Pancreatic islets have been studied by the electron microscopy of ultrathin sections in mature guinea pigs and cats. Alpha cells contain specific granules which are notably dense after fixation with osmium tetroxide. Their substance is homogeneous. They are enclosed within a vacuolar membrane. Other organelles such as the Golgi apparatus, endoplasmic reticulum, RNA granules and mitochondria are present inconspicuously, and without particular specialization. Beta cells of the cat and guinea pig differ in important respects. In the cat the specific granules contain an angular crystalloid element. The homologous structure in the guinea pig is spherical. These granules are enclosed in membranous envelopes. The densityof the granules is substantially less than those of alpha cells. Beta cells also differ from alpha cells in having a richer development of the endoplasmic reticular system, Golgi apparatus and RNA granules. The guinea pig beta cells also show a marked polarization of organelles.
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Bencosme et al. (1958) studied this question.