The islets of Langerhans contain from a few hundred to several thousand hormone-secreting endocrine cells. The islets are found throughout the pancreas but are most abundant in the tail region of the gland. The human pancreas contains, on average, about 1 million islets, which vary in size from 50 to 300 |xm in diameter. Each islet is separated from the surrounding acinar tissue by a connective tissue sheath.
Islets are composed of four hormone-producing cell types: insulin-secreting beta cells, glucagon-secreting alpha cells, somatostatin-secreting delta cells, and pancreatic polypeptide-secreting F cells. Immunofluorescent staining techniques have shown that the four cell types are arranged in each islet in a pattern suggesting a highly organized cellular community, in which paracrine influences may play an important role in determining hormone secretion rates (Fig. 35.1). Further evidence that cell-to-cell communica-
tion within the islet may play a role in regulating hormone secretion comes from the finding that islet cells have both gap junctions and tight junctions. Gap junctions link different cell types in the islets cells and potentially provide a means for the transfer of ions, nucleotides, or electrical current between cells. The presence of tight junctions between outer membrane leaflets of contiguous cells could result in the formation of microdomains in the interstitial space, which may also be important for paracrine communication. Although the existence of gap junctions and tight junctions in pancreatic islets is well documented, their exact function has not been fully defined.
The arrangement of the vascular supply to islets is also consistent with paracrine involvement in regulating islet secretion. Afferent blood vessels penetrate nearly to the center of the islet before branching out and returning to the surface of the islet. The innermost cells of the islet, therefore, receive arterial blood, while those cells nearer the surface receive blood-containing secretions from inner cells. Since there is a definite anatomical arrangement of cells in the islet (see Fig. 35.1), one cell type could affect the secretion of others. In general, the effluent from smaller islets passes through neighboring pancreatic acinar tissue before entering into the hepatic portal venous system. By contrast, the effluent from larger islets passes directly into the venous system without first perfusing adjacent acinar tissue.
Therefore, islet hormones arrive in high concentrations in some areas of the exocrine pancreas before reaching peripheral tissues. However, the exact physiological significance of these arrangements is unknown.
Neural inputs also influence islet cell hormone secretion. Islet cells receive sympathetic and parasympathetic innervation. Responses to neural input occur as a result of activation of various adrenergic and cholinergic receptors (described below). Neuropeptides released together with the neurotransmitters may also be involved in regulating hormone secretion.
Beta Cells. In the early 1900s, M. A. Lane established a histochemical method by which two kinds of islet cells could be distinguished. He found that alcohol-based fixatives dissolved the secretory granules in most of the islet cells but preserved them in a small minority of cells. Water-based fixatives had the opposite effect.He named cells containing alcohol-insoluble granules A cells or alpha cells and those containing alcohol-soluble granules B cells or beta cells. Many years later, other investigators used immunofluorescence techniques to demonstrate that beta cells produce insulin and alpha cells produce glucagon.
Insulin-secreting beta cells are the most numerous cell type of the islet, comprising 70 to 90% of the endocrine cells. Beta cells typically occupy the most central space of the islets (see Fig. 35.1). They are generally 10 to 15 |xm in diameter and contain secretory granules that measure 0.25 |xm.
Alpha Cells. Alpha cells comprise most of the remaining cells of the islets. They are generally located near the periphery, where they form a cortex of cells surrounding the more centrally located beta cells. Blood vessels pass through the outer zone of the islet before extensive branching occurs. Inward extensions of the cortex may be present along the axes of blood vessels toward the center of the islet, giving the appearance that the islet is subdivided into small lobules.
Delta Cells. Delta cells are the sites of production of so-matostatin in the pancreas. These cells are typically located in the periphery of the islet, often between beta cells and the surrounding mantle of alpha cells. Somatostatin produced by pancreatic delta cells is identical to that previously described in a neurotransmitter role (see Chapter 3) and as a hypothalamic hormone that inhibits growth hormone secretion by the anterior pituitary (see Chapter 32).
F Cells. F cells are the least abundant of the hormone-secreting cells of islets, representing only about 1% of the total cell population. The distribution of F cells is generally similar to that of delta cells. F cells secrete pancreatic polypeptide.
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