HUMAN BIOLOGY

308 Chapter 16

The Ovarian Cycle: Oocytes Develop

FSH and LH stimulate cells outside the zona pellu cida

to make estrogens, so estrogen-rich fluid builds up in

the follicle. The blood level of estrogen also rises. Several

hours before it is ovulated, an oocyte completes the cell

division, meiosis I, that was arrested years before. Now,

there are two cells. The smaller one, called the “first polar

body,” may divide again. (Polar bodies contain unneeded

material and eventually disintegrate.) The larger cell, the

secondary oocyte, gets most of the cytoplasm. It now

begins another round of meiosis (meiosis II). As before, this

division is not completed. That happens only if the oocyte

is fertilized.

About halfway through the ovarian cycle, a woman’s

pituitary gland detects the rising estrogen level. It releases

LH, which causes changes that make the follicle swell. The

surge also causes enzymes to break down the bulging fol-

licle wall. When the follicle ruptures—the event we call

ovulation—fluid escapes, along with the secondary oocyte

and polar body (Figure 16.2, step 4).

Once it is in the abdominal cavity, the secondary oocyte

normally enters an oviduct. Long, ciliated projections from

the oviduct (called fimbriae) extend over part of the ovary.

Movements of the projections and cilia sweep the oocyte

into the channel. If fertilization takes place, the oocyte will

finish meiosis II and become a mature egg.

The ovarian and menstrual cycles dovetail

You may remember from Section 16.1 that estrogens

released early in the menstrual cycle stimulate growth of

the endometrium and its blood vessels and glands. These

changes pave the way for a possible pregnancy. Just before

the midcycle LH surge, cells of the follicle wall start releas-

ing estrogens and progesterone. Table 16.1 summarizes

n    As the menstrual cycle advances, a cycle in the ovaries

forms an oocyte that may develop into an egg.

n    Links to Limbic system 13.8, Hormones from

the hypothalamus and pituitary 15.3

Hormones guide ovulation

A newborn girl’s ovaries contain about 2 million cells called

primary oocytes (“first egg-forming cells”). All but about

300 are later resorbed, although the ovaries may make

fresh oocytes later on. In each oocyte,

meiosis I begins but then is stopped

by genetic controls. This gamete-

forming type of cell division restarts,

usually in one oocyte at a time, with

each of a woman’s menstrual cycles.

The shift is part of the ovarian cycle,

in which a primary oocyte matures

and is ovulated (Figure 16.2).

Step 1 shows a primary oocyte

near an ovary’s surface. It is sur-

rounded by a layer of cells that nour-

ish it. This layer and the primary

oocyte make up a follicle. At this

point, the hypothalamus is secret-

ing enough GnRH, a releasing hor-

mone, to make the anterior pituitary

release more FSH (follicle stimulat-

ing hormone) and LH (luteinizing

hormone). As the blood level of those

two hormones rises, the follicle grows. More cell layers form

around it. In between, proteins form a thick layer called the

zona pellucida (“transparent girdle”).

Figure 16.2 Animated! Oocytes develop by way of cyclic changes in the ovary. In the cycle’s first phase, a follicle grows and matures.

When a secondary oocyte is released from an ovary, it will enter an oviduct (fallopian tube), the channel to the uterus. (© Cengage Learning)

16.2

corpus luteum Tempo-

rary structure that secretes

hormones that prepare the

uterus for an embryo.

follicle A primary oocyte

and the layer of cells that

nourish it.

ovarian cycle Cycle in

which a primary oocyte

matures.

ovulation Release of a

secondary oocyte from an

ovary.

secondary oocyte The

developmental stage of an

oocyte that is ovulated.

zona pellucida The protein

layer around an ovarian

follicle.

primary

oocyte

follicle
ovary cells

secondary

oocyte

polar

body

secondary

oocyte

corpus

luteum

1 One of many imma-

ture follicles in an ovary.

Each consists of a

primary oocyte and the

surrounding follicle cells.

2 A fluid-filled cavity

begins to form in the

follicle’s cell layer.

3 The primary oocyte

completes meiosis I

and divides unequally,

forming a secondary

oocyte and a polar body.

4 Ovulation. Rupture of

the mature follicle releases

a secondary oocyte

coated with secreted

protein and follicle cells.

5 A corpus luteum

develops from follicle

cells left behind after

ovulation.

6 If pregnancy

does not occur,

the corpus luteum

degenerates.

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