Human Physiology, 14th edition (2016)

Reproduction 727

leaving another polar body which, like the first, degenerates

( fig. 20.30 ).

Changes continue in the ovary following ovulation. The

empty follicle, under the influence of luteinizing hormone

from the anterior pituitary, undergoes structural and biochemi-

cal changes to become a corpus luteum ( 5 yellow body).

Unlike the ovarian follicles, which secrete only estradiol, the

corpus luteum secretes two sex steroid hormones: estradiol and

progesterone. Toward the end of a nonfertile cycle, the cor-

pus luteum regresses to become a nonfunctional corpus albi-

cans. These cyclic changes in the ovary are summarized in

figure 20.31.

Pituitary-Ovarian Axis

The term pituitary-ovarian axis refers to the hormonal inter-

actions between the anterior pituitary and the ovaries. The

anterior pituitary secretes two gonadotropic hormones—follicle-

stimulating hormone (FSH) and luteinizing hormone (LH)—

both of which promote cyclic changes in the structure and

function of the ovaries. The secretion of both gonadotropic hor-

mones, as previously discussed, is controlled by a single releas-

ing hormone from the hypothalamus—gonadotropin-releasing

hormone (GnRH)—and by feedback effects from hormones

secreted by the ovaries. The nature of these interactions will be

described in detail in the next section.

Because one releasing hormone stimulates the secretion

of both FSH and LH, one might expect to always see parallel

changes in the secretion of these gonadotropins. However,

FSH secretion is slightly greater than LH secretion during

an early phase of the menstrual cycle, whereas LH secre-

tion greatly exceeds FSH secretion just prior to ovulation.

These differences are believed to result from the feedback

effects of ovarian sex steroids, which can change the amount

of GnRH secreted, the pulse frequency of GnRH secretion,

and the ability of the anterior pituitary cells to secrete FSH

and LH. These complex interactions result in a pattern of

hormone secretion that regulates the phases of the menstrual

cycle.

Figure 20.30 Oogenesis. During meiosis, each primary
oocyte produces a single haploid gamete. If the secondary
oocyte is fertilized, it forms a second polar body and its nucleus
fuses with that of the sperm cell to become a zygote. Also see
figure 20.40.

Oogonium

(46 chromosomes)

Primary oocyte

(46 chromosomes)

Secondary oocyte

(23 chromosomes)

Spermatozoon fertilizes

secondary oocyte

Fertilization

Zygote

Ovulation

Meiosis arrested

at metaphase II

First meiotic division

Second meiotic

division starts

Second meiotic

division completed

First polar body

degenerates

If no fertilization,

secondary oocyte

disintegrates

Second

polar body

degenerates

| CHECKPOINT

8a. Compare the structure and contents of a primary

follicle, secondary follicle, and graafian follicle.

8b. Describe oogenesis and explain why only one mature

ovum is produced by this process.

9a. Define ovulation and describe the changes that

occur in the ovary following ovulation in a nonfertile

cycle.

9b. Compare the hormonal secretions of the ovarian

follicles with those of a corpus luteum.