Birthdates of neurons in induced microgyria.

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