REACTIVE NEUROGENESIS DURING REGENERATION OF THE LESIONED MEDIAL CEREBRAL-CORTEX OF LIZARDS

This study reports that lesion of the adult lizard medial cortex (liza rd hippocampal fascia dentata) induces a short period of intensive neu rogenesis which we have termed reactive neurogenesis; a cell prolifera tion event that occurs in the subjacent ependyma. Specific lesion of t he medial cortex was achieved by intraperitoneal injection of the neur otoxin 3-acetylpyridine and proliferating cells were detected using tr itiated thymidine or 5-bromodeoxiuridine pulse labelling. After lesion , granule neurons in the lizard medial cortex cell layer appeared pykn otic and died; they were then removed and progressively replaced by a set of new neurons. These neurons were mostly generated from the secon d to the seventh day post-lesion. A dramatic temporal increment of lab elled ependymal cells was detected when either tritiated thymidine or 5-bromodeoxiuridine pulses were delivered in that period. The maximum of about five thousand labelled cells per hemisphere was reached by th e fourth day after the lesion. Beyond the seventh day post-lesion, the numbers of labelled cells returned to a level of about four hundred p er hemisphere, similar to that of the control specimens. Electron micr oscopy revealed that the recently generated cells were neuroblasts or immature neurons with a characteristic pattern of chromatin condensati on and a high number of ribonucleic granules.