Electron Microscope Studies of the Langerhans Islets in the Toad Pancreas

Abstract

The pancreatic islet cells of the toads (Bufo vulgaris formosus) captured in December was observed by the electron microscope.
1. Pancreatic islet cells of the toad can be classified into the A-, B-, third, fourth and fifth types. The A- and B-cells are the most common, whereas the third and the fifth type cells are less common and the fourth type cells are the least common.
2. The α-granules of the A-cells of the toad agree in ultrastructural properties with those of other animal species of the vertebrate. Each of them consists of a spherical granule core of high density and a smooth limiting membrane applied closely to it; the electron lucent space between the both is in general narrow. A smaller number of α-granules with less dense core are also found. It is one of the characteristics of the A-cells that they contain a considerable amount of glycogen granules dispersed in the whole cytoplasm.
3. The β-granules of the B-cells of the toad consist of several, irregularly arranged, rod-shaped and crystalloid cores of low density and of the limiting membrane loosely surrounding them. The presence of filament bundles in the cytoplasm represents a characteristic of the B-cells. In the B-cells elongated mitochondria with unique appearance occur sometimes singly or in small number in which longitudinal cristae mitochondriales are densely packed in a lamellar structure. Huge elongated crystalloid bodies encountered exclusively in the B-cells may correspond to an enlarged and modified type of those special mitochondria.
4. The third and fourth type islet cells are regarded as the variants of the A-cells, since they possess specific granules similar to α-granules and contain in cytoplasm variable amounts of glycogen granules. The specific granules of the third type cells are remarkably small and the limiting membrane is attached closely to the dense granule core, so that the light space between membrane and core is usually obscure. Specific granules with a less dense core are frequently observed. In this islet cell type the specific granules are dispersed only sporadically and the mitochondria are generally smaller than in other islet cell types. In the fourth type cells the numerous specific granules coincide in their size and ultrastructural properties with α-granules, although their cores are generally less dense and show coarser texture; in addition frequent elliptic granules are characteristic.
5. The fifth type islet cells are characterized by the poorly developed Golgi apparatus, by the mitochondria with indistinct mitochondrial membrane and by the numerous free ribosomes in spite of the absence of the cisterns of endoplasmic reticulum. The cores of the specific granules are in general moderately lucent showing coarse texture and the limiting membranes are closely applied to the cores, but specific granules with a dense core may also be encountered. The fifth type islet cells seem to correspond to the D-cells described in previous electron microscopic observations on the pancreatic islets in other animals.
6. The third and fifth type islet cells are each gathered in small cell groups which, juxtaposed to each other, are located between the A- and the B-cell groups in the islet. This location coincide with that of the silber-positive or argyrophil islet cells (A₁-cells of HELLMAN and his collaborators) observed in the light microscopy. It may be therefore possible that the third and fifth type islet cells, both or one of them, correspond to the silber-positive cells.
7. The specific granules of the islet cells were confirmed to be formed in the Golgi complex, but the extrusion mechanism of them could not been clarified in the present study. However, the following finding seems to be worthy of attention: The limiting membranes of certain specific granules frequently become discontinuous and sometimes even disappear, so that the granule cores can be discharged