Supraependymal cell clusters in the rat hypothalamus.

The fine structure of supraependymal cell clusters of the median eminence was studied with TEM. The cluster cells were identified on the basis of ultrastructure and histochemical determination of glial fibrillar acidic protein (GFA). The phagocytic properties were also studied by means of intraventricular injections of HRP. Neurons, neuroglia cells and degenerating ependyma- and glial cells were found. The extrusion of degenerating infundibular elements into the ventricle is a constant phenomenon but its precise localization and intensity are variable. The close proximity of the clusters to capillary loops is stressed. Because of the broken ependyma at the neck of the cluster, the permeability of the infundibular lining for HRP is increased. Clusters may be seen as sites lacking a brain--CSF barrier.