Page 276 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 261

(A) (B)
VetBooks.ir Artery Supraoptic and paraventricular nuclei
of hypothalamus

Anterior Capillary plexus Artery
pituitary
Posterior pituitary

Portal vessels
Vein

Capillary plexus

Figure 13-3. The relationship between the hypothalamus, neurohypophysis (posterior pituitary gland)
and adenohypophysis (anterior pituitary gland). (A) The network of the hypothalamohypophysial portal
system. (B) The direct neural connection between the hypothalamus and the neurohypophysis.

adenohypophysis either to stimulate or to microscopic appearance of nervous tissue,
inhibit the release of other hormones consisting of unmyelinated axons and sup­
(Table 13‐1). These neurotransmitters portive glial cells.
travel only a short distance to stimulate Historically, the pituitary gland was
target cells, but they are transmitted via known as the master gland because of the
the blood from their site of origin to their large number of hormones secreted and
site of action. They are therefore consid­ their wide‐ranging effects. Several hor­
ered endocrine hormones, but they are mones from the adenohypophysis stimu­
also referred to as inhibiting or releasing late distant endocrine glands to increase
factors. All of these neurotransmitter production of their own hormones
hormones except dopamine are small (Table 13‐1). These stimulatory adenohy­
peptides, and the small peptides are rap­ pophyseal hormones are often called
idly degraded after passing through the trophic or tropic hormones.
hypothalamohypophysial portal system The hypothalamus functions as a cru­
and entering the general circulation. The cial interface between the nervous and
unique pituitary portal vascular system endocrine systems, where sensory infor­
permits the delivery of a relatively high mation is integrated and used to regulate
concentration of the releasing or inhibit­ the endocrine output of the pituitary
ing factors to the adenohypophysis so that gland. Much of this information is related
a biologic effect is possible. the status of the internal environment in
The different cell types of the adenohy­ question (e.g., extracellular fluid osmolal­
pophysis exhibit different histologic staining ity, blood glucose concentration, body
characteristics, depending on the hormone temperature, and metabolic rate). Release
they produce. As a consequence, cells are of adenohypophyseal hormones can also
characterized as basophils, acidophils, or be regulated through more direct feed­
chromophobes, among other specific types. back loops based on the blood concentra­
These endocrine cells are arranged in cords tions of the hormones involved. The
or clusters around blood‐filled sinusoids, in hormone produced by the target endo­
keeping with their role as an endocrine crine gland of a specific trophic hormone
organ. The neurohypophysis has the typical can act on (1) the hypothalamus to reduce