
Just deep to the perimetrium, is the myometrium, or muscular layer, composed of a thick layer of smooth
muscle. The myometrium accounts for the bulk of the uterine wall and is the thickest layer of smooth muscle
in the body. In the cervix, the muscular layer contains less muscle and denser connective tissue. The cervix is
therefore more rigid and less contractile than the rest of the uterus.
The myometrium is the middle thick muscular layer composed of bundles of smooth muscle that contracts
rhythmically during childbirth to expel the baby from the mother's body.
c. Endometrium
The innermost spongy layer of the uterus is the endometrium, or mucous membrane, which consists of a
simple columnar epithelial lining and a layer of connective tissue called the lamina propria. If fertilization
occurs, the young embryo takes root into the endometrium (implants) and resides there for the rest of its
development.
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Cervix
The lower one third, or neck, of the uterus is called the cervix. This tubular part extends downward into the
upper part of the vagina. The cervix surrounds the opening called the cervical orifice (ostium uteri), through
which the uterus opens to the vagina. It is normally blocked by a plug of mucus.
Vagina
The vagina is a fibro muscular tube, about 8-10 centimeters long, that extends from the uterus to the outside.
Vagina is the female organ of copulation, receiving the penis during intercourse. It also allows menstrual flow
and childbirth (often called the birth canal). A thin mucous membrane called the hymen covers the vaginal
opening.
External Reproductive Organs
The external accessory organs of the female reproductive system include the labia majora, the labia minora,
the clitoris, and the vestibular glands. These structures that surround the openings of the urethra and vagina
compose the vulva.
20.5 Oogenesis
Learning outcomes
After reading this section, you should be able to
a. Discuss the development of the oocyte and the follicle and describe ovulation and fertilization.
b. What are the main steps in the process of oogenesis?
The process of egg formation in females is called oogenesis. The process of oogenesis takes years to complete.
The formation of female gametes begins in the fetus. By the fourth month of development, the ovaries contain
5 million oogonia (the cells from which oocytes develop).
Formation of primary oocyte
During development of the fetus, many oogonia begin meiosis, but stop in prophase I and are now called
primary oocytes. They remain in this state until puberty. The primary oocytes become surrounded by a single
layer of follicle cells. At birth there are about 2 million of them. The first meiotic division completes just
before ovulation.
From birth to puberty, the number of primary oocytes decreases to around 300,000–400,000. Only about
400 primary oocytes will complete development and give rise to the secondary oocytes that are eventually
released from the ovaries.
Formation of secondary oocyte and first polar body
Just before ovulation primary oocyte completes meiosis I and results in the formation of a large secondary
oocyte, and the other, small first polar body. In humans, the secondary oocyte arrests in metaphase II, if the
ovulated secondary oocyte is fertilized by a sperm it quickly completes meiosis II forming ovum otherwise the
secondary oocyte deteriorates.
20.6 Menstrual Cycle (female reproductive cycle)
Learning outcomes
After reading this section, you should be able to
a. Discuss various events of menstrual cycle.
b. Describe corpus luteum as a temporary endocrine gland.
c. Shortly describe menopause.
d. Differentiate monstrous cycle estrous cycle.
e. Describe corpus luteum as temporary endocrine gland.
f. What is the fate of corpus luteum?
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g.
h.
i.
j.
Describe menstrual cycle in detail.
What is the cause of multiple births?
What are the main events occurring in menstrual phase of menstrual cycle?
Define menarche.
There are two closely linked reproductive cycles in human females.
a. Ovarian cycle
The cyclic events in the ovaries define the ovarian cycle.
b. Menstrual cycle
The reproductive cycle in human and other primates is called menstrual cycle, also called the uterine
cycle. It involves changes in the uterus. Menstrual cycles average 28 days. The first reproductive cycle
begins at puberty near the thirteenth year of life and continues into middle age, and then ceases. A
female’s first reproductive cycle, called menarche, which occurs after the ovaries and other organs of
the female reproductive system mature and begin responding to certain hormones.
The female reproductive cycle is characterized by regular, recurring changes in the endometrium,
which culminate in menstrual bleeding (menses). These endometrial changes are coordinated with the
phases of the ovarian cycle, which are controlled by gonadotropins released by the anterior pituitary.
