Comparative fine structure of vertebrate neuroglia: Teleosts and reptiles

Electron microscopic examination of aldehyde perfused fish and reptile brains reveals the presence of three distinct varieties of glial elements; ependyma, astrocytes and oligodendrocytes. Ependymal cells line the ventricular cavity, possess cilia and extend long fibrillar processes to the pial surface. Several cytoplasmic organelles and specialized plasma membrane contacts are unique to this glial variety. Astrocytes possess long processes which can be traced as pericapillary and sub‐pial end‐feet. Their cytoplasm contains fine filaments (less than 100 Å) and quantities of glycogen granules. Oligodendrocytes display only occasional long processes and usually have thin cytoplasmic rims, especially when related to myelinated axons. A wide range of nuclear and cytoplasmic matrix densities, numerous free ribosomes and an extensive microtubular system characterize these cells. On the basis of distinctive cytoplasmic organelles it appears possible to establish the homology of lower vertebrate and mammalian glial elements.