332
Chapter 11
Three other anterior pituitary hormones also stimulate the
secretion of another hormone but have an additional function
as well. Follicle-stimulating hormone and luteinizing hormone
stimulate the gonads to secrete the sex hormones—estradiol and
progesterone from the ovaries, or testosterone from the testes—
but in addition, they regulate the growth and development of ova
and sperm. The actions of FSH and LH are described in detail
in Chapter 17. Growth hormone stimulates the liver to secrete
a growth-promoting peptide hormone known as
insulin-like
growth factor 1 (IGF-1),
and in addition, exerts direct effects
on metabolism (Section E in this chapter).
Prolactin is unique among the six classical anterior pitu-
itary hormones in that its major function is not to exert con-
trol over the secretion of a hormone by another endocrine
gland. Its most important action is to stimulate development
of the mammary glands and milk production by direct effects
upon the breasts. During lactation, prolactin exerts a second-
ary action to inhibit gonadotropin secretion, thus decreasing
fertility when a woman is breast-feeding. In the male, prolac-
tin may facilitate several components of reproductive function,
although its precise roles are uncertain.
Hypophysiotropic Hormones
As stated previously, secretion of the anterior pituitary hor-
mones is largely regulated by hormones produced by the hypo-
thalamus and collectively called hypophysiotropic hormones.
These hormones are secreted by neurons that originate in
discrete nuclei of the hypothalamus and terminate in the
median eminence around the capillaries that are the origins
of the hypothalamo-pituitary portal vessels. The generation
of action potentials in these neurons causes them to secrete
their hormones, much as action potentials cause other neu-
rons to release neurotransmitters by exocytosis. Hypothalamic
hormones, however, enter the capillaries and are carried by the
hypothalamo-pituitary portal vessels to the anterior pituitary
(
Figure 11–16
). There, they act upon the various anterior
pituitary cells to control their hormone secretions.
Thus, these hypothalamic neurons secrete hormones
in a manner identical to that described previously for the
hypothalamic neurons whose axons end in the posterior
pituitary. In both cases the hormones are synthesized in
Target cells of hormone 3
Respond to hormone 3
Plasma hormone 3
Plasma hormone 2
Plasma hormone 1
(in hypothalamo-pituitary portal vessels)
Stimulus
Third endocrine gland
Hormone 3 secretion
Anterior pituitary
Hormone 2 secretion
Hypothalamus
Hormone 1 secretion
Figure 11–14
Typical sequential pattern by which a hypophysiotropic hormone
(hormone 1 from the hypothalamus) controls the secretion of an
anterior pituitary hormone (hormone 2), which in turn controls the
secretion of a hormone by a third endocrine gland (hormone 3). The
hypothalamo-pituitary portal vessels are illustrated in Figure 11–13.
LH
Growth hormone
TSH
Prolactin
ACTH
Gonads
Secrete hormones
Female
Estradiol,
progesterone
Male
Testos-
terone
Germ cell
development
Female
Ovum
Male
Sperm
Liver and
other cells
Secrete
IGF-1
Adrenal
cortex
Secretes
cortisol
Many organs
and tissues
Protein synthesis,
carbohydrate
and lipid
metabolism
Thyroid
Secretes
thyroxine,
triiodothyronine
Breasts
Breast
development and
milk production
(in male may
facilitate
reproductive
function)
FSH
Anterior pituitary
Figure 11–15
Targets and major functions of the six classical anterior pituitary hormones.
hormone induces the thyroid to secrete thyroxine and triiodo-
thyronine. Adrenocorticotropic hormone, meaning “hormone
that stimulates the adrenal cortex,” stimulates that gland to
secrete cortisol.
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