Morphology Of The Endocrine Pancreas

The 1 to 2 million islets of the human pancreas range in size from about 50-500 mm in diameter and contain from

50-300 endocrine cells. Collectively the islets comprise only 1-2% of the pancreatic mass. They are highly vascular, with each cell seemingly in direct contact with a capillary. Blood is supplied by the pancreatic artery and drains into the portal vein, which thus delivers the entire output of pancreatic hormones to the liver. The islets are also richly innervated with both sympathetic and parasympathetic fibers that terminate on or near the secretory cells.

Histologically, the islets consist of three cell types. Beta cells, which synthesize and secrete insulin, make up about 60-75% of a typical islet. Alpha cells are the source of glucagon and comprise perhaps as much as 20% of islet tissue. Delta cells, which are considerably less abundant, produce somatostatin. An additional but rarer cell type, the F cell, may also appear in the exocrine part of the pancreas. It contains and secretes a compound of called pancreatic polypeptide, which inhibits pancreatic exocrine cell functions.

Beta cells occupy the central region of the islet or microlobules within islets, whereas alpha cells occupy the outer rim. Delta cells are interposed between them and are thus in contact with both types (Fig. 1). Gap junctions link alpha cells to each other, beta cells to each other, and alpha cells to beta cells. Although experimental proof is lacking, this arrangement may account for synchronous secretory activity. There are also tight junctions between various islet cells. These sites of close apposition or actual fusion of plasma membranes of adjacent cells may affect diffusion of substances into or out of intercellular spaces. This arrangement could either facilitate or hinder paracrine communication between alpha, beta, and delta cells. Blood flows through an anastomosing network of capillaries from the center of the islet toward the periphery. This arrangement favors intra-islet delivery of insulin from the centrally located beta cells to the peripherally located alpha cells. The physiologic consequences of these complex anatomic specializations are not understood.

FIGURE 1 Arrangement of cells in a typical islet. The clear cells in the center of the islet are the beta (insulin secreting) cells. The stippled cells in the periphery are the alpha (glucagon secreting) cells, and the solid black cells are the delta (somatostatin secreting) cells. (From Orci L, Unger R., Functional subdivision of islets of Langerhans and possible role of D cells. Lancet, 1975;2:1243-1244.)

FIGURE 1 Arrangement of cells in a typical islet. The clear cells in the center of the islet are the beta (insulin secreting) cells. The stippled cells in the periphery are the alpha (glucagon secreting) cells, and the solid black cells are the delta (somatostatin secreting) cells. (From Orci L, Unger R., Functional subdivision of islets of Langerhans and possible role of D cells. Lancet, 1975;2:1243-1244.)

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