A scanning electron microscopic study of the ependymal surface of the third ventricle of the rabbit, rat, mouse and human brain

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 epen‐dymal 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.