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Young dogs (6-14 weeks) were perfused with buffered aldehydes by standard techniques. The inferior medullary velum and tela choroidea were removed intact from the fourth ventricle, post-osmicated, dehydrated, critical point dried, coated with palladium-gold and examined in a Cambridge Stereoscan S4 scanning electron microscope. Selected samples after thorough scanning were routinely prepared for transmission electron microscopy (TEM). This study depicts numerous epiplexus (Kolmer cell) macrophages that exhibit extreme polymorphism. Because of the pleomorphic nature of these cells and their processes, classification based on external morphology is not practical. TEM shows their internal structure to be that of a macrophage. Bleb-like structures and bulbous projections appear to emerge from certain ependymal cell surfaces which seem to be completely or partially devoid of cilia.

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Young dogs were perfused w i t h b u f f e r e d aldehydes a t f l u i d pressures approaching normal p h y s i o l o g i c a l values. Surface t i s s u e from t h e temporal horns o f t h e l a t e r a l v e n t r i c l e s were prepared f o r scanning e l e c t r o n microscopy, u s i n g postosmication, c r i t i c a l p o i n t d r y i n g and gold-palladium c o a t i n g .The ependymal surfaces a r e m o s t l y c i l i a t e d b u t n o n c i l i a t e d areas occur along t h e medial w a l l s . The l a t t e r areas a r e c h a r a c t e