851
CHAPTER TWENTY-TWO
Reproductive System
Ovulation
As a follicle matures, its primary oocyte undergoes meiosis
I, giving rise to a secondary oocyte and a F rst polar body. A
process called
ovulation
(o
vu-la
shun) releases these cells
from the follicle.
Release of LH from the anterior pituitary gland triggers
ovulation, which rapidly swells the mature follicle and weak-
ens its wall. Eventually the wall ruptures, and the follicular
fl uid, accompanied by the secondary oocyte, oozes outward
from the surface of the ovary.
Figure 22.25
shows expulsion
of a mammalian oocyte.
Although as many as twenty primary follicles may begin
maturing at any one time, one dominant follicle usually out-
grows the others. Typically, only the dominant follicle fully
develops, and the other follicles degenerate
(f
g. 22.24)
.
Certain drugs used to treat female infertility, such as Clomid (clomi-
phene), may cause a woman to “superovulate.” More than one follicle
grows, more than one secondary oocyte is released, and if all of these
secondary oocytes are fertilized and complete prenatal development,
multiple births may result.
FIGURE 22.22
Light micrograph of the surface of a mammalian ovary (200×).
Maturing
follicle
Primary oocyte
Primordial follicles
(a)
(b)
Secondary
oocyte
Zona
pellucida
Corona
radiata
Fluid-filled
antrum
Granulosa
cells
Theca
externa
Theca
interna
FIGURE 22.23
Ovarian follicle. (
a
) Structure of a mature (GraaF
an) follicle. (
b
) Light micrograph of a mature follicle (250×).
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