Biology 12

Chapter 6 The Endocrine System • MHR 173

substances are referred to as tropic hormones. For
example, the thyroid-stimulating hormone (TSH) is
a tropic hormone produced by the pituitary gland
that stimulates — as its name implies — the thyroid
gland to produce and secrete thyroid hormone. TSH
regulates thyroid secretion through a negative
feedback system.
The pituitary gland is typically referred to as the
“master gland” of the endocrine system. This

designation reflects the role of the pituitary gland

in regulating the activity of the other hormone-

producing glands of the endocrine system. The

pituitary gland is actually composed of two glands

• the anterior and posterior pituitary, as shown in
  Figure 6.8. As the human embryo develops, the
  anterior pituitary is formed from cells from the roof
  of the mouth that migrate toward the brain. The
  posterior pituitary is composed of neural tissue.

hypothalamus

posterior pituitary anterior pituitary

Neurosecretory

cells produce ADH

and oxytocin. Neurosecretory cells produce

releasing and release-inhibiting

hormones.

These hormones move

down axons to axon endings.

These hormones are

secreted into a portal system.

Each type of hypothalamic

hormone either stimulates or

inhibits production and secretion

of an anterior pituitary hormone.

The anterior pituitary secretes

its hormones into the bloodstream.

When appropriate, ADH and

oxytocin are secreted from

axon endings into the bloodstream.

antidiuretic

hormone (ADH)

kidney tubules

oxytocin

oxytocin

smooth muscle

in uterus mammary

glands

mammary

glands

thyroid

stimulating

hormone (TSH)

adrenocortico-

tropin (ACTH)

thyroid adrenal cortex

prolactin (PRL)

human growth

hormone (HGH)

bones, tissues

gonadotropins

(FSH and LH)

ovaries, testes

portal

system

Figure 6.8The hypothalamus produces two hormones, ADH and oxytocin, which
are stored and secreted by the posterior pituitary. The hypothalamus controls the
secretions of the anterior pituitary, and the anterior pituitary controls the secretions
of the thyroid, adrenal cortex, and gonads (which are also endocrine glands).