Oogenesis and Ovarian Cycle
The production
of female gametes is a process called oogenesis. Oogenesis
occurs as part of the monthly ovarian cycle. The female stem
cells, called oogonia (sing. oogonium), complete their
mitotic divisions before birth and by birth have already begun
the process of meiosis as primary oocytes. However, the
first reduction division of meiosis freezes during
prophase and roughly 2 million primary oocytes, frozen in
prophase, are present at birth. From birth to puberty, primary
oocytes degenerate until at puberty only 400,000 primary oocytes
remain. This process by which primary oocytes disappear is
called atresia (adj. atretic). |
The primary
oocytes are surrounded by simple squamous epithelium in a
structure called a primordial follicle. The primordial
follicles are found in the outer edge of the cortex in clusters
known as egg nests. At puberty, rising levels of
follicle stimulating hormone (FSH) begins the ovarian cycle
by which a select number of primordial follicles begin further
development. |
Complete
development of a follicle is divided into the following stages:
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Primary follicle
A primordial follicle becomes a primary follicle
when the squamous follicular cells divide and
become cuboidal. At the same time the primary
oocyte becomes bigger. When there are two or
more layers of cells surrounding the primary
oocyte they are called granulosa cells.
As the primary follicle gets bigger, a fluid
filled space containing macromolecules, called
the zona pellucida, appears between the
primary oocyte and granulosa cells. |
Growth of the follicle is also associated with
development of the cells immediately surrounding
the follicle called thecal cells. Some
thecal cells along with granulosa cells secrete
female sex hormones called estrogens, of
which estradiol is the most important.
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Secondary follicle
A few of the primary follicles continue to grow
while most degenerate through atresia. In the
follicles that remain, the granulosa cells
secrete a fluid called follicular fluid.
This fluid coalesces into a fluid-filled cavity
called the antrum. With the appearance of
an antrum the follicle is called a secondary
follicle. |
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Mature Graafian follicle
Usually only one follicle remains midway into
the ovarian cycle. This follicle enlarges partly
as the result of further accumulation of fluid
into the antrum. The primary oocyte projects
into antrum in a mound of granulosa cells called
the cumulus oophorus. The follicle is now
large enough to span the width of the cortex and
creates a conspicuous bulge on the surface of
the ovary. The follicle is now called a
mature Graafian follicle. |
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Ovulation
At about 14 days, or midway into the ovarian
cycle, a sudden rise in luteinizing hormone
(LH) released by the pituitary causes
ovulation. About 3 hours before ovulation the
primary oocyte resumes the first division of
meiosis. The division results in a secondary
oocyte that receives all the cytoplasm and a
polar body that contains only the genetic
material and not much else. The polar body is
essentially discarded. |
As a
result of the upsurge of LH, the cumulus
oophorus detaches from the follicular wall, the
fluid pressure within the follicle increases and
the follicular wall weakens. The follicular wall
finally ruptures and the secondary oocyte
is extruded. Granulosa cells remain attached to
the zona pellucida of the secondary oocyte and
form the corona radiata. |
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Corpus luteum
The ruptured follicle collapses and the
granulosa cells and internal thecal cells
transform into steriod-hormone-producing cells.
Though some estrogens continue to be synthesized
by these cells, these cells now synthesize
progestins of which progesterone is
the most important. Progesterone promotes the
secretory phase of the uterus. The accumulation
of a yellow pigment in these cells is the reason
this structure is called the corpus luteum
(yellow body). |
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Corpus albicans
If pregnancy does not occur the corpus luteum
begins to degenerate after 12 days. Fibroblasts
invade the deteriorating structure and form pale
scar tissue that is called a corpus albicans
(white body). |
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