The fine structure of the ependymal tanycytes at the level of the arcuate nucleus

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Abstract
The ependymal tanycytes of the third ventricle at the level of the arcuate nucleus were studied with the electron microscope. Criteria were discovered for identifying the tanycyte process as it courses through the arcuate neuropil, making it possible to describe the entire ependymal cell from the apical surface to the end foot. The apical membranes of the tanycytes are characterized by a lack of cilia and by numerous microvilli. The apical lateral membranes may be extensively interdigitated, and the junctional complexes between adjacent apical lateral membranes often lack maculae occludentes. The cells are well endowed with large Golgi apparatus. The endoplasmic reticulum is principally smooth. The tanycyte processes contain small paraxially oriented mitochondria and numerous paraxial microtubules as well as lipid droplets and accumulations of smooth endoplasmic reticulum in the form of concentric rings. Some of the tanycytes end on capillaries in the arcuate nucleus. These pericapillary ependymal sheaths contain irregular inclusions of varying densities. The ependymal tanycytes have been implicated as secretory cells involved in hypothalamic control of the anterior pituitary gland. This concept is discussed in relation to the present fine structural findings.