Phases of menstrual cycle
Various phases of menstrual cycle are as follows:
a. Menstrual phases (Days 1-5)
b. Proliferative or Pre-ovulatory phase (Days 6-14)
c. Secretory or post-ovulatory phase (Days 15-28)
a. Menstrual phase (Days 1-5):
In menstruation phase, arteries in the endometrium to constrict, much of the uterine lining disintegrates, small
blood vessels in the endometrium constrict, releasing blood that is shed along with endometrial tissue and
fluid. The result is menstruation—the menstrual flow phase of the uterine cycle.
The thick, hormone-dependent functional part of the endometrium detaches from the uterine wall and
passes out through the vagina
b. Proliferative or Pre-ovulatory phase (Days 6-14)
Prior to ovulation, ovarian steroid hormones, estradiol is secreted in increasing amount that stimulates the
endometrium to thicken and form new functional layer in order to support the developing embryo. The
endometrium becomes velvety, thick, and well vascularized.
At the end of proliferative phase (Day 14) ovulation (the release of an oocyte from ovary) occurs in
response to the release of LH from anterior lobe of pituitary gland. LH also converts the ruptured follicle to a
corpus luteum.
c. Secretory or post-ovulatory phase (Days 15-28)
After ovulation in secretory phase estradiol and progesterone secreted by the corpus luteum stimulate
continued development and maintenance of the uterine lining, including enlargement of arteries and growth of
endometrial glands. During the secretory phase the endometrium prepares for implantation of an embryo.
Fate of corpus luteum
If fertilization has not occurred, the corpus luteum begins to degenerate toward the end of the secretory phase.
Upon disintegration of the corpus luteum, the rapid drop in ovarian hormone levels causes arteries in the
endometrium to constrict, much of the uterine lining disintegrates, Small blood vessels in the endometrium
constrict, releasing blood that is shed along with endometrial tissue and fluid. The result is menstruation—the
menstrual flow phase of the uterine cycle.
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20.6 PHASES OF MENSTRUAL CYCLE
Menopause
Menopause is the cessation of ovulation and menstruation. During this interval, the ovaries lose their
responsiveness to FSH and LH, resulting in a decline in estradiol production. After about 500 cycles, a woman
undergoes menopause, Menopause usually occurs between the ages of 46 and 54. About 50% of women reach
menopause by age fifty, and
85% reach it by age fifty-two.
Cyclic menstruation is an indicator of normal reproductive life of females.
Multiple birth and twins
In 1-2% of all ovulations, more than one oocyte is ovulated. This phenomenon, which increases with age, can
result in multiple births. Since, in such cases, different oocytes are fertilized by different sperm, the siblings
are fraternal, or non-identical, twins. In some cases a single oocyte is fertilized by a single sperm, followed by
the splitting and separation of fertilized eggs daughter cells, resulting in the development is two identical and
genetically similar organisms.
Menstrual Versus Estrous Cycles
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In all female mammals, the endometrium thickens before ovulation, but only humans and some other primates
have menstrual cycles. Other mammals have estrous cycles, in which in the absence of a pregnancy, the uterus
reabsorbs the endometrium and no extensive fluid flow occurs. Whereas human females may engage in sexual
activity throughout the menstrual cycle, mammals with estrous cycles usually copulate only during the period
surrounding ovulation. This period, called estrus (from the Latin oestrus, frenzy, passion), is the only time the
female is receptive to mating. It is often called “heat,” and the female’s temperature does increase slightly.
The length and frequency of estrous cycles vary widely among mammals. Bears and wolves have one estrous
cycle per year; elephants have several. Rats have estrous cycles throughout the year, each lasting only 5 days.
20.7 Infertility
Learning outcomes
After reading this section, you should be able to
a. Define infertility.
b. Discuss infertility in male.
c. Discuss infertility in female.
d. How problem of infertility can be solved?
e. What is IVF?
f. Describe the procedure of IVF.
g. Discuss ethics related to IVF.
h. Describe miscarriage.
i. Define abortion.
Infertility is the inability to have children after one year of trying, is most often due to problems in the man,
such as underproduction of sperm. Female infertility can arise from a lack of eggs or a failure to ovulate.
Technologies can help treat many forms of infertility. About 15% of couples who want children are unable to
conceive.
Female infertility
Female infertility can result from;
a. Lack of eggs due to failure of ovaries to ovulate.
b. Blocked oviducts (often due to sexually transmitted diseases or pelvic inflammatory disease) resulting
in failure of egg to move towards uterus.
c. Sperm fails to fertilize egg.
d. Failure of uterus to support the embryo.
Male infertility
Male infertility can result from
a. Low sperm count; in this case the testes may not produce enough number of sperms.
b. Varicocele: This happens with the dilation of veins associated with the spermatic cord in the testes.
