Human Physiology, 14th edition (2016)

Reproduction 725

disappears. This unequal division of cytoplasm ensures that the

ovum will be large enough to become a viable embryo should

fertilization occur. The secondary oocyte then begins the sec-

ond meiotic division, but meiosis is arrested at meta-phase II.

The second meiotic division is completed only by an oocyte

that has been fertilized.

The secondary oocyte, arrested at metaphase II, is con-

tained within a graafian follicle. Some of the granulosa cells

of this follicle form a mound called the cumulus oopho-

rus that supports the oocyte. Other granulosa cells form a

ring around the oocyte called a corona radiata. Between

the oocyte and the corona radiata is a thin gel-like layer

of proteins and polysaccharides called the zona pellucida

(see fig.  20.26 b ). The zona pellucida is significant because

it presents a barrier to the ability of a sperm to fertilize an

ovulated oocyte.

The follicles grow and develop under the influence of

FSH from the anterior pituitary. The theca interna of the ovar-

ian follicles (see fig. 20.26 b ) produce testosterone in response

to LH stimulation, similar to the response of Leydig cells to

LH in the testes of males. In females, the testosterone pro-

duced by the theca diffuses to the granulosa cells of the folli-

cles, which have the enzyme aromatase to convert testosterone

to estradiol. FSH stimulates increased amounts of aromatase

in the granulosa cells, so that the ovaries secrete increasing

amounts of estradiol into the blood as the follicles grow under

FSH stimulation.

Ovulation

Usually by the 10th to 14th day after the first day of men-

struation only one follicle has continued its growth to become

a fully mature graafian follicle ( fig.  20.28 ). Other secondary

follicles during that cycle regress and become atretic —a term

The ovaries of a newborn girl contain about 2 million
oocytes. Each is contained within its own hollow ball of cells,
the ovarian follicle. By the time a girl reaches puberty, the
number of oocytes and follicles has been reduced to 400,000.
Only about 400 of these oocytes will ovulate during the wom-
an’s reproductive years, and the rest will die by apoptosis.
Oogenesis ceases entirely at menopause (the time menstrua-
tion stops).
Primary oocytes that have not yet been stimulated to
complete the first meiotic division are contained within tiny
primary follicles ( fig. 20.26 a ). Scientists have long believed
that the primary oocytes and primary follicles could not be
renewed postnatally—that a girl is born with all she will ever
have, and those that are lost cannot be replaced. There has
been a controversial report of the postnatal production of new
oocytes and primary follicles in mice, but this observation has
not yet been made in humans.
Immature primary follicles consist of only a single layer
of follicle cells. In response to FSH stimulation, some of these
oocytes and follicles get larger, and the follicular cells divide
to produce numerous layers of granulosa cells that surround
the oocyte and fill the follicle. Some primary follicles will be
stimulated to grow still more, and they will develop a number of
fluid-filled cavities called vesicles; at this point, they are called
secondary follicles ( fig.  20.26 a ). Continued growth of one of
these follicles will be accompanied by the fusion of its vesicles
to form a single fluid-filled cavity called an antrum. At this
stage, the follicle is known as a mature, or graafian, follicle
( fig. 20.26 b ).
As the follicle develops, the primary oocyte completes its
first meiotic division. This does not form two complete cells,
however, because only one cell—the secondary oocyte —gets
all the cytoplasm. The other cell formed at this time becomes a
small polar body ( fig. 20.27 ), which eventually fragments and

Vesicle Primary follicles

Secondary

follicle

(a)

Granulosa

cells

Corona

radiata

Zona

pellucida

Antrum

Theca

interna

Cumulus

oophorus

Secondary

oocyte

(b)

Figure 20.26 Photomicrographs of the ovary. ( a ) Primary follicles and one secondary follicle and ( b ) a graafian follicle are
visible in these sections.