Human Physiology, 14th edition (2016)

726 Chapter 20

blister—and extrude its oocyte into the uterine tube in the pro-

cess of ovulation ( fig. 20.29 ).

The released cell is a secondary oocyte surrounded by the

zona pellucida and corona radiata. If it is not fertilized, it will

degenerate 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 oocyte will then complete the sec-

ond meiotic division. In this process, the cytoplasm is again

not divided equally; most remains in the zygote (fertilized egg),

that means “without an opening,” in reference to their failure to
rupture. Follicle atresia, or degeneration, is a type of apoptosis
that results from a complex interplay of hormones and para-
crine regulators. The gonadotropins (FSH and LH), as well as
various paracrine regulators and estrogen, act to protect folli-
cles from atresia. By contrast, paracrine regulators that include
androgens and FAS ligand (chapter 3, section 3.5) promote
atresia of the follicles.
The follicle that is protected from atresia and that develops
into a graafian follicle becomes so large that it forms a bulge
on the surface of the ovary. Under proper hormonal stimula-
tion, this follicle will rupture—much like the popping of a

Figure 20.27 A secondary oocyte with its first
polar body. The first polar body is formed by the first meiotic
division. This will degenerate and a second meiotic division will
not progress beyond metaphase unless the secondary oocyte is
fertilized. In that event, the second meiotic division will produce a
second polar body, which also will degenerate (see fig. 20.30 ).

(a)

(b)

First polar
body

Secondary
oocyte

Figure 20.28 An ovary containing follicles at

different stages of development. This cat ovary shows a fully

mature (graafian) follicle, an atretic follicle that started to develop

but then stopped, and a corpus albicans that formed from a

corpus luteum of a previous cycle.

Corpus

albicans

Atretic

follicle

Mature

(graafian) follicle

Figure 20.29 Ovulation from a human ovary. Notice

the cloud of fluid and granulosa cells surrounding the ovulated

oocyte.

Oocyte

Ovary