This heats the testicles. The heat can affect the number or shape of the sperm.
c. Failure of the sperms to reach egg; the sperm may not be vigorous enough to reach an egg.
d. Impotence; impotence also called erectile dysfunction is the inability to maintain an erection.
Temporary impotence can result from alcohol or drug use or from psychological problems. Permanent
impotence can result from nervous system or circulatory problems.
e. Cystic Fibrosis; Cystic Fibrosis, a genetic disorder can also contribute to male infertility.
Treatment of infertility
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Hormone therapy
Hormone therapy can sometimes increase sperm or egg production.
Surgery
Surgery operations can often correct ducts that have failed to form properly or have become blocked. Penile
implants can treat impotence.
Artificial conception
In this technique sperm are collected, concentrated, and then injected into the woman’s uterus via the vagina.
Sperm Banks
If a man produces no functioning sperm, the couple may elect to use another man’s sperm from a sperm bank.
If a woman has no eggs of her own, they can be obtained from a donor for fertilization and injection into the
uterus.
Hormone therapy
If a woman has normal eggs that are not being released properly, hormone injections can induce ovulation;
such treatments frequently result in multiple pregnancies (twins, triplets, or more).
Surrogate mother
If a woman cannot maintain pregnancy, she and her partner may enter into a legal contract with a surrogate
mother who agrees to carry the couple’s child to birth.
IVF; In-vitro fertilization
The technique of in vitro fertilization (IVF) involves mixing oocytes and sperm in culture dishes.
In-vitro fertilization
In-vitro fertilization (IVF) sometimes called the creation of a “test-tube baby” is a modern reproductive
technique that involves mixing oocytes and sperm in culture dishes. The fertilized eggs are incubated and
transferred to female uterus where further development occurs.
Steps
a. IVF begins with the administration of drugs that promote the development of multiple eggs (instead of
the one egg that typically occurs during each ovulation).
b. The eggs are surgically removed and then fertilized with sperm in a petri dish.
c. Fertilized eggs are allowed to develop for several days until they have formed at least eight cells
d. Injected into a woman’s uterus. There, one or more embryos may successfully implant and continue
development.
e. The sperm and eggs, as well as the embryos created, can be used immediately or frozen for later use.
To date, evidence indicates that abnormalities arising as a consequence of IVF procedures are rare.
20.7 IN-VITRO FERTILIZATION
The Ethics of IVF
Recent research has shown that IVF babies have a slightly greater risk of birth defects than babies born from
natural conception. Despite such risks and the high cost (around $10,000 for each attempt, whether it succeeds
or not), IVF is now routinely performed at medical centers throughout the world and results in the birth of
thousands of babies each year.
One major difference between natural and artificial fertilization is that technology offers choices that
nature does not. For example, sperm can be sorted based on whether they contain an X or Y chromosome,
increasing the likelihood of creating a zygote of a particular sex.
Parents may thus have a lot of information they can use in choosing which embryos to implant.
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An infertile couple could purchase eggs from a woman, sperm from a man, and then hire a surrogate to carry
the baby. Who, then, are the baby’s true parents? Each new reproductive technology raises moral and legal
questions. Many of these have not yet been addressed, much less resolved, by society.
Abortion and Miscarriage (spontaneous abortion)
Abortion
Abortion is defined as the termination of pregnancy by the removal or expulsion of a foetus or embryo from
the uterus before it is viable. An abortion can occur spontaneously, in which case it is usually called a
miscarriage, or it can be purposely induced. The term abortion most commonly refers to the induced abortion
of a human pregnancy.
Miscarriage
Miscarriage also called spontaneous abortion is a naturally occurring event characterized by loss of fetus
before it can survive outside the uterus. It usually occurs before the 20th week of pregnancy. Most miscarriages
occur during the first 7 weeks of pregnancy.
It is a common cause of failure to achieve reproductive success even when the couple is fertile.
Perhaps as many as one-third of all pregnancies end in miscarriages, some of them so quickly that a
woman has no idea she ever conceived and was pregnant.
Causes of miscarriage
a. Most miscarriages are caused by chromosome problems.
b. Trisomy 13 and 18 are the most common autosomal aneuploids and usually result in miscarriage.
c. Public health officials suspect that some pollutants contribute to miscarriages, skin rashes, nervous
disorders, and birth defects.
d. Usually, these problems are unrelated to the mother or father's genes.
e. Other possible causes for miscarriage include: Drug and alcohol abuse, hormone problems, infections,
obesity, physical problems with the mother's reproductive organs, and problem with the body's
immune response.
