HUMAN BIOLOGY

reprodUCtIVe systeMs 309

corpus luteum

breaks down

Progesterone

A FSH and LH levels in blood

B Follicle changes in an ovary

C Estrogen and progesterone levels in blood

D Changes in uterine lining

FSH

LH

FSH and LH stimulate

follicle maturation

LH surge

triggers ovulation

follicle matures ovulation

corpus

luteum

forms

Estrogen

follicle secretes

estrogens

corpus luteum

secretes estrogens,

progesterone

estrogens,

progesterone,

cause uterine

lining to thicken

menstrual

fl ow

0246810121416182022242628

low estrogen

Days of cycle

Follicular phase Luteal phase

What hormonal changes trigger the groWth

and release of a secondary oocyte from

an ovary?

• Shifts in FSH and LH cause a follicle (primary oocyte and
  support cells) to grow.


• A midcycle surge of LH triggers ovulation, in which a secondary
  oocyte is released from the ovary.


• The cyclic release of estrogen and progesterone helps pave the
  way for fertilization of an egg and pregnancy.

taKe-hoMe Message

these events. When ovulation occurs, the estrogens act
on tissue around the cervical canal, which opens into the
vagina. The cervix starts to secrete large amounts of a thin,
clear mucus, which is ideal for sperm to swim through.
As diagrammed in Figure 16.3A and 16.3B, the mid-
cycle surge of LH triggers formation of a corpus luteum
(“yellow body”). This structure develops from cells left
behind in the follicle, and it secretes some estrogen and
progesterone (Figure 16.3C). The progesterone prepares the
uterus for an embryo. For example, it causes mucus in the
cervix to become thick and sticky, which may prevent bac-
teria from entering the uterus. Progesterone also maintains
the endometrium during a pregnancy.
A corpus luteum lasts for about 12 days. In that time,
the hypothalamus signals for a decrease in FSH, which
prevents other follicles from developing. If no embryo
implants in the endo metrium, the corpus luteum begins to
disintegrate. After it breaks down, progesterone and estro-
gen levels drop, so the endometrium also breaks down and
menstruation begins (Figure 16.3D).

Figure 16.3 Animated! Hormones govern the menstrual and

ovarian cycles. A GnRH from the hypothalamus stimulates the

anterior pituitary to secrete FSH and LH. B FSH and LH stimulate

a follicle to grow, an oocyte to mature, and the ovaries to secrete

progesterone and estrogens that stimulate the endometrium to

rebuild. C A midcycle LH surge triggers ovulation and the formation

of a corpus luteum. D Progesterone and some estrogens released

by the corpus luteum maintain the endometrium, but if no pregnancy

occurs, they stop being released and the corpus luteum breaks down.

(© Cengage Learning)

hypothalamus

anterior pituitary

GnRH

Days of

Phase Events the Cycle*

Menstrual phase Menstruation; endometrium 1–5

breaks down

Follicle matures in ovary; 6–13

endometrium rebuilds

Proliferative phase Endometrium begins to 14

thicken, ovulation occurs

Progestational Lining of endometrium 15–28

phase develops to receive a

possible embryo

Table 16.1 Phases of the Menstrual Cycle

*Assumes a 28-day cycle

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