Extracellular spaces of the rat pars intermedia as outlined by lanthanum tracer.

The extracellular spaces of the rat pars intermedia were examined after perfusion with fixation containing lanthanum. The tracer reveals complex and extensive interdigitations among secretory cells. Spaces appear continuous with extracellular "channels" between the epithelial cells lining the hypophyseal cleft. Some suggestions for close contiguity between endocrine cells was present as "narrowing" or discontinuous areas within lanthanum-filled spaces. Extracellular regions surrounding nerve fibers and terminals appear continuous with the spaces between secretory cells. Areas between cells are thought to provide access to vascular and local neurotransmitter input, as well as to provide an extensive area for extrusion of peptide hormones and endogeneous opiates.