In all species studied, the upper two-thirds of the third ventricle is lined by ependymal cells with a profusion of long cilia that extend into the lumen. In the rabbit, rat, and mouse, an abrupt transition to a sparsely ciliated ependymal surface occurs along the lower one-third of the ventricular wall, at the level of the underlying ventromedial (VM) nucleus. Unique to the rat transition zone, however, is a predominance of microvilli. In the human, a subtle transition to a sparsely ciliated ependymal surface occurs somewhat higher at about the level of the underlying dorsomedial (DM) nucleus. Along the upper expanse of the rabbit infundibular recess (IR) ciliary bundles are infrequent, although occasional solitary cilia are seen. The characteristic features of this region, however, are the non-ciliated ependymal cells with their many irregular bleb or microvilli-like surface protrusions. In contrast to the rabbit, the same region in the rat and mouse is distinguished by an irregular ependymal surface with stout bleb-like protrusions and many single cilia. Similarly in the human, cilia become progressively fewer toward the IR. Although the non-ciliated ependymal surface in this region resembles that of the rabbit, the surface blebs and microvilli-like protrusions are fewer in number and less prominent. The lateral walls of the third ventricle display a consistent regional variation in the morphology of the luminal ependymal surface in all species studied. Although it is not possible to establish unequivocally the nature or significance of these regional differences, the structural modifications of the ependymal surface in the region of the IR are entirely consistent with the secretory or absorptive function attributed to ependyma in this area.The lining of the third ventricle consists largely of a single layer of cuboidal or columnar ependymal cells; however in certain areas, a monolayer of flattened ependyma or a multilayered ependymal lining may also be found (Brightman and Palay, '63; Vigh, Aros, Wenger, KoritsAnszky, and CeglCdi, '63; Rinne, '66; Tennyson and Pappas, '68; Knowles and Anand Kumar, '69). In addition, on the basis of morphological and cytochemical observations a specialized variety of ependymal cell (the tanycyte) , has also been distinguished within circumscribed areas of the ventricular wall. More recent studies have suggested structural and possible functional differences among tanycytes themselves (Luppa and Feustel, '71; Millhouse, '71). ANAT. REC., 174: 407420.The significance of ependymal tanycytes derives largely from the connections they establish between the ventricular lumen and the cerebral vasculature. Particular attention has been focused on the tanycytes of the third ventricle whose processes extend to the primary capillary network of the median eminence, and to the pars tuberalis. This intriguing structural relationship has led to the accumulation of much data in support of a secretory-transport role for ependyma (Vigh et al., '63; Leonhardt, '66; Rodriguez, '69 '70) ...

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