20.8 Sexually transmitted diseases (STDs)
Learning outcomes
After reading this section, you should be able to
a. Define sexually transmitted disease. Give examples.
b. Write a short note on syphilis.
c. Write a short note on gonorrhea.
d. What are the symptoms of syphilis?
e. What are the symptoms of gonorrhea?
f. Write a note on AIDS.
g. Discus mode of transmission of AIDS.
h. Name the causative agent of gonorrhea and Syphilis
Sexually transmitted diseases (STDs)
STDs are contagious diseases that can be spread by sexual contact. Viral STDs (such as AIDS, genital herpes,
and genital warts) cannot be cured, whereas STDs caused by bacteria, Protists, and fungi are generally curable
with drugs. Abstinence or the use of latex condoms can prevent STDs.
Gonorrhea
Causative agent
Gonorrhea is caused by bacterium Neisseria gonorrhea which invades the mucosae of the reproductive and
urinary tracts
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Symptoms
In women usually there are no symptoms some time accompanied by abdominal discomfort, vaginal discharge,
abnormal uterine bleeding, in men it is associated with painful urination discharge of puss from penis.
Number of cases reported in U.S.(2006)
350,000
Effects on the fetus
This disease causes Blindness in fetus, and may also result in stillbirth.
Treatment
The disease can be treated by introducing Antibiotics such as Penicillin and tetracycline.
Complications
Complications include Arthritis, rash, infertility and pelvic inflammatory disease.
Syphilis
Syphilis is caused by a bacterium Treponema pallidum. This bacterium easily penetrates the mucosa and
abraded skin.
Symptoms
Initial chancre sore appears usually on genitals or mouth; rash occurs about 6 months later; the disease is
asymptomatic for several years and finally damage to heart, liver, nerves, brain. In females the lesion remains
undetected in vagina and cervix. The chancre ulcerates and becomes crusty; then it heals spontaneously and
disappears within a few weeks.
Number of cases reported in U.S. (2006)
37,000
Effects on the fetus
Syphilis results in Miscarriage, premature birth, still birth and birth defects.
Treatment
Disease can be controlled through Antibiotics. Penicillin is the treatment of choice.
Complications
Complications include dementia. Fever and joint pain are common. Tertiary syphilis is characterized by
gummas, destructive lesions of the CNS, blood vessels, bones, and skin.
Acquired Immune Deficiency Syndrome (AIDS)
AIDS (Acquired Immune Deficiency Syndrome) is a serious condition that weakens the body's immune
system, leaving it unable to fight against infectious microorganisms.
The disease is caused by Human Immunodeficiency Virus (HIV or AIDS virus). The disease includes a
number of unusual and severe infections, cancers, severe weight loss, diseases affecting the brain and central
nervous system.
Cases of AIDS were first identified in San Francisco and New York about twenty years ago. Now there are an
estimated 42 million people living with HIV or AIDS worldwide, and more than 3 million die every year from
AIDS-related illnesses.
What happens when HIV invades human body?
When HIV is introduced into the body, this virus is too strong for the helper T cells and killer T cells
(lymphocytes). The virus then invades these cells and starts to reproduce itself, thereby not only killing the
CD4 T cells, but also infect other healthy cells. The HIV virus cannot be destroyed and lives in the body
undetected for months or years before any sign of illness appears. Gradually, over many years, as the T cells
become progressively destroyed or inactivated, other viruses, parasites or cancer cells (called "opportunistic
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diseases”) which would not have been able to get past a healthy body's defense, can multiply within the body
without fear of destruction.
HIV infection damages the immune system, leaving the body susceptible to infection with a wide
range of bacteria, viruses, fungi and protozoa. This condition is called acquired immune deficiency syndrome
(AIDS).
Transmission of AIDS
AIDS is transmitted via following main routes:
1. The most common mode of transmission is the transfer of body secretions through sexual contact.
2. Blood transfusion or blood products can transmit the virus, most often through the sharing of
contaminated syringes and needles.
3. HIV can be spread during pregnancy from mother to fetus.
4. HIV is also transmitted through procedures, such as acupuncture, tattooing, ear piercing, etc., when
contaminated needles or other non-sterile instruments are used.
5. People who have another sexually transmitted disease, such as syphilis, genital herpes, gonorrhea, or
bacterial vaginosis are at greater risk for getting HIV during sex with infected partners.
6. If a woman with HIV is pregnant, her newborn baby can catch the virus from her before birth, during
the birth process, or from breast feeding.
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