Page 285 - Anatomy and Physiology of Farm Animals, 8th Edition

270 / Anatomy and Physiology of Farm Animals

levels adequate for nerve and muscle
VetBooks.ir function. Calcium is removed from the
bone matrix, and this decalcification can
lead to bone deformities and osteoporo-
sis. This creates the medical condition
nutritional secondary hyperparathy-
roidism. In young animals, growth may
be disrupted and limb and spine abnor-
malities may develop, a manifestation of
hyperparathyroidism sometimes called
rickets. A particular form of nutritional
hyperparathyroidism commonly called
bran disease or bighead is seen in horses
fed excessive amounts of cereal by‐prod- Figure 13-11. Pancreatic islet (encircled) sur­
ucts (e.g., bran) in conjunction with cal- rounded by cells of the exocrine pancreas. Source:
cium‐poor hay. Bone tissue that has been courtesy of Sandra Pitcaithley, DVM.
decalcified by high levels of circulating
PTH is frequently replaced with soft, islet), and they produce the hormone insu-
bulky fibrous tissue; this is especially lin. Glucagon is produced by the α‐cells.
prominent in the flat bones of the face β‐Cells are sensitive to increases in
and mandible, giving rise to the appear- blood glucose (sugar), such as occur after a
ance of an enlarged head. Prompt cor- meal containing digestible carbohydrates,
rection of the dietary imbalance may and they release insulin in response to such
improve most symptoms (e.g., lameness) increases in blood glucose. Glucose is able
of bran disease, although the enlarged to move in to cells by either facilitated
facial features are usually permanent. diffusion or active transport and insulin
stimulates the uptake of glucose via certain
glucose transporters, thus lowering blood
Pancreatic Islets glucose levels. The primary targets for
insulin‐mediated glucose transporter
The pancreas of domestic animals is a uptake are skeletal and adipose tissues.
bilobed gland adjacent to the proximal The brain is considered to be an “insulin
part of the duodenum (small intestine). insensitive” organ because the glucose
The pancreas is an important exocrine transporters maintain a basal rate of glu­
gland whose enzymatic secretions are cose uptake; however, recent evidence sug­
delivered to the lumen of the duodenum gests that insulin does have neuroprotective
by one or two ducts. Scattered through­ effects in the CNS and may convey infor­
out the substance of the pancreas are mation about energy reserves in the body.
small masses of endocrine tissue called Insulin also stimulates skeletal muscle and
pancreatic islets (formerly the islets of liver cells to synthesize glycogen, the stor­
Langerhans). age form of glucose. Insulin affects the
The pancreatic islets are clumps of pale‐ metabolism of amino acids and lipids, for it
staining cells, arranged in irregular cords stimulates protein synthesis in skeletal
separated by capillaries (Fig. 13‐11). muscle and liver and the deposition of
Special stains are used to demonstrate the lipids in adipose tissue. Insulin is also the
types of epithelial cells found in the major endocrine stimulus for the state of
pancreatic islets. The known number of anabolism that exists after a meal is
distinct kinds of cells is still growing, but digested and nutrients are absorbed. When
the two best characterized are the α‐cells blood glucose decreases (such as during
and β‐cells. The β‐cells are the more fasting), the stimulus for insulin secretion
numerous (about 75% of all cells in the is lost and insulin levels are extremely low.