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The organs of the female reproductive system produce and sustain the female
sex cells (egg cells or ova), transport these cells to a site where they may
be fertilized by sperm, provide a favorable environment for the developing fetus,
move the fetus to the outside at the end of the development period, and
produce the female sex hormones. The female reproductive system includes the
ovaries, Fallopian tubes, uterus, vagina, accessory glands, and external
genital organs. Select a topic below to learn more about the female
reproductive system.
Ovaries
The primary female reproductive organs, or gonads, are the two ovaries. Each
ovary is a solid, ovoid structure about the size and shape of an almond, about
3.5 cm in length, 2 cm wide, and 1 cm thick. The ovaries are located in
shallow depressions, called ovarian fossae,
one on each side of the uterus, in the lateral walls of the pelvic cavity.
They are held loosely in place by peritoneal ligaments.
Structure
The ovaries are covered on the outside by a layer of simple cuboidal
epithelium called germinal (ovarian) epithelium. This is actually the visceral
peritoneum that envelops the ovaries. Underneath this layer there is a dense
connective tissue capsule, the tunica albuginea. The substance of the ovaries
is distinctly divided into an outer cortex and an inner medulla. The cortex
appears more dense and granular due to the presence of numerous ovarian
follicles in various stages of development. Each of the follicles contains an
oocyte, a female germ cell. The medulla is loose connective tissue with
abundant blood vessels, lymphatic vessels, and nerve fibers.
Oogenesis
Female sex cells, or gametes, develop in the ovaries by a form of meiosis
called oogenesis. The sequence of events in oogenesis is similar to the
sequence in spermatogenesis, but the timing and final result are different.
Early in fetal development, primitive germ cells in the ovaries differentiate
into oogonia. These divide rapidly to form thousands of cells, still called
oogonia, which have a full complement of 46 (23 pairs) chromosomes.
Oogonia then enter a growth phase, enlarge, and become primary oocytes. The
diploid (46 chromosomes) primary oocytes replicate their DNA and begin the
first meiotic division, but the process stops in prophase and the cells remain
in this suspended state until puberty. Many of the primary oocytes degenerate
before birth, but even with this decline, the two ovaries together contain
approximately 700,000 oocytes at birth. This is the lifetime supply, and no
more will develop. This is quite different than the male in which
spermatogonia and primary spermatocytes continue to be produced throughout the
reproductive lifetime. By puberty the number of primary oocytes has further
declined to about 400,000.
Beginning at puberty,
under the influence of follicle-stimulating hormone, several primary oocytes
start to grow again each month. One of the primary oocytes seems to outgrow
the others and it resumes meiosis I. The other cells degenerate. The large
cell undergoes an unequal division so that nearly all the cytoplasm,
organelles, and half the chromosomes go to one cell, which becomes a secondary
oocyte. The remaining half of the chromosomes go to a smaller cell called the
first polar body. The secondary oocyte begins the second meiotic division, but
the process stops in metaphase. At this point ovulation occurs. If
fertilization occurs, meiosis II continues. Again this is an unequal division
with all of the cytoplasm going to the ovum, which has 23 single-stranded
chromosome. The smaller cell from this division is a second polar body. The
first polar body also usually divides in meiosis I to produce two even smaller
polar bodies. If fertilization does not occur, the second meiotic division is
never completed and the secondary oocyte degenerates. Here again there are
obvious differences between the male and female. In spermatogenesis, four
functional sperm develop from each primary spermatocyte. In oogenesis, only
one functional fertilizable cell develops from a primary oocyte. The other
three cells are polar bodies and they degenerate.
Ovarian Follicle Development
An ovarian follicle consists of a developing oocyte surrounded by one or
more layers of cells called follicular cells. At the same time that the oocyte
is progressing through meiosis, corresponding changes are taking place in the
follicular cells. Primordial follicles, which consist of a primary oocyte
surrounded by a single layer of flattened cells, develop in the fetus and are
the stage that is present in the ovaries at birth and throughout childhood.
Beginning at puberty follicle-stimulating hormone stimulates changes in the
primordial follicles. The follicular cells become cuboidal, the primary oocyte
enlarges, and it is now a primary follicle. The follicles continue to grow
under the influence of follicle-stimulating hormone, and the follicular cells
proliferate to form several layers of granulose cells around the primary
oocyte. Most of these primary follicles degenerate along with the primary
oocytes within them, but usually one continues to develop each month. The
granulosa cells start secreting estrogen and a cavity, or antrum, forms within
the follicle. When the antrum starts to develop, the follicle becomes a
secondary follicle. The granulose cells also secrete a glycoprotein substance
that forms a clear membrane, the zona pellucida, around the oocyte. After
about 10 days of growth the follicle is a mature vesicular (graafian)
follicle, which forms a "blister" on the surface of the ovary and
contains a secondary oocyte ready for ovulation.
Ovulation
Ovulation, prompted by luteinizing hormone from the anterior pituitary,
occurs when the mature follicle at the surface of the ovary ruptures and
releases the secondary oocyte into the peritoneal cavity. The ovulated
secondary oocyte, ready for fertilization is still surrounded by the zona
pellucida and a few layers of cells called the corona radiata. If it is not
fertilized, the secondary oocyte degenerates in a couple of days. If a sperm
passes through the corona radiata and zona pellucida and enters the cytoplasm
of the secondary oocyte, the second meiotic division resumes to form a polar
body and a mature ovum
After ovulation and in response to luteinizing hormone, the portion of the
follicle that remains in the ovary enlarges and is transformed into a corpus
luteum. The corpus luteum is a glandular structure that secretes progesterone
and some estrogens.
Its fate depends on whether fertilization occurs. If fertilization does not
take place, the corpus luteum remains functional for about 10 days then it
begins to degenerate into a corpus albicans, which is primarily scar tissue,
and its hormone output ceases. If fertilization occurs, the corpus luteum
persists and continues its hormone functions until the placenta develops
sufficiently to secrete the necessary hormones. Again, the corpus luteum
ultimately degenerates into corpus albicans, but it remains functional for a
longer period of time.
Genital Tract
Fallopian Tubes
There are two uterine tubes, also called Fallopian tubes or oviducts. There
is one tube associated with each ovary. The end of the tube near the ovary
expands to form a funnel-shaped infundibulum,
which is surrounded by fingerlike extensions called fimbriae. Because there is
no direct connection between the infundibulum and the ovary, the oocyte enters
the peritoneal cavity before it enters the Fallopian tube. At the time of
ovulation, the fimbriae increase their activity and create currents in the
peritoneal fluid that help propel the oocyte into the Fallopian tube. Once
inside the Fallopian tube, the oocyte is moved along by the rhythmic beating
of cilia on the epithelial lining and by peristaltic action of the smooth
muscle in the wall of the tube. The journey through the Fallopian tube takes
about 7 days. Because the oocyte is fertile for only 24 to 48 hours,
fertilization usually occurs in the Fallopian tube.
Uterus
The uterus is a muscular organ that receives the fertilized oocyte and
provides an appropriate environment for the developing fetus. Before the first
pregnancy, the uterus is about the size and shape of a pear, with the narrow
portion directed inferiorly. After childbirth, the uterus is usually larger,
then regresses after menopause.
The uterus is lined with the endometrium. The stratum functionale of the
endometrium sloughs off during menstruation. The deeper stratum basale
provides the foundation for rebuilding the stratum functionale.
Vagina
The vagina is a fibromuscular tube, about 10 cm long, that extends from the
cervix of the uterus to the outside. It is located between the rectum and the
urinary bladder. Because the vagina is tilted posteriorly as it ascends and
the cervix is tilted anteriorly, the cervix projects into the vagina at nearly
a right angle. The vagina serves as a passageway for menstrual flow, receives
the erect penis during intercourse, and is the birth canal during childbirth.
External Genitalia
The external genitalia are the accessory structures of the female
reproductive system that are external to the vagina. They are also referred to
as the vulva or pudendum. The external genitalia include the labia majora,
mons pubis, labia minora, clitoris, and glands within the vestibule.
The clitoris is an erectile organ, similar to the male penis, that responds
to sexual stimulation. Posterior to the clitoris, the urethra, vagina,
paraurethral glands and greater vestibular glands open into the vestibule.
Sexual Response
The female sexual response includes arousal and orgasm, but there
is no ejaculation. A woman may become pregnant without having an
orgasm.
Follicle-stimulating hormone, luteinizing hormone, estrogen, and
progesterone have major roles in regulating the functions of the
female reproductive system.
At puberty, when the ovaries and uterus are mature enough to
respond to hormonal stimulation, certain stimuli cause the
hypothalamus to start secreting gonadotropin-releasing hormone. This
hormone enters the blood and goes to the anterior pituitary gland where it
stimulates the secretion of follicle-stimulating hormone and luteinizing
hormone. These hormones, in turn, affect the ovaries and uterus and the
monthly cycles begin. A woman's reproductive cycles last from menarche to
menopause.
The monthly ovarian cycle begins with the follicle development during the
follicular phase, continues with ovulation during the ovulatory phase, and
concludes with the development and regression of the corpus
luteum during the luteal phase.
The uterine cycle takes place simultaneously with the ovarian cycle. The
uterine cycle begins with menstruation during the menstrual phase, continues
with repair of the endometrium during the proliferative phase, and ends with
the growth of glands and blood vessels during the secretory phase.
Menopause occurs when a woman's reproductive cycles stop. This period is
marked by decreased levels of ovarian hormones and increased levels of
pituitary follicle-stimulating hormone and luteinizing hormone. The changing
hormone levels are responsible for the symptoms associated with menopause.
Source:
SEER's Training Website.
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