Human Physiology, 14th edition (2016)

Reproduction 729

the day of ovulation. After ovulation, the ovaries are in the luteal
phase until the first day of menstruation. The cyclic changes in the
endometrium are called the menstrual, proliferative, and secretory
phases. These will be discussed separately. It should be noted that
the time frames used for the following discussion are only aver-
ages. Individual cycles may exhibit considerable variation.

Follicular Phase

Menstruation lasts from day 1 to day 4 or 5 of the average cycle.
During this time the secretions of ovarian steroid hormones are
at their lowest, and the ovaries contain only primordial and pri-
mary follicles. During the follicular phase of the ovaries, which
lasts from day 1 to about day 13 of the cycle (this duration is
highly variable), some of the primary follicles grow, develop
vesicles, and become secondary follicles. Toward the end of
the follicular phase, one follicle in one ovary reaches maturity
and becomes a graafian follicle. As follicles grow, the granulosa
cells secrete an increasing amount of estradiol (the principal
estrogen), which reaches its highest concentration in the blood
two days before ovulation at about day 12 of the cycle.
The growth of the follicles and the secretion of estradiol are
stimulated by, and dependent upon, FSH secreted from the ante-
rior pituitary. The amount of FSH secreted during the early fol-
licular phase is believed to be slightly greater than the amount
secreted in the late follicular phase ( fig.  20.32 ), although this
can vary from cycle to cycle. FSH stimulates the production of
FSH receptors in the granulosa cells, so that the follicles become
increasingly sensitive to a given amount of FSH. This increased
sensitivity is augmented by estradiol, which also stimulates the
production of new FSH receptors in the follicles. As a result,
the stimulatory effect of FSH on the follicles and their secretion
of estradiol increases despite the lack of an increase in FSH lev-
els during the follicular phase. Toward the end of the follicular
phase, FSH and estradiol also stimulate the production of LH
receptors in the graafian follicle. This prepares the graafian fol-
licle for the next major event in the cycle.
The rapid rise in estradiol secretion from the granulosa
cells during the follicular phase acts on the hypothalamus to
increase the frequency of GnRH pulses. In addition, estradiol
augments the ability of the pituitary to respond to GnRH with
an increase in LH secretion. As a result of this stimulatory, or
positive feedback, effect of estradiol on the pituitary, there is
an increase in LH secretion in the late follicular phase that cul-
minates in an LH surge ( fig. 20.32 ).
The LH surge begins about 24 hours before ovulation
and reaches its peak about 16 hours before ovulation. It is this
surge that acts to trigger ovulation. Because GnRH stimulates
the anterior pituitary to secrete both FSH and LH, there is a
simultaneous, smaller surge in FSH secretion. Some investiga-
tors believe that this midcycle peak in FSH acts as a stimulus
for the development of new follicles for the next month’s cycle.

Ovulation

Under the influence of FSH stimulation, the graafian follicle
grows so large that it becomes a thin-walled “blister” on the

surface of the ovary. The growth of the follicle is accompanied

by a rapid increase in estradiol secretion. This rapid increase in

estradiol, in turn, triggers the LH surge at about day 13. Finally,

the surge in LH secretion causes the wall of the graafian follicle

to rupture at about day 14 ( fig. 20.33 , top ). In the course of ovu-

lation, a secondary oocyte, arrested at metaphase II of meiosis,

is released from the ovary and swept by cilia into a uterine tube.

The ovulated oocyte is still surrounded by a zona pellucida and

corona radiata as it begins its journey to the uterus.

Figure 20.32 Hormonal changes during the

menstrual cycle. Sample values are indicated for LH,

FSH, progesterone, and estradiol during the menstrual

cycle. The midcycle peak of LH is used as a reference day.

(IU  5  international unit.)

See the Test Your Quantitative Ability section of the Review

Activities at the end of this chapter.

100

200

0

5

10

Estradiol(pg/ml)

Progesterone

(ng/ml)

0

10

20

FSH

(mlU/ml)

20

40

60

LH

(mlU/ml)

15 10 50 5 10 15

Days from LH peak

Before After