Human Physiology, 14th edition (2016)

732 Chapter 20

epithelium (the upper cells die and become filled with keratin).

High levels of estradiol also cause the production of a thin,

watery cervical mucus that can easily be penetrated by sperma-

tozoa. During the luteal phase of the cycle, the high levels of

progesterone cause the cervical mucus to thicken and become

sticky after ovulation has occurred.

Effects of Pheromones,

Stress, and Body Fat

Since GnRH stimulates the anterior pituitary to secrete FSH and

LH, the GnRH-releasing neurons of the hypothalamus might

be considered the master regulators of the reproductive system.

However, the release of GnRH is itself regulated by feedback

effects of ovarian hormones and by input from higher brain cen-

ters. Because of input to GnRH neurons from the olfactory sys-

tem, pheromones can cause the menstrual cycle of roommates

The endometrium is therefore well prepared to accept and nour-
ish an embryo should fertilization occur.
The menstrual phase occurs as a result of the fall in ovarian
hormone secretion during the late luteal phase. Necrosis (cellular
death) and sloughing of the stratum functionale of the endome-
trium may be produced by constriction of the spiral arteries. It
would seem that the spiral arteries are responsible for menstrual
bleeding, since animals that lack these arteries do not bleed when
they shed their endometrium. The phases of the menstrual cycle
are summarized in figure 20.35 and in table 20.6.
The cyclic changes in ovarian secretion cause other cyclic
changes in the female reproductive tract. High levels of estra-
diol secretion, for example, cause cornification of the vaginal

Table 20.6 | Phases of the Menstrual Cycle

Phase of Cycle Hormonal Changes Tissue Changes

Ovarian Endometrial Pituitary Ovary Ovarian Endometrial

Follicular (days 1–4) Menstrual FSH and LH secretion low Estradiol and

progesterone

remain low

Primary follicles grow Outer two-thirds of

endometrium is shed

with accompanying

bleeding

Follicular (days 5–13) Proliferative FSH slightly higher than

LH secretion in early

follicular phase

Estradiol secretion

rises (due to FSH

stimulation of

follicles)

Follicles grow;

graafian follicle

develops (due to

FSH stimulation)

Mitotic division

increases thickness of

endometrium; spiral

arteries develop (due to

estradiol stimulation)

Ovulatory (day 14) Proliferative LH surge (and increased

FSH) stimulated by

positive feedback from

estradiol

Estradiol secretion

falls

Graafian follicle

ruptures and

secondary oocyte

is extruded into

uterine tube

No change

Luteal (days 15–28) Secretory LH and FSH decrease

(due to negative

feedback from steroids)

Progesterone

and estrogen

secretion

increase, then fall

Development of

corpus luteum (due

to LH stimulation);

regression of

corpus luteum

Glandular development

in endometrium (due to

progesterone

stimulation)

CLINICAL APPLICATION

Polycystic ovarian syndrome ( PCOS ) is a common endo-

crine disorder in women during the reproductive years. Its

cause is unknown, but it appears to be a genetic disorder.

The ovaries contain follicles that become fluid-filled cysts

visible in an ultrasound. Common symptoms include amen-

orrhea (the absence of menstruation), dysmenorrhea (pain-

ful menstruations), reduced fertility, excessive growth of

body hair, male pattern baldness on the head, and acne.

The ovaries secrete an excessive amount of androgens that

produce some of these symptoms, and there is also an ele-

vated secretion of insulin from the pancreatic islets coupled

with an increased insulin resistance. Women with PCOS

are in danger of developing type 2 diabetes and metabolic

syndrome.

Clinical Investigation CLUES

Linda was told that she has polycystic ovarian syndrome

(PCOS), and the physician advised her to lose weight

and exercise.

• What is PCOS, and what symptoms of it did Linda
  display?


• What aspect of PCOS likely caused her symptoms?


• Why did the physician recommend that Linda lose
  weight and exercise?