Human Physiology, 14th edition (2016)

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

3. 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:

5. Distinguish between the anterior and posterior
   pituitary, and identify the hormones secreted by
   each part.


6. Explain how the hypothalamus regulates both the
   posterior and anterior pituitary glands.


7. 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 ).