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.