Ependymal astrocytes in the frog cerebellum.

We have examined the ependymal astrocytes of the frog cerebellar cortex in thin sections and freeze-fracture replicas. The somata border the fourth ventricle and give rise to basal processes whose daughter branches cross the molecular layer and terminate as subpial endfeet. Irregular lamellar appendages arise from the basal processes and their branches. In the molecular layer the appendages selectively ensheath apposed parallel fiber boutons and Purkinje cell dendritic spines. Other appendages ramify throughout the neuropil, some contributing to extensive pericapillary sheaths. Freeze-fractured ependymal astrocyte plasma membrane consistently has a greater concentration of intramembranous particles (IMPs) and IMPs of larger mean size than neuronal plasma membrane in the same replicas. Like the astrocytes of the mammalian central nervous system, frog ependymal astrocytes form numerous gap junctions with each other. However, orthogonal arrays of IMPs ("assemblies") were not observed. Ependymal cells in the frog cerebellum combine the morphology, and probably the functions, of both ependymal cells and astrocytes.