Ultrastructural evidence for sinusoid spaces and coupling between pituicytes in the rat.

The pericapillary palisade of the rat neurohypophysis was examined by means of thin-section and freeze-etch electron-microscopy. Special attention was given to pituicyte processes intermingled with neurosecretory terminals. These processes are identified by the presence of lipid droplets and ribosomes. Extracellular spaces are conspicuously enlarged in circumscribed regions between fingerlike protrusions of pituicyte processes. Neurosecretory axons seem to have free access to these enlarged spaces. Zonulae occludentes often combined with small gap junctions are found at the border of these sinusoid spaces. Gap junctions and occasionally intermediate junctions are seen between pituicyte processes. The topographic relationship and the functional significance of these structural features remain to be further elucidated.