Ganong's Review of Medical Physiology, 23rd Edition

378
SECTION IV
Endocrine & Reproductive Physiology

below). To avoid redundancy, this chapter will focus particularly

on growth hormone and its role in growth and facilitating the

activity of other hormones, along with a number of general con-

siderations about the pituitary. The melanocyte-stimulating hor-

mones (MSHs) of the intermediate lobe of the pituitary,

α

-MSH

and

β

-MSH, will also be touched upon.

MORPHOLOGY

GROSS ANATOMY

The anatomy of the pituitary gland is summarized in Figure
24–1 and discussed in detail in Chapter 18. The posterior pi-
tuitary is made up largely of the endings of axons from the su-
praoptic and paraventricular nuclei of the hypothalamus and
arises initially as an extension of this structure. The anterior
pituitary, on the other hand, contains endocrine cells that
store its characteristic hormones and arises embryologically as
an invagination of the pharynx
(Rathke’s pouch).
In species
where it is well developed, the intermediate lobe is formed in
the embryo from the dorsal half of Rathke’s pouch, but is
closely adherent to the posterior lobe in the adult. It is separat-
ed from the anterior lobe by the remains of the cavity in Rath-
ke’s pouch, the
residual cleft.

HISTOLOGY

In the posterior lobe, the endings of the supraoptic and
paraventricular axons can be observed in close relation to
blood vessels.
Pituicytes,
stellate cells that are modified astro-
cytes, are also present.
As noted above, the intermediate lobe is rudimentary in
humans and a few other mammalian species. In these species,
most of its cells are incorporated in the anterior lobe. Along the
residual cleft are small thyroid-like follicles, some containing a
little colloid. The function of the colloid, if any, is unknown.
The anterior pituitary is made up of interlacing cell cords and
an extensive network of sinusoidal capillaries. The endothelium
of the capillaries is fenestrated, like that in other endocrine
organs. The cells contain granules of stored hormone that are
extruded from the cells by exocytosis. Their constituents then
enter the capillaries to be conveyed to target tissues.

CELL TYPES IN THE ANTERIOR PITUITARY

Five types of secretory cells have been identified in the anterior

pituitary by immunocytochemistry and electron microscopy.

Traditionally, they were also characterized by their affinity for

either acidic or basic histological stains. The cell types are the

somatotropes, which secrete growth hormone; the lactotropes

(also called mammotropes), which secrete prolactin; the corti-

cotropes, which secrete ACTH; the thyrotropes, which secrete

TSH; and the gonadotropes, which secrete FSH and LH. The

characteristics of these cells are summarized in Table 24–1.

Some cells may contain two or more hormones. It is also nota-

ble that the three pituitary glycoprotein hormones, FSH, LH,

and TSH, while being made up of two subunits, all share a com-

mon

α

subunit that is the product of a single gene and has the

same amino acid composition in each hormone, although their

carbohydrate residues vary. The

α

subunit must be combined

with a

β

subunit characteristic of each hormone for maximal

physiologic activity. The

β

subunits, which are produced by sep-

arate genes and differ in structure, confer hormonal specificity.

The

α

subunits are remarkably interchangeable and hybrid

molecules can be created. In addition, the placental glycopro-

tein gonadotropin human chorionic gonadotropin (hCG) has

α

and

β

subunits (see Chapter 25).

The anterior pituitary also contains folliculostellate cells

that send processes between the granulated secretory cells.

These cells produce paracrine factors that regulate the growth

and function of the secretory cells discussed above. Indeed,

the anterior pituitary can adjust the relative proportion of

secretory cell types to meet varying requirements for different

hormones at different life stages. This plasticity has recently

been ascribed to the presence of a small number of pluripo-

tent stem cells that persist in the adult gland.

PROOPIOMELANOCORTIN

& DERIVATIVES

BIOSYNTHESIS

Intermediate-lobe cells and corticotropes of the anterior lobe

both synthesize a large precursor protein that is cleaved to

form a family of hormones. After removal of the signal pep-

tide, this prohormone is known as

proopiomelanocortin

(POMC).

This molecule is also synthesized in the hypothala-

mus, the lungs, the gastrointestinal tract, and the placenta. The

structure of POMC, as well as its derivatives, is shown in Fig-

ure 24–2. In corticotropes, it is hydrolyzed to ACTH and a

polypeptide of unknown function called

β

-lipotropin (LPH),

FIGURE 24–1
Diagrammatic outline of the formation of the
pituitary (left) and the various parts of the organ in the adult (right).

Pars

tuberalis

Anterior

lobe

Posterior

lobe

Rathke’s pouch Intermediate lobe

Third ventricle Third ventricle