Human Physiology, 14th edition (2016)

728 Chapter 20

shedding of the endometrium is accompanied by bleeding. There

is no bleeding when most other mammals shed their endome-

trium, and therefore their cycles are not called menstrual cycles.

In human females and other primates that have menstrual

cycles, coitus (sexual intercourse) may be permitted at any time

of the cycle. Nonprimate female mammals, by contrast, are sex-

ually receptive (in “heat” or “estrus”) only at a particular time in

their cycles, shortly before or after ovulation. These animals are

therefore said to have estrous cycles. Bleeding occurs in some

animals (such as dogs and cats) that have estrous cycles shortly

before they permit coitus. This bleeding is a result of high estro-

gen secretion and is not associated with shedding of the endome-

trium. The bleeding that accompanies menstruation, by contrast,

is caused by a fall in estrogen and progesterone secretion.

Phases of the Menstrual Cycle:

Cyclic Changes in the Ovaries

The duration of the menstrual cycle is typically about 28 days.

Because it is a cycle, there is no beginning or end and the changes

are generally gradual. However, it is convenient to call the first

day of menstruation “day 1” of the cycle, because the flow of

menstrual blood is the most apparent of the changes that occur. It

is also convenient to divide the cycle into phases based on changes

that occur in the ovary and in the endometrium. The ovaries are

in the follicular phase from the first day of menstruation until

20.5 MENSTRUAL CYCLE

Cyclic changes in the secretion of gonadotropic hormones

from the anterior pituitary cause ovarian changes during a

monthly cycle. The ovarian cycle is accompanied by cyclic

changes in the secretion of estradiol and progesterone,

which cause changes in the endometrium of the uterus

during a menstrual cycle.

Figure 20.31 Stages of ovum and follicle development. This diagram illustrates the stages that occur in an ovary during
the course of a monthly cycle. The arrows indicate changes with time.

Primary

follicle

Growing

primary

follicle

Primary

oocyte

Secondary

follicle

Mature

(graafian)

follicle Antrum

Time

Cumulus oophorous

Corona radiata

Secondary oocyte

Zona pellucida

Ovulation

Corpus

luteum

Germinal

epithelium

Corpus

albicans

Secondary oocyte

LEARNING OUTCOMES

After studying this section, you should be able to:

10. Describe the changes that occur in the ovaries and
    endometrium during each phase of the menstrual
    cycle.


11. Explain the hormonal regulation of ovulation, corpus
    luteum formation, and menstruation.

Humans, apes, and Old-World monkeys have cycles of ovarian
activity that repeat at approximately one-month intervals; hence
the name menstrual cycle ( menstru 5  monthly). The term men-
struation is used to indicate the periodic shedding of the stra-
tum functionale of the endometrium, which becomes thickened
prior to menstruation under the stimulation of ovarian steroid
hormones. In primates (other than New-World monkeys) this