THE CONTINUITY OF LIFE: REPRODUCTION
All mammals reproduce sexually. As you will recall, sexual reproduction involves two events: meiosis (the
division of the cell nucleus to produce sex cells or gametes) and fertilization (the fusion of two gametes). In
vertebrates, meiosis produces two types of gametes: gametes specialized for motility called sperm and
gametes specialized for production and storage of nutrients called eggs or ova. In many invertebrates, such
as insects, the generations do not overlap; but in vertebrates, parents not only survive after their young are
produced but often are essential to the rearing of the young. This evolutionary trend toward increasing
parental care becomes pronounced among birds and reaches its fullest expression among some of the
mammals, ourselves included.
In most species of fish and in amphibians, as in many invertebrates, fertilization is external. Among
organisms that produce amniote eggs (reptiles, birds and monotreme mammals), fertilization is internal. The
outer protective shell, produced as the egg moves down through the female reproductive tract, is laid down
after the egg cell is fertilized, enclosing the embryo (organism at an early stage of development) and its
membranes. Fertilization is also internal among marsupial and placental mammals, in which the embryo
develops within the mother and is nourished by her.
HUMAN REPRODUCTION AND DEVELOPMENT
In humans, as in other vertebrates, the reproductive system produces, stores, nourishes and releases
specialized sex cells known as gametes. These cells are released in ways that make possible the fusion of
sperm and egg, the male and female gametes. All the cells of the human body are derived from the single
fertilized cell known as a zygote.
Sexual Development
For the first six weeks after fertilization, human male and female embryos are identical in appearance; in
other words, the gonads, or reproductive organs, look the same in all human embryos. Then, during the
seventh week of development, major changes occur. Activation of genes on the sex chromosomes triggers
the gonads to develop into either testes or ovaries. The testes (singular: testis), which are the primary
reproductive organs of a male embryo, begin to produce steroid hormones known as androgens. The tissues
of the embryo respond to these hormones by developing into the male reproductive organs.
If the embryo is female, the reproductive organs of a female embryo called the ovaries produce
steroid hormones known as estrogens. In response to these hormones, the tissues of the embryo develop in
a pattern that produces the female reproductive organs. The male and female reproductive organs develop
from exactly the same tissues in the embryo.
After birth the testes and the ovaries continue to produce small amounts of androgens and
estrogens, respectively. These sex hormones continue to influence the development of the reproductive
organs. Although gonads and accessory organs are already formed at birth, they have not reached their full
size and neither testes nor ovaries are capable of producing active reproductive cells (gametes) until the
onset of puberty. Puberty is a period of rapid growth and sexual maturation during which the reproductive
system becomes fully functional and an individual develops secondary sexual traits (see the accompanying
handout “Physiological and Psychological Changes of Puberty”). Among the secondary sexual traits are the
amount and distribution of body fat, hair and skeletal muscle. Those traits are associated with maleness and
femaleness but play no direct role in reproduction. At the completion of puberty, the male and female
gonads are fully developed. The onset of puberty varies considerably among individuals. It may occur
anytime from age 9 to 16. Generally, puberty begins about a year earlier in females than in males.
Puberty begins with a change in the hypothalamus, the part of the brain that regulates the
secretions of the pituitary gland. This change causes the pituitary gland to produce increased levels of two
hormones that affect the gonads. These hormones, named for their effects on the female, are folliclestimulating hormone (FSH) and luteinizing hormone (LH).
Primary Reproductive Organs
The human reproductive system consists of a pair of primary reproductive organs, or gonads, and accessory
glands and ducts. The male gonads are the testes; the female gonads are the ovaries. Testes and ovaries
have the same general functions: they produce gametes and the secrete sex hormones, which influence
reproductive functions and the development of secondary sexual traits. The male gametes are sperm; the
female gametes are oocytes or “eggs.”
Male Reproductive System
The primary male reproductive organs, the testes, develop within the abdominal cavity. Just before birth
(and sometimes just after) the testes descend through a canal into an external sac called the scrotum. The
testes remain in the scrotum, outside the body cavity, where the temperature is about 1° to 3°C lower than
the internal temperature of the body (37°C). Sperm development in the testes requires the lower
temperature.
The testes consist of hundreds of tiny tubules called seminiferous tubules. The word seminiferous
means seed-bearing, making it an appropriate name for these tubules because it is there that sperm are
produced. The seminiferous tubules are tightly coiled and twisted together to form a compact organ. When
the pituitary gland begins to release FSH and LH, these hormones stimulate the testes to make the principal
male sex hormone testosterone.
Cells that can respond to testosterone are found all over the body. Testosterone produces a number
of secondary sex characteristics that appear in males at puberty. (The primary sex characteristics are
those that pertain to the reproductive system itself — the reproductive tract and external features.) A
boy’s voice becomes deeper, he grows a beard and more body hair, his chest broadens, and he may find it
easier to develop large muscles. He will continue to grow for several years after his female classmates have
stopped growing.
FSH and testosterone stimulate the development of sperm. When large numbers of sperm have been
produced in the testes, the developmental process of puberty is completed. The reproductive system is now
functional, meaning the the male can produce and release sperm.
Sperm Development
Sperm are derived from special cells within the testes that go through the process of meiosis to form the
mature sperm that have half the number of chromosomes of all other body cells. A sperm cell consists of a
head, which contains the highly condensed nucleus; a midpiece, which is packed with energy-releasing
mitochondria cells; and a tail, or flagellum, which propels the cell forward. At the tip of the head is a small
cap that contains an enzyme vital to the process of fertilization because it allows the sperm cell to enter the
egg cell.
Mature sperm travel from the seminiferous tubules into the epididymis, a comma-shaped structure in
which they fully mature and are stored. After brief storage in the epididymis, the sperm are forced into a
tube known as the vas deferens. The vas deferens passes into the abdominal cavity where three glands —
prostate gland, bulbourethral glands (sometimes called Cowper’s gland) and seminal vesicles — produce
seminal fluid in which the sperm are suspended. The combination of sperm and seminal fluid is known as
semen.
The number of sperm present in even a few drops of semen is astonishing. Between 100 and 200
million sperm are present in 1 milliliter of semen. That’s about 5 million sperm per drop! Eventually, the vas
deferens merges with the urethra, the tube that leads to the outside of the body through the penis and
serves both for the excretion of urine and the ejaculation of sperm in males. The function of the penis is to
deposit sperm cells within the reproductive tract of the female. The human penis is formed of three
cylindrical masses of spongy erectile tissue, each of which contains a large number of small spaces, each
about the size of a pinhead. One of these masses is enlarged at the far end to form the glans penis, which is
a smooth protective cap over the spongy tissues. The exterior portion of the penis is covered by a loose,
thin layer of skin, which at the end forms an encircling fold over the glans. This fold, called the foreskin, is
sometimes surgically removed (circumcision). The urethra terminates in a slitlike opening in the penis.
Sperm Release
Erection of the penis, which can be elicited by a variety of stimuli, is caused by dilation of the blood vessels
carrying blood to the spongy tissues, resulting in the collection of blood within the spaces. As the tissues
become distended, they compress the veins and so inhibit the flow of blood out of the tissues. With
continued stimulation, the penis becomes hard and enlarged.
Erection is accompanied by discharge of a small amount of fluid from the bulbourethral glands (peashaped organs at the base of the penis). This serves as a lubricant to facilitate the movement of
spermatozoa along the male urethra and to aid penetration of the penis into the female. Continued
stimulation characteristically leads to contraction of the muscles in the scrotum (raising the testes close to
the body) and of the muscles encircling the epididymis and vas deferens. These contractions move the
spermatozoa toward and into the urethra. As this occurs, the seminal vesicles secrete a fructose-rich fluid
that nourishes the sperm cells. This fluid contains a high concentration of prostaglandins, which cause
contractions in the musculature of the female uterus and oviducts and so may assist the sperm in reaching
the egg. The prostate gland adds a thin, milky, alkaline fluid that helps neutralize the normally acidic pH of
the female reproductive tract. Finally, the muscles at the base of the penis are stimulated, and these
muscle contractions propel the semen out through the urethra. The process is called ejaculation.
Because ejaculation is regulated by the autonomic nervous system, it is not completely voluntary.
Approximately 2 to 3 milliliters of sperm are released in an ejaculation. If the 300 million or so sperm are
released in the reproductive tract of a female, the chances of a single sperm fertilizing an ovum (egg), if one
is available, are quite good.
Female Reproductive System
The primary reproductive organs in the female are the ovaries (singular: ovary). The ovaries are located in
the abdominal cavity, suspended by ligaments (bands of connective tissue) and mesenteries. The oocytes,
from which eggs develop, are in the outer layer of the ovary. Whereas the testes may produce several
hundred million reproductive cells each day, the ovaries usually produce only one ovum per month. But in
addition to producing ova, the female reproductive system has another important job to perform. Each time
an ovum is released, the body of the female must be prepared to nourish a developing embryo.
The uterus (also called the womb) is a hollow, muscular, pear-shaped organ slightly smaller in size
than a clenched fist in the nonpregnant female. It lies almost horizontally in the abdominal cavity and is on
top of the bladder. The uterus is lined by the endometrium, which has two principal layers, one of which is
shed at menstruation and another from which the shed layer is regenerated. The sphincter muscle that
forms the opening to the uterus is called the cervix. The sperm pass through this opening on their way
toward the ovum. The cervix dilates (or enlarges) to allow the fetus to emerge at the time of birth. A fetus
is defined as a developing human from about the second month of gestation (pregnancy) until birth.
The vagina is a muscular tube about 7.5 centimeters long that leads from the cervix of the uterus to
the outside of the body. It is the organ that receives the penis and is also the birth canal. Its exterior
opening is between the urethra, the tube leading from the urinary bladder, and the anus. (Note that, unlike
the urethra in males, the female urethra functions only as a duct to remove urine from the body.) The
vaginal tract is mildly acidic, with a pH between 4 and 5.
The external genital organs of the female are collectively known as the vulva. The clitoris, which
corresponds to the penis in the male (in the early embryo, the structures are identical), is about 2
centimeters long and, like the penis, composed chiefly of erectile tissue. The labia are folds of skin that
surrounding the opening to the vagina. The labia majora are fleshy and, in the adult, covered with pubic hair.
They enclose and protect the underlying, more delicate structures, the labia minora. (Embryologically, they
are homologous with the scrotum in the male.)
As in males, puberty in females starts with changes in the hypothalamus that cause the release of
FSH and LH from the pituitary gland. These are the same kinds of hormones that are found in males,
although their target cells and the effects they produce are quite different.
FSH (follicle-stimulating hormone) stimulates cells within the ovaries to produce the hormones known
as estrogens. The estrogens cause the reproductive system to complete its development and also to produce
the secondary sex characteristics that appear in females. These characteristics include enlargement of the
breasts and reproductive organs, widening of the hips, and growth of hair in the armpits and pubic area.
Oogenesis or Ova Development
Each ovary contains from 400,000 to 2,000,000 primary follicles, which are clusters of cells surrounding a
single ovum. The function of a follicle is to prepare a single ovum for release into the part of the
reproductive system where it can be fertilized.
Ova mature within their follicles. Although a female is born with as many as 2 million immature ova
(primary follicles) in her ovaries — and does not produce any new ova during her lifetime — fewer than 500
will reach maturity, usually one at a time, about every 28 days after puberty to develop into mature egg cells
(ova).
Ovulation
When a follicle has completely matured, the ovum is released. This process is called ovulation. The follicle
literally ruptures, and the ovum is swept from the surface of the ovary into the opening of one of the two
Fallopian tubes (or oviducts). The ovum moves through the fluid-filled Fallopian tube, pushed along by
microscopic cilia attached to the cells that line the walls of the tube. It is during its journey through the
Fallopian tube that an egg can be fertilized. After a few days, the ovum passes from the Fallopian tube into
a larger cavity, the uterus. The endometrium of the uterus is specially designed to receive a fertilized ovum,
if fertilization has occurred.
Ovulation begins at puberty and usually continues until a female is in her late forties, when
menopause occurs. After menopause, follicle development and menstruation no longer occur and a female is
no longer capable of bearing a child.
The Menstrual Cycle
In females, the interaction of the reproductive system and the endocrine system takes the form of a
complex series of periodic events called the menstrual cycle. This name is quite appropriate: The cycle
takes an average of about 28 days, and the word menstrual comes from the Latin word mensis, meaning
“month.”
The menstrual cycle, which is controlled by hormones operating on a negative-feedback mechanism,
involves the development and the release of an egg for fertilization and the preparation of the uterus to
receive a fertilized egg. If an egg is not fertilized, it is discharged along with the lining of the uterus. The
menstrual cycle has four phases: follicle phase, ovulation, luteal phase and menstruation.
QUESTIONS: Answer on a separate sheet using complete sentences. Restate the question in each answer.
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Define the term “gametes” and give two examples of human gametes.
What are testes? What are two functions of the testes?
What are ovaries? What are two functions of the ovaries?
Define the term “puberty.” At what age does puberty occur?
What is the scrotum? Why are the scrotum and testes kept outside the body cavity, when this
position means they can be more readily injured?
Give several examples of secondary sex characteristics in males and females.
What is the function of the epididymis?
What is the function of the prostate gland, bulbourethral glands and seminal vesicles?
How many sperm typically are present in one drop of semen?
What is the uterus, and what is its function?
What is the term used to describe the birth canal of a human female?
Fertilization of an oocyte occurs where in the female body?