REMOVAL OF COBALT-LABELED NEURONS AND NERVE-FIBERS BY MICROGLIA FROM THE FROGS BRAIN AND SPINAL-CORD

We investigated the microglial reaction around cobalt-labeled degenera ting neurons and nerve fibers in the frog central nervous system. The aim of these studies was to reveal the routes of migrating microglial cells during debris removal and the effect of seasonal changes on this process in a cold-blooded animal. Oculomotor and spinal motoneurons w ere filled with cobaltous-lysine complex through their axons. In the t orus semicircularis and the isthmic nucleus, neurons were labeled with iontophoretically applied cobaltous-lysine complex through their inju red dendrites and axons. The animals were left to survive for 1 to 50 days. During the summer, oculomotor neurons disintegrated by the seven th postoperative day. The debris from the neurons were phagocytosed by microglia-like cells identified by the presence of cobalt in their cy toplasm. Some of these cells were wedged between ependymoglial cells o f the cerebral aqueduct, others appeared at the pial surface of the me sencephalon. The speed of this process was twice as fast during the su mmer as during the winter. Part of cobalt-labeled microglial cells in the torus semicircularis and the isthmic nucleus moved toward the epen dyma of the optic ventricle and the cerebral aqueduct, respectively. C obalt-loaded microglial cells did not move toward the surface in the s pinal cord and the deep part of mesencephalic tegmentum, and left the brain probably via blood vessels. We conclude that microglial cells lo aded with phagocytosed tissue debris may leave the brain tissue via th ree routes and their activity depends on the environmental temperature . (C) 1996 Wiley-Liss, Inc.