NATURALLY-OCCURRING CELL-DEATH IN THE DEVELOPING CEREBRAL-CORTEX OF THE RAT - EVIDENCE OF APOPTOSIS-ASSOCIATED INTERNUCLEOSOMAL DNA FRAGMENTATION

Naturally occurring dead cells in the developing rat neocortex, subcor tical white matter and hippocampus, which increase in number during th e first postnatal week and decrease thereafter to disappear by the end of the first month, were examined by in situ labeling of nuclear DNA fragmentation. These cells showed peripheral chromatin condensation or extremely dark, often fragmented, nuclei. Southern hybridization foll owing agarose gel electrophoresis of DNA extracted from the developing cortex, but not from adult brain, showed a 'ladder' pattern which is typical of internucleosomal DNA fragmentation. Taken together these re sults show that naturally occurring cell death (programmed cell death) in the developing cerebral cortex has the morphology of apoptosis and is associated with endonuclease activation.