Electron microscopic study on the human pancreatic islets.

Abstract

In biopsy specimens of pancreas taken from five patients with gastric ulcer the pancreatic islets were electron microscopically examined.1. The human islet cells are classified into B, A and D cells chiefly on account of the ultrastructural differences of their specific granules. In β-granules (300-370mμ) one or several crystalloid cores of moderate electron density are loosely encased in a membrane sac, though atypical β-granules containing one spherical core are not uncommon. The α-granules (450-650mμ) contain a single spherical core of high electron density which is in larger granules loosely, but in smaller ones closely, encompassed by a membrane sac. The space between the core and the sac appears empty after single fixation in osmic acid but moderately electron opaque after double fixation with glutaraldehyde and osmic acid. The δ-granules (450-650mμ) are, in the osmic acid fixed preparations, composed of a spherical core of low electron density and an interrupted membrane sac applied very closely to the core; after doublefixation the core appears more electron dense and a continuous sac is attached directly to it.2. The release of specific granules by emiocytosis has not been supported by the present observations, but figures suggesting a diacrine secretion through intracytoplasmic dissolving of the granules, e.g. density decrease and disintergration of the core and perforation or disappearance of its membrane sac, have been common to all islet cell types. Especially in the D cells the density of the δ-granules are conspicuously lowered as they become matured and their membranous sacs are often perforated or lost in the most electron lucent ones.3. The Golgi complex which is largest in B cells is located either on the vascular side of the cell with an accumulation of the specific granules or on the opposite side. The formation of the specific granules within the Golgi complex seems to be probable in human three islet cell types; minute granules appear in dilated portions of the Golgi cisternae or, contained in a smooth membraneous sac, in close proximity to the Golgi complex. A single centriole is found occasionally within the Golgi area.4. All three islet cell types are rich in the elements of granular endoplasmic reticulum and free ribosomes aggregated usually into small clusters.5. Mitochondria are variable in shape and size especially in the B cells; round and irregular mitochondria with obscure cristae may occur.6. The human islet cells are characterized by occurrence of lipoidal bodies. In the B cells they (2-3μ in diameter) occur most abundantly and are bounded by a double membrane similar to mitochondrial membrane; their matrix of moderate density contains vesicles or vacuoles usually in a considerable number which do not bulge into the surrounding cytoplasm. In A and D cells the lipoidal bodies occur less frequently; their matrix is highly electron dense and contains relatively few vacuoles of larger size and vesicles which bulge extensively into the surrounding cytoplasm. The lipoidal bodies of the B cells seem to originate from the swollen mitochondria whereas those in A and D cells from the Golgi complex. The fate and significance of the lipoidal bodies of the human islet cells have been discussed.7. It has been emphasized that the human A and D cells are similar in the ultrastructural properties of their secretory granules and the lipoidal bodies. In the human endocrine pancreas both islet cell types seem to be akin to each other.