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Tanycytes are thought by most investigators to be a special variety of ependymal cell that has a long basal (tail) process that extends radially through the neuropil for a substantial distance (reviewed in Peters et al. 1976). They occur in clusters and are most numerous in the floor and ventral walls of the posterior third ventricle where relatively short distances separate the ventricular and pial surfaces of the brain. Endings of tanycytes contact blood vessels, neurons, and the pial surface. When described in Golgi preparations and electron micrographs, tanycytes had a somatic portion that lies in the ependyma and contains the nucleus, a neck region that extends into the subependymal neuropil, and a tail process. Regional differences in the morphology of tanycytes and the distribution of their endings were described and related to functions suggested for this interesting cell type (Altman and Bayer 1978; Walsh et al. 1978).
The origin and division pattern of tanycytes was explored in autoradiographic experiments. Labeling of these cells with 3 H-thymidine occurred late in embryonic development and reached a peak postnatally (Altman and Bayer 1978). Depending on the region examined, cytodifferentiation of tanycytes may continue for several weeks postnatally; among the last to achieve adult form are those adjacent to the dorsal zone of the ventromedial nucleus of the hypothalamus (Walsh et al. 1978). GFAP expression began later in development in tanycytes than in ependymal cells, was found postnatally in many cells, and was detected in adult tanycytes (Hajos and Basco 1984; Levitt and Rakic 1980; Roessmann et al. 1980).