BIRTH-DATES OF NEURONS IN INDUCED MICROGYRIA

Freezing injury to the cortical plate of the newborn rat results in th e formation of a focal region of cerebrocortical microdysgenesis resem bling, in many ways, human 4-layered microgyria. Previous research has shown that neurons born during embryonic day (E) 20 migrate through t he initial damage and take their place in the cell-dense layer of the microgyric lesion. The current study was conducted to determine: (1) w hether neurons generated earlier in development would be found in micr ogyric cortex, and (2) whether the freezing injury would stimulate pro duction of neurons postnatally. Rat pups from mothers who were injecte d with S-phase markers on E15, E17, E19, and E21 were subjected to fre ezing injury of the cortex to induce microgyria on postnatal day (P) 1 . Other pups received a freezing lesion and then pulse or cumulative i njections of S-phase markers for the next 72 h. Neurons born on E17 an d E19 were found scattered throughout the cell-dense layer of the micr ogyric cortex. Early (E15) generated neurons were nearly absent in the microgyric cortex, and there was no evidence of postnatal induction o f cortical neurogenesis. These results are considered in light of rece nt work demonstrating postnatal neocortical neurogenesis in response t o early neocortical injury.