Transitory disappearance of microglia during the regeneration of the lizard medial cortex.

In normal lizards, microglial cells populate the medial cortex (a zone homologous to the hippocampal fascia dentata), with a preferential distribution along the border between the granular cell layer and the plexiform layers. Intraperitoneal injection of the neurotoxin 3-acetylpyridine (3AP) induces a selective lesion in the medial cortex with a rapid degeneration of the granular layer and its zinc-enriched axonal projection. Within 6-8 weeks, the granular layer is, however, repopulated by a new set of neurons generated in the subjacent ependyma and the cell debris is removed. The aim of this study was to determine to what extent microglia were involved in the scavenging processes during the regeneration process. To this end we studied the brains of regenerating lizards at different times after 3AP lesion, visualising microglial cells by the nucleoside diphosphatase (NDPase) histochemical reaction. Surprisingly, we found that stained microglial cells disappeared 6-8 hours after 3AP injection and remained absent until 10-15 days after injection. One month postlesion an increased population of microglial cells was found scattered throughout all plexiform layers of the cortex. Thorough examination of semithin and ultrathin sections confirmed the absence of microglia in the medial cortex of recent lesioned animals but the presence of an exuberant population after 1 month postlesion. In the tissue, phagocytotic scavenging was carried out by radial ependymocytes, not by microglia.