Cogeneration of neurons with a unique molecular phenotype in layers V and VI of widespread lateral neocortical areas in the rat.

Monoclonal antibody PC3.1 detects a unique subpopulation of neurons located mainly in layer VI and, to a lesser extent, in layer V within the lateral neocortical areas in the rat. In an attempt to characterize these neurons, we determined the time of their generation in selected neocortical areas by a double-labeling experiment combining quantitative long-survival 3H-thymidine autoradiography and immunohistochemistry for the PC3.1 antigen. We found that the vast majority of PC3.1-positive neurons in both layers V and VI were generated concurrently at embryonic day 15 in all areas examined, demonstrating a strict correlation between the molecular identity of neurons and the time of their generation, irrespective of their final positions along the radial and tangential axes. In contrast, PC3.1-negative neurons, which should represent more diverse phenotypic identities, were generated during a more extended