Review of Medical Physiology - William F. Ganong 2002

Endocrine System, Metabolism, and Reproductive Function
Endocrine Functions of the Pancreas and Regulation of Carbohydrate Metabolism
Structure of Islet Cells

The islets of Langerhans (Fig. 19-1) are ovoid, 76 x 175 µm Cell clusters scattered throughout the Pancreas; they are more abundant in the tail than in the body and HEAD. They account for about 2% of the pancreatic volume, whereas the exocrine portion makes up 80%; the remainder consists of ducts and Blood Vessels. The human pancreas contains 1 to 2 million islets. Each islet is well vascularized; unlike other Endocrine glands (but similarly to the gastrointestinal tract), blood from the islets drains into the portal vein of the Liver.

Islet Cells can be classified into distinct types based on their staining and morphological characteristics. Humans have at least four Major Cell Types: A, B, D, and F. A, B, and D cells are also designated as α, β, and δ. However, this nomenclature can cause confusion when Greek letters are used to denote other body structures, such as adrenergic receptors (see Chapter 4). A cells secrete Glucagon, B cells secrete Insulin, D cells secrete Somatostatin, and F cells secrete Pancreatic Polypeptide. B cells, which are the most prevalent type accounting for 60-75% of islet cells, are typically located in the center of each islet. They are surrounded by A cells, which comprise 20% of the total cell population, along with fewer D and F cells. Islets located in the tail, body, and the anterior and superior portions of the human pancreatic head contain numerous A cells and only a few (or no) F cells in their periphery. In contrast, islets in the posterior region contain a relatively high number of F cells and few A cells, particularly in rats and possibly humans. Islets rich in A cells (glucagon-secreting) originate embryologically from the dorsal pancreatic bud, whereas F-cell-rich islets (pancreatic polypeptide-secreting) arise from the ventral bud. These buds develop independently from the duodenum.

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Fig. 19-1. Islet of Langerhans in the rat pancreas. The dark-stained cells are B cells. The surrounding pancreatic acinar tissue appears light in color (magnification 400x; courtesy of LL Bennett).

B-cell granules represent insulin storage packages within the Cell Cytoplasm. Each package is enclosed in a membrane-bound vesicle (Fig. 19-2); typically, a light zone (halo) is visible between the vesicle wall and the package. The shape of the package varies among species; in humans, some are round while others are rectangular. Within B cells, insulin molecules form polymers and complexes with zinc. Variations in granule shape are likely due to differences in the size of the polymers and zinc-insulin aggregates. A-cell granules, which contain glucagon, are relatively uniform in shape across different species (Fig. 19-3). D Cells also contain A large number of relatively homogeneous granules.