Elimination of cobalt from the frog brain introduced into the optic centres through the optic nerve.

One optic nerve in several frogs was filled with cobaltous-lysine complex, and the animals were left to survive from 1 day to 52 days. Degenerated cobalt-filled retinal fibres were phagocytosed by ependymo-glial, and microglial cells. The cobalt appeared in the ependymo-glial cells in the 4th postoperative day, and its amount was greatly reduced by the 52nd day. Within 12 days the labelled axons were replaced by cobalt-loaded microglial cells in the termination sites of optic fibres. By the end of the experimental period, the number of labelled cells increased in the periventricular layers, and decreased in places where retinal fibres had terminated. These processes were accompanied by the appearance of cobalt in the choroid plexus. It is supposed that glial cells dischargd the cobalt into brain ventricles, and the metal left the nervous tissue via the cerebrospinal fluid.