Brain phagocytes may empty tissue debris into capillaries

The possibility that brain phagocytes may empty remnants of degenerated neurons into capillaries has been studied in frogs. Degeneration of nerve fibers was brought about by transectioning the optic tract, the tectothalamic and tectoisthmic tracts, the postoptic commissure or the radial nerve. To help identification of phagocytozed degenerated neuronal elements, the transected fibers were filled either with horseradish peroxidase (HRP) or cobaltous-lysine complex. The survival times were 3, 4, 7, 27, 47 and 70 days after the application of the markers. The HRP-labeled structures were identified in 60 microm thick sections using diaminobenzidine as chromogen, while cobalt was precipitated in the form of cobaltous sulfide. Small pieces of these sections were further processed for electron microscopy. In each area of the brain and spinal cord investigated, microglial cells and astrocytic processes containing fragments of degenerated neuronal elements could be seen close to capillaries. In some cases a microglial or astrocytic process pierced the capillary basal lamina and seemingly delivered inclusion bodies into the cytoplasm of capillary endothelial cells and pericytes. In the inclusion bodies, which were usually large vesicles, fragments of HRP or cobalt-labeled or unlabeled membranes with a foamy appearance, or condensed myelin lamellae could be observed. These vesicles protruded the luminal membrane of the endothelial cell that was disrupted in some cases suggesting that the content of the inclusion body was discharged into the lumen of the capillary. These results give support to Penfield's hypothesis (1925) that glial cells may empty phagocytozed materials into capillaries.