Ultrastructural evidence for the presence of a glial sheath investing the islets of Langerhans in the pancreas of mammals.

Ultrastructural evidence is presented for a peri-insular glial or Schwann cell sheath surrounding islets of Langerhans in different mammals. The sheath covers substantial areas of the islet periphery. In the sand rat (Psammomys obesus) there are islets where 1/2 to 3/4 of the surface is invested by a virtually continuous glial sheath. Usually the sheath covers one of the poles of the islets and is thus differentiated from the peri-insular connective tissue and pancreatic acini. The sheath is composed of long, exceptionally thin outgrowths of single satellite cells located at the periphery of the islets. The sheath is a monolayer and, along the course of continuous tracts, its width never exceeds 20-40 nm, hence its more adequate demonstration by methods that increase the electron density of the intercellular spaces. Previous descriptions of glial sheaths surrounding single endocrine cells in the islets represent only a fragment of the peri-insular sheath. The islet satellite cells differ significantly from those of autonomic ganglia but resemble more closely those found in extramedullary paraganglia.