618
Chapter 17
the ovaries are still primordial, a relatively constant number
of preantral and early antral follicles are also always present.
At the beginning of each menstrual cycle, 10 to 25 of these
preantral and early antral follicles begin to develop into larger
antral follicles. About one week into the cycle, a further selec-
tion process occurs: only one of the larger antral follicles, the
dominant follicle,
continues to develop. The exact process
by which a follicle is selected for dominance is not known, but
it is likely related to the amount of estrogen produced locally
within the follicle. (This is probably why hyperstimulation of
infertile women with gonadotropin injections can result in the
development of many follicles.) The nondominant follicles (in
both ovaries) that had begun to enlarge undergo a degenera-
tive process called
atresia
which is an example of programmed
cell death, or apoptosis. The eggs in the degenerating follicles
also die.
Atresia is not limited to just antral follicles, however,
for follicles can undergo atresia at any stage of development.
Indeed, this process is already occurring in the female fetus, so
that the 2 to 4 million follicles and eggs present at birth repre-
sent only a small fraction of those present earlier in gestation.
Atresia then continues all through prepubertal life so that only
200,000 to 400,000 follicles remain when active reproductive
life begins. Of these, all but about 400 will undergo atresia
during a woman’s reproductive life. Therefore, 99.99 percent
of the ovarian follicles present at birth will undergo atresia.
The dominant follicle enlarges mainly as a result of an
increase in fl uid, causing the antrum to expand. As this occurs,
the granulosa cell layers surrounding the egg form a mound
that projects into the antrum and is called the
cumulus ooph-
orous
(see Figure 17–16). As the time of ovulation approaches,
the egg (a primary oocyte) emerges from meiotic arrest and
completes its fi rst meiotic division to become a secondary
oocyte. The cumulus separates from the follicle wall so that it
and the oocyte fl oat free in the antral fl
uid. The mature follicle
(also called a
graafi
an follicle
) becomes so large (diameter
about 1.5 cm) that it balloons out on the surface of the ovary.
Ovulation occurs when the thin walls of the follicle
and ovary rupture at the site where they are joined because of
enzymatic digestion. The secondary oocyte, surrounded by its
tightly adhering zona pellucida and granulosa cells, as well as
the cumulus, is carried out of the ovary and onto the ovarian
surface by the antral fl uid. All this happens on approximately
day 14 of the menstrual cycle.
Primordial
follicle
Primary
follicle
Mature follicle
Fully grown
oocyte
Granulosa
cells
Zona
pellucida
Nucleus
of oocyte
Zona
pellucida
Oocyte
Granulosa
cells
Early theca
Theca
Granulosa cells
Oocyte
Antrum
Antrum
Cumulus
oophorous
Oocyte
Zona pellucida
Theca
Granulosa cells
Preantral
follicle
Early antral
follicle
Fluid
Fluid
Figure 17–16
Development of a human oocyte and ovarian follicle. The fully mature follicle is 1.5 cm in diameter. Blood vessels are not shown.
Adapted from Erickson et al.
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