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Postnatal maturation of the islets of Langerhans in sheep. Light microscopic, immunohistochemical, morphometric, and ultrastructural investigations with particular reference to the transient appearance of argyrophil insulin immunoreactive cells.

The endocrine pancreas of lambs was found to offer an interesting animal model for some aspects of normal and pathological postnatal maturation of the islet parenchyma also in man. Normal features in lambs, previously observed in man, were the high volume density (5-8%) of the islet parenchyma, the high incidence of D (somatostatin) cells (about 30% even in young adult sheep), and the concentration of PP cells to the posterior part of the head of the pancreas. Particularly interesting was the presence in newborn lambs of large islet bodies (0.2-2 mm in diameter), consisting (to more than 90%) of large, argyrophil (Grimelius), granulated, parenchymal cells which were found to be insulin immunoreactive. Their secretion granules were smaller and more electron dense than those of the mature non-argyrophil B (insulin) cells. The argyrophil insulin immunoreactive cells often formed intercellular cavities and they could be equipped with microvilli and even cilia. At birth, these large islet bodies constituted more than half of the total islet parenchyma, but already during the first 10 days they underwent marked regressive changes (i.e. with large haemorrhages; "Blutinseln"); in 2-month-old lambs and in young adult sheep they had practically disappeared. These argyrophil insulin immunoreactive islet cells were considered to represent immature B cells with some persisting foetal characters. In 1- and 4-day-old lambs some non-argyrophil B cells in small islets of Langerhans harboured secretion granules of both types of B cells, indicating that a transformation could occur from 1 type to the other in this dual population of islet B cells. It was speculated that the argyrophil insulin immunoreactive cells could be homologues to the argyrophil islet cells often occurring in insulin-producing islet-cell neoplasms and in nesidiodysplasia in man.