Biology 12

Chapter 6 The Endocrine System • MHR 175

Excess HGH production during adult years produces
acromegaly, symptoms of which include excessive
thickening of bone tissue. This thickening causes
abnormal growth of the head, hands, and feet as
shown in Figure 6.10, as well as spinal
deformities. Treatment of patients diagnosed with
acromegaly involves surgical removal of the
tumour, radiation therapy, injection of a growth
hormone blocking drug, or a combination of these
treatments. The development of a tumour within
the pituitary gland is the most common cause of
both gigantism and acromegaly.
HGH shares many structural and functional
similarities with prolactin, the next pituitary
hormone to be described in this section.
Prolactin This substance is a non-steroid
hormone produced by the anterior pituitary and,
in smaller quantities, by the immune system, the
brain, and the pregnant uterus. Prolactin stimulates
the development of mammary gland tissue and
milk production (lactogenesis).
The hypothalamic regulation of prolactin
production is somewhat unusual. The hypothalamus
secretes the neurotransmitter dopamine, which
inhibits rather than stimulates the production and
secretion of prolactin by the pituitary. Severing the
connection between the hypothalamus and the
pituitary gland results in an increase in prolactin
production. After birth, however, the stimulation of
nerve endings in the nipples during infant feeding
will trigger the release of prolactin-secreting
hormones by the hypothalamus. This spinal reflex
(known as a neuroendocrine reflex) stimulates the

production of prolactin. Increasing estrogen levels

also stimulate prolactin production in late

pregnancy to prepare the mammary glands for

lactation after the birth of a baby. Increased

prolactin levels in pregnancy also inhibit ovulation

by suppressing the production of LH. Figure 6.11

illustrates one common application of increasing

prolactin levels, in milk-producing cows.

Figure 6.11This milking machine stimulates prolactin release

so milk production continues after the calf has been weaned.

Figure 6.10Acromegaly is caused by overproduction of HGH in the adult. It is
characterized by an enlargement of the bones in the face, fingers, and toes of
an adult. Today, various therapies are used to treat this disorder.

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