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Endocrine system (systema endocrinum) 177

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9.20 Carotid body (dog). The carotid body is a para- 9.21 Pancreatic islet (cat). Pancreatic islets constitute
sympathetic paraganglion composed of light chief the endocrine component of the pancreas. They con-
cells, dense support cells, a capillary network and tain α-, β, δ, C and PP cells and a delicate network of
numerous nerve fibres. Haematoxylin and eosin stain sinusoidal capillaries. Haematoxylin and eosin stain
(x225). (x400).

A (α) cells contain dense, argyrophilic granules (200– silver stain. D cells give off slender cytoplasmic processes.
300 nm). Their nucleus is usually distinctly indented. These The electron microscope reveals small endocrine granules
cells comprise 5–30% of pancreatic islet cells. In the pig, (150–300 nm) of limited density. The granules contain
their number decreases rapidly after birth (from 50 to somatostatin, a growth-inhibiting factor that inhibits the
20%). In the horse, they accumulate in the centre of the activity of A and B cells. Somatostatin is also produced in
islet; in the ox they are located peripherally. Their granules the D cells of the gastrointestinal tract, in the hypothala-
contain glucagon. mus and in the C cells of the thyroid gland.
B (β) cells are larger than A cells and have an oval, PP (F cells) constitute a very small group of het-
loosely structured nucleus. They stain with Gomori’s erogeneous endocrine cells that synthesise a variety of
aldehyde fuchsin. At the electron microscope level, their gastro-pancreatic polypeptide hormones (e.g. pancreatic
granules are observed to have a distinctive, crystalline polypeptide; vasoactive intestinal polypeptide, VIP; chol-
internal structure. The cytoplasm is rich in rough endo- ecystokinin/pancreozymin, CCK).
plasmic reticulum, mitochondria and Golgi fields. In most All pancreatic islet hormones are synthesised as precur-
domestic mammals, B cells account for 60–80% of islet sors in the rough endoplasmic reticulum, packaged into
cells (up to 98% in the sheep). B cells tend to be located at secretory granules in the Golgi apparatus and transported
the periphery of islets in the horse and more centrally in as active hormone to the cell surface. Movement of the
the ox. Their granules contain insulin. granules, along intracellular microtubules, is regulated
Glucagon and insulin act antagonistically in regulating by calcium. The hormones are released by exocytosis for
blood glucose concentration. Insulin secreted by B cells uptake by the capillary network.
increases the uptake of blood glucose by the liver, muscle
and fat. Triggered by an increase in plasma glucose con- Enteroendocrine system
centration, the release of insulin causes the concentration Located within the epithelium of the gastrointestinal sys-
of glucose in the blood to fall. Glucagon, the product of A tem are numerous individual secretory cells that signal by
cells, has the opposite effect, raising the concentration of the paracrine mode. Together, these are referred to as the
blood glucose by activating glycogenolysis. enteroendocrine system. Cell types of the enteroendo-
C cells have been proposed to represent undifferenti- crine system include the well-described enterochromaffin
ated stem cells. They may also be inactive forms of A or cell, which secretes serotonin, and a constantly grow-
B cells. ing list of cell types that synthesise peptide hormones.
D (δ) cells are relatively rare (5% in the dog). With the Currently, over 30 of such peptide hormones have been
light microscope, they can be identified using Grimelius’ identified. They are stored, together with a carrier pro-

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