642
Chapter 17
III. During the late follicular phase, plasma estrogen increases to
elicit a surge of LH, which then causes, via the granulosa cells,
completion of the egg’s fi rst meiotic division and cytoplasmic
maturation, ovulation, and formation of the corpus luteum.
IV. During the luteal phase, under the infl uence of small amounts
of LH, the corpus luteum secretes progesterone and estrogen.
Regression of the corpus luteum results in a cessation of the
secretion of these hormones.
V. Secretion of GnRH and the gonadotropins is inhibited during
the luteal phase by the combination of progesterone, estrogen,
and inhibin.
Uterine Changes in the Menstrual Cycle
I. The ovarian follicular phase is equivalent to the uterine
menstrual and proliferative phases, the fi rst day of
menstruation being the fi rst day of the cycle. The ovarian
luteal phase is equivalent to the uterine secretory phase.
a. Menstruation occurs when the plasma estrogen and
progesterone levels decrease as a result of regression of the
corpus luteum.
b. During the proliferative phase, estrogen stimulates growth
of the endometrium and myometrium and causes the
cervical mucus to be readily penetrable by sperm.
c. During the secretory phase, progesterone converts the
estrogen-primed endometrium to a secretory tissue and
makes the cervical mucus relatively impenetrable to sperm.
It also inhibits uterine contractions.
Other Effects of Estrogen and Progesterone
I. The many effects of estrogen and progesterone are summarized
in Table 17–8.
Androgens in Women
I. Androgens are produced in women and have several functions
including growth of pubic and axillary hair.
II. Excess androgen can cause virilism.
Puberty
I. At puberty, the hypothalamic-anterior-pituitary-gonadal axis
becomes active as a result of a change in brain function that
permits increased secretion of GnRH.
II. The fi rst signs of puberty are changes in nipples and the
appearance of pubic or axillary hair.
Female Sexual Response
I. Sexual intercourse results in increases in blood fl ow and
muscular contractions throughout the body.
II. Androgens appear to be important in libido in women.
Pregnancy
I. After ovulation, the egg is swept into the fallopian tube, where
a sperm, having undergone capacitation and the acrosome
reaction, fertilizes it.
II. Following fertilization, the egg undergoes its second meiotic
division, and the nuclei of the egg and sperm fuse. Reactions
in the ovum block penetration by other sperm and trigger cell
division and embryogenesis.
III. The conceptus undergoes cleavage, eventually becoming
a blastocyst, which implants in the endometrium on
approximately day 7 after ovulation.
a. The trophoblast gives rise to the fetal part of the placenta,
whereas the inner cell mass develops into the embryo
proper.
b. Although they do not mix, fetal blood and maternal
blood both fl ow through the placenta, exchanging gases,
nutrients, hormones, waste products, and other substances.
c. The fetus is surrounded by amniotic fl uid in the amniotic sac.
IV. The progesterone and estrogen required to maintain the
uterus during pregnancy come from the corpus luteum for
the fi rst two months of pregnancy, their secretion stimulated
by human chorionic gonadotropin produced by the
trophoblast.
V. During the last seven months of pregnancy, the corpus
luteum regresses, and the placenta itself produces large
amounts of progesterone and estrogen.
VI. The high levels of progesterone, in the presence of estrogen,
inhibit the secretion of GnRH and thereby that of the
gonadotropins, so that menstrual cycles do not occur.
VII. Delivery occurs by rhythmical contractions of the uterus,
which fi rst dilate the cervix and then move the infant,
followed by the placenta, through the vagina. The
contractions are stimulated in part by oxytocin, released
from the posterior pituitary in a refl ex triggered by uterine
mechanoreceptors, and by uterine prostaglandins.
VIII. The breasts develop markedly during pregnancy as a result of
the combined infl
uences of estrogen, progesterone, prolactin,
and human placental lactogen.
a. Prolactin secretion is stimulated during pregnancy by
estrogen acting on the anterior pituitary, but milk is not
synthesized because high concentrations of estrogen
and progesterone inhibit the milk-producing action of
prolactin on the breasts.
b. As a result of the suckling refl ex, large bursts of prolactin
and oxytocin are released during nursing. The prolactin
stimulates milk production and the oxytocin causes milk
ejection.
Menopause
I. Around the age of 50, a woman’s menstrual cycles become
less regular and ultimately disappear—menopause.
a. The cause of menopause is a decrease in the number
of ovarian follicles and their hyporesponsiveness to the
gonadotropins.
b. The symptoms of menopause are largely due to the
marked decrease in plasma estrogen concentration.
Additional Clinical Examples
I. Amenorrhea is either the absence of menarche (primary) or
the loss of normal menstrual cycles (secondary).
II. Congenital adrenal hyperplasia is caused by an adrenal
enzyme defect and a shift of substrate to the androgen
pathway. Androgen production can lead to virilization of an
XX fetus.
III. Precocious puberty is the early onset of puberty often caused
by LH and FSH overproduction.
previous page 670 Vander's Human Physiology The Mechanisms of Body Function read online next page 672 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off