| Abstract: | The absorbing capability of the choroidal epithelium in the third and the fourth ventricles of the teleost Leuciscus rutilus was studied by using the electron dense tracers lanthanum and peroxidase. The tracers were either injected into the third ventricle or applied onto the leptomeninx. Peroxidase was rapidly absorbed by coated vesicles after being injected into the ventricle. This reaction product was retained in multivesicular bodies which remained localized at the apical pole of the cell. However, when peroxidase was applied onto the leptomeninx only a limited uptake was observed. Lanthanum was not absorbed by the epithelial cells. The zonulae occludentes between two adjacent cells prevented the tracers from reaching the ventricular or vascular sides. Thus, the epithelium of the saccus dorsalis and the plexus choroideus posterior act as a barrier prohibiting the transport of these tracers from the blood to the cerebrospinal fluid or also in the reverse direction. |
