495
CHAPTER THIRTEEN
Endocrine System
Prolactin
is a protein, and as its name suggests, it pro-
motes milk production. No normal physiological role in
human males has been f
rmly established, although prolac-
tin may help maintain normal sperm production. In contrast,
abnormally elevated levels oF the hormone can disrupt sex-
ual Function in both sexes.
Prolactin secretion is mostly under inhibitory control
by dopamine From the hypothalamus, also called
prolactin-
release inhibiting hormone
(PIH). The hypothalamus likely
releases more than one
prolactin-releasing factor
(PR±).
Growth hormone can stimulate elongation oF bone tissue
directly, but its eFFect on cartilage requires a mediator substance,
insulin-like growth Factor-1 (IG±-1). Growth hormone releases
IG±-1 From the liver and other tissues. Clinical Application 13.2
discusses some clinical uses oF growth hormone.
Hypophyseal veins
Hypophyseal
portal veins
Optic chiasma
Capillary bed
Neurosecretory cells
that secrete posterior
pituitary hormones
Neurosecretory
cells that secrete
releasing hormones
Superior hypophyseal
artery
Inferior hypophyseal
artery
Posterior lobe of pituitary gland
Sella turcica of
sphenoid bone
Third ventricle
Hypothalamus
Anterior lobe of pituitary gland
Capillary bed
Secretory cells
of anterior
pituitary gland
FIGURE 13.12
Hypothalamic releasing hormones stimulate cells of the anterior lobe to secrete hormones. Nerve impulses originating in the
hypothalamus stimulate nerve endings in the posterior lobe of the pituitary gland to release hormones.
Anterior pituitary
Peripheral endocrine gland
Hypothalamus
Releasing
hormone
(Hormone 1)
Target cells
(Hormone 3)
Anterior pituitary
hormone
(Hormone 2)
+
+
+
Stimulation
Inhibition
cells
Secretory
FIGURE 13.14
Light micrograph of the anterior pituitary gland (240×).
FIGURE 13.13
Hypothalamic control of the peripheral endocrine
glands may use as many as three types of hormones, with multiple
negative feedback controls. (
+
= stimulation;
= inhibition)
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