Page 279 - Anatomy and Physiology of Farm Animals, 8th Edition

264 / Anatomy and Physiology of Farm Animals

certain types of human pituitary dwarf- a a
VetBooks.ir ism, and the bovine product is used to b b
increase both lactation and food
efficiency.

Adrenocorticotropic Hormone
c c
The release of adrenocorticotropic hor-
mone (ACTH) from the corticotrope cells of
the adenohypophysis is regulated by the
hypothalamic factor corticotropic‐releasing
hormone (CRH). The primary target cells of
ACTH are the cells of the adrenal cortex d d
(outer region of the adrenal glands) that pro­
duce glucocorticoids. Glucocorticoids are
steroid hormones that function in the regula­
tion of metabolism (discussed later).
Figure 13-5. Adrenal gland of the horse. The
Adrenal Glands. The two adrenal glands cortical zones (a, zona glomerulosa; b, zona fascic­
are located close to the kidneys (ad, toward; ulate; c, zona reticularis) can be recognized, as can
ren, kidney). Shape, size, and exact location the medullary region (d). Source: Dellmann, 1971.
vary from one species to another. Each gland
consists of an outer region, the adrenal granules prior to release. These endocrine
cortex, and an inner region, the adrenal cells are termed chromaffin cells because
medulla. These parts of the adrenal gland of their affinity for chromium stains. As was
come from separate embryonic precursors described in Chapter 11 in the section on
and have distinctly different functions, in the autonomic nervous system, epinephrine
spite of their close physical association and norepinephrine are released from the
within a single connective tissue capsule. adrenal medulla in times of stress, and this
The blood supply to the adrenal gland release is regulated via the autonomic nerv­
varies, but in general, small arteries enter
the capsule surrounding the gland. These ous system. A tumor of chromaffin cells is
arteries are derived directly from the aorta termed a pheochromocytoma, and this
or from its branches, including the renal, neoplasia typically results in excessive
intercostal, and lumbar arteries. Veins secretion of epinephrine and norepi-
from the adrenal gland drain to the caudal nephrine. The resulting clinical signs are
vena cava. those of excessive stimulation of the sym-
Three zones or regions of the adrenal pathetic nervous system (e.g., increased
cortex can be identified by light micros­ heart rate and blood pressure and
copy in most mammals, and each zone is increased metabolic rate).
the source of different hormones. From Hormones of the Adrenal Cortex. The
outermost to innermost, the three layers or zona glomerulosa secretes mineralocorti-
zones are zona glomerulosa, zona fascic- coids (primarily aldosterone) that function
ulata, and zona reticularis (Fig. 13‐5). All in the regulation of sodium and potassium
hormones secreted from all three zones balance. The regulation of balance is primar­
are steroid hormones, so cells of all zones ily accomplished by controlling the loss of
have ultrastructural characteristics of ster­ sodium and potassium in urine; more details
oid‐secreting cells. on their specific functions are given in
The hormones secreted by the adrenal Chapters 18 and 23. Mineralocorticoid secre­
medulla (epinephrine and norepinephrine) tion is not regulated by ACTH but rather by
are amines and are stored in secretory the serum potassium concentration and the