Early regional specification for a molecular neuronal phenotype in the rat neocortex

The timing of neocortical regional specification was examined using a monoclonal antibody, designated PC3.1, that binds a 29-kDa polypeptide and recognizes a neuronal subpopulation located in the lateral but not dorsomedial neocortex in the rat. When lateral cortical tissue fragments at embryonic days 12 and 16 were maintained in an organotypic culture system, a substantial number of neurons became PC3.1-immunopositive. In marked contrast, considerably fewer, if any, PC3.1-positive neurons were observed in cultures of dorsal cortical tissue. The selective appearance of PC3.1-immunopositive neurons was also observed in dissociated cultures derived from the lateral, but not dorsal, cortical primordium at embryonic day 13 and later. In light of previous reports showing that the interactions between developing neocortical neurons and cortical afferents begin at embryonic day 14 or later, our findings imply that some regional specification occurs well before these interactions and suggest the importance of elements intrinsic to the neocortex in establishing neocortical regional specificity. Furthermore, [3H]thymidine birth-dating experiments revealed that the majority of presumptive PC3.1-immunopositive neurons underwent their final mitosis around embryonic day 15, suggesting that the regional specification events for these neurons occur before their neurogenesis.