CHANGES IN APICAL ORGANIZATION OF CHOROIDAL CELLS IN RATS ADAPTED TO SPACEFLIGHT OR HEAD-DOWN TILT

Structural changes observed in choroid plexuses from rats dissected ab oard a space shuttle, on day 13 of an orbital flight (NASA STS-58 miss ion, SLS-2 Experiments) demonstrated that choroidal epithelial cells d isplay a modified organization in a microgravitational environment. Re sults were compared with ultrastructural observations of choroid plexu s from rats maintained under anti-orthostatic restraint (head-down til t) for 14 days. In both experiment types, the main alterations observe d by transmission electron microscopy, at the level of choroidal epith elial cells from the third and fourth ventricles, concerned the format ion and the organization of apical microvilli, whereas pseudopod-like structures appeared. Immunocytochemical distribution of ezrin, a cytos keletal protein involved in apical cell differentiation in choroid ple xus, confirmed the structural alteration of microvilli in head-down ti lted rats. Kinocilia tended to disappear from the apical surface, sugg esting a partial loss of cell polarization. In addition, large amounts of clear vesicles were gathered in the apical cytoplasm of choroidal epithelial cells. Disorganization of apical microvilli, accumulations of apical vesicles and partial loss of cell polarity showed that long- stays in weightlessness induced alterations in the fine structure of c horoid plexus! consistent with a marked reduction of cerebrospinal flu id production.