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.
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