Ontogeny of progesterone receptor expression in the subplate of fetal and neonatal rat cortex.

The progesterone receptor (PR) is transiently expressed in the rat cortex during development and its expression is initiated in the developmentally critical layer, the subplate. As subplate neurons pioneer thalamocortical and corticofugal connectivity, the expression of PR in this layer suggests an important function for PR in cortical development. Using immunocytochemistry for PR, the present study determined the precise ontogeny of PR expression in subplate neurons. The number of cells containing PR immunoreactivity (PRir) within the subplate was quantified from embryonic day (E) 17 through postnatal day (P) 14. The subplate was positively identified by the marker calretinin and by BrDU birthdating. The results demonstrate that PRir is undetectable in fetal cortex on E17, but is first observed in the subplate on E18. The number of PRir cells peaks on P2 and then steadily declines, until PRir is once again not detectable in subplate by P14. This developmental window of PR expression within the subplate coincides with establishment of early cortical circuitry and the gradual demise of subplate cells, suggesting that PR may play a critical role in mediating these fundamental developmental processes.