Biology 12

Chapter 6 The Endocrine System • MHR 199

400 will mature to release its ovum during her
reproductive life.
As shown in Figure 6.27, during the initial
follicular stageof the menstrual cycle, increased
levels of FSH stimulate the follicles to release
increased quantities of estrogen into the bloodstream.
Estrogen stimulates the endometrium of the uterus
to thicken and increase the blood supply in
preparation for a possible pregnancy. Estrogen also
inhibits the levels of FSH, forming a negative
feedback loop. At the same time, estrogen
stimulates the hypothalamus to release large

amounts of LH, which trigger the release of an

ovum from one of the developing follicles.

Ovulationusually occurs at the midpoint (day

14) of a 28-day menstrual cycle. The release of the

ovum triggers a rapid biochemical change in the

follicle that released the ovum. The follicle changes

to become the corpus luteum, a group of cells that

produces the hormone progesterone.

Ovulation signals the beginning of the luteal

stageof the menstrual cycle. LH stimulates the

corpus luteum to produce progesterone, which

inhibits the development of other follicles. Thus,

pituitary

LH gland

FSH

0714 21 28 days

0714 21 28 days

0714 21 28 days

Levels of

reproductive

hormones in blood

Ovarian cycle

Hormone blood levels

Endometrial changes

during menstrual cycle

developing follicles ovulation corpus luteum

luteal

regression

follicular phase luteal phase

estrogen

progesterone

menstrual

phase

proliferative

phase ovulation

secretory

phase

menstrual

phase

Figure 6.27The growth and thickening of the

uterine lining (the endometrium) is governed

by rising levels of progesterone. Menstruation

(the sloughing off of blood-rich tissue) is

initiated by falling progesterone levels.