Reproduction
615
Unlike the continuous sperm production of the male, the matu-
ration of the female gamete, the egg, followed by its release
from the ovary—
ovulation
—is cyclical. The structure and
function of the female reproductive system (e.g., the uterus) are
synchronized with these ovarian cycles. In human beings, these
cycles are called
menstrual cycles.
The length of a menstrual
cycle varies considerably from woman to woman and even in
any particular woman, but averages about 28 days. The fi rst day
of menstrual fl ow (
menstruation
) is designated as day 1.
Menstruation is the result of events occurring in the
uterus. However, the uterine events of the menstrual cycle are
due to cyclical changes in hormone secretion by the ovaries.
The ovaries are also the sites for the maturation of gametes.
One oocyte normally becomes fully mature and is ovulated
around the middle of each menstrual cycle.
It is the interactions among the ovaries, hypothalamus,
and anterior pituitary gland that produce the cyclical changes
in the ovaries that result in (1) maturation of a gamete each
cycle, and (2) hormone secretions that cause cyclical changes
in all of the female reproductive organs (particularly the
uterus). The uterine changes prepare this organ to receive and
nourish the developing conceptus, and only when there is no
pregnancy does menstruation occur. After describing the full
menstrual cycle in the absence of pregnancy, we describe the
events of pregnancy, delivery, and lactation.
Anatomy
The female reproductive system includes the two ovaries and
the female reproductive tract—two
fallopian tubes
(or ovi-
ducts), the uterus, the cervix, and the vagina. These structures
are termed the
female internal genitalia
(
Figure 17–13
).
Unlike in the male, the urinary and reproductive duct systems
of the female are entirely separate from each other.
The ovaries are almond-sized organs in the upper pelvic
cavity, one on each side of the uterus. The ends of the fal-
lopian tubes are not directly attached to the ovaries but open
into the abdominal cavity close to them. The opening of each
fallopian tube is funnel-shaped and surrounded by long, fi n-
gerlike projections (the
mbriae
) lined with ciliated epithe-
lium. The other ends of the fallopian tubes are attached to the
uterus and empty directly into its cavity. The
uterus
is a hol-
low, thick-walled, muscular organ lying between the urinary
bladder and rectum. It is the source of menstrual fl ow, and it
is where the fetus develops during pregnancy. The lower por-
tion of the uterus is the
cervix.
A small opening in the cervix
leads to the
vagina,
the canal leading from the uterus to the
outside.
The
female external genitalia
(
Figure 17–14
) include
the mons pubis, labia majora, labia minora, clitoris, ves-
tibule of the vagina, and vestibular glands. The term
vulva
is another name for all these structures. The mons pubis is
the rounded fatty prominence over the junction of the pubic
bones. The labia majora, the female homolog of the scrotum,
are two prominent skin folds that form the outer lips of the
vulva. (The terms
homolog
and
analogous
mean that the two
structures are derived embryologically from the same source
and/or have similar functions.) The labia minora are small
skin folds lying between the labia majora. They surround the
urethral and vaginal openings, and the area thus enclosed is
the vestibule, into which glands empty. The vaginal opening
lies behind the opening of the urethra. Partially overlying
the vaginal opening is a thin fold of mucous membrane, the
hymen.
The
clitoris,
the female homolog of the penis, is an
erectile structure located at the front of the vulva.
Ovarian Functions
As noted at the beginning of this chapter, the ovary, like the
testis, serves several functions: (1)
oogenesis,
the produc-
tion of gametes during the fetal period, (2) maturation of the
oocyte, (3) expulsion of the mature oocyte (ovulation), and
(4) secretion of the female sex steroid hormones, (estrogen
and progesterone), as well as the peptide hormone inhibin.
Before ovulation, the maturation of the oocyte and endocrine
functions of the ovaries take place in a single structure, the
follicle. After ovulation, the follicle, now without an egg, dif-
ferentiates into a corpus luteum, which only has an endocrine
function. For comparison, recall that in the testes, the produc-
tion of gametes and the secretion of sex steroids take place in
different compartments—in the seminiferous tubules and in
the Leydig cells, respectively.
Oogenesis
The female germ cells, like those of the male, have different
names at different stages of development. However, the term
egg
will be used to refer to the germ cells at any stage.
At birth, the ovaries contain an estimated total of 2 to
4 million eggs, and no new ones appear after birth. Thus, in
marked contrast to the male, the newborn female already has
all the germ cells she will ever have. Only a few, perhaps 400,
will be ovulated during a woman’s lifetime. All the others
degenerate at some point in their development so that few, if
any, remain by the time a woman reaches approximately 50
years of age. One result of this developmental pattern is that
the eggs ovulated near age 50 are 35 to 40 years older than
those ovulated just after puberty. It is possible that certain
chromosomal defects more common among children born to
older women are the result of aging changes in the egg.
During early
in utero
development, the primitive germ
cells, or
oogonia
(singular,
oogonium
), a term analogous to
spermatogonia in the male, undergo numerous mitotic divi-
sions (
Figure 17–15
). Around the seventh month after con-
ception, the fetal oogonia cease dividing. Current thinking is
that from this point on, no new germ cells are generated.
During fetal life, all the oogonia develop into
pri-
mary oocytes
(analogous to primary spermatocytes), which
then begin a fi rst meiotic division by replicating their DNA.
They do not, however, complete the division in the fetus.
Accordingly, all the eggs present at birth are primary oocytes
SECTION C
Female Reproductive Physiology
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