Endocrine Glands 331
Figure 11.12 The structure of the pituitary
gland. The anterior lobe is composed of glandular tissue,
whereas the posterior lobe is composed largely of neuroglia and
nerve fibers.
Posterior lobe
(neurohypophysis)
Pars distalis
Pars
intermedia
(fetus only)
Infundibulum
Pars tuberalis
Anterior lobe
(adenohypophysis):
Optic chiasma
Hypothalamus
The pituitary gland is structurally and functionally divided
into an anterior lobe, or adenohypophysis, and a posterior lobe
called the neurohypophysis. These two parts have different
embryonic origins. The adenohypophysis is derived from a pouch
of epithelial tissue (Rathke’s pouch) that migrates upward from
the embryonic mouth, whereas the neurohypophysis is formed as
a downgrowth of the brain. The adenohypophysis consists of two
parts in adults: (1) the pars distalis, also known as the anterior
pituitary, is the rounded portion and the major endocrine part
of the gland, and (2) the pars tuberalis is a sheath of tissue that
partially wraps around the infundibulum. These parts are illus-
trated in figure 11.12. A pars intermedia, a strip of tissue between
the anterior and posterior lobes, exists in the fetus. During fetal
development, its cells mingle with those of the anterior lobe, and
in adults they no longer constitute a separate structure.
The neurohypophysis is the neural part of the pituitary
gland. It consists of the pars nervosa, also called the posterior
pituitary, which is in contact with the adenohypophysis, and
the infundibulum. Nerve fibers extend through the infundibu-
lum along with small neuroglia-like cells called pituicytes.
Pituitary Hormones
The hormones secreted by the anterior pituitary (the pars distalis
of the adenohypophysis) are called trophic hormones. The term
trophic means “feed.” Although the anterior pituitary hormones
are not food for their target organs, this term is used because high
concentrations of the anterior pituitary hormones cause their
target organs to hypertrophy, while low levels cause their tar-
get organs to atrophy. When names are applied to the hormones
The complexity of different second-messenger systems is
needed so that different signaling molecules can have varying
effects. For example, insulin uses the tyrosine kinase second-
messenger system to stimulate glucose uptake into the liver
and its synthesis into glycogen, whereas glucagon (another
hormone secreted by the pancreatic islets) acts on the same
cells to promote opposite effects—the hydrolysis of glycogen
and secretion of glucose—by activating a different second-
messenger system that involves the production of cAMP.
| CHECKPOINT
- Using diagrams, describe how steroid hormones and
thyroxine exert their effects on their target cells.
4a. Use a diagram to show how cyclic AMP is produced
within a target cell in response to hormone
stimulation and how cAMP functions as a second
messenger.
4b. Describe the sequence of events by which a
hormone can cause a rise in the cytoplasmic Ca^2 1
concentration and explain how Ca^2 1 can function as
a second messenger.
4c. Explain the nature and actions of the receptor
proteins for insulin and the growth factors.
11.3 Pituitary Gland
The pituitary gland includes the anterior pituitary and pos-
terior pituitary. The posterior pituitary stores and releases
hormones that are actually produced by the hypothala-
mus, whereas the anterior pituitary produces and secretes
its own hormones. The anterior pituitary, however, is regu-
lated by hormones secreted by the hypothalamus, as well
as by feedback from the target gland hormones.
LEARNING OUTCOMES
After studying this section, you should be able to:
- Distinguish between the anterior and posterior
pituitary, and identify the hormones secreted by
each part. - Explain how the hypothalamus regulates both the
posterior and anterior pituitary glands. - Describe negative feedback inhibition in the
regulation of hypothalamic and anterior pituitary
hormones.
The pituitary gland, or hypophysis, is located on the inferior
aspect of the brain in the region of the diencephalon (chapter 8).
Roughly the size of a pea—about 1.3 cm (0.5 in.) in diameter—
it is attached to the hypothalamus by a stalklike structure called
the infundibulum ( fig. 11.12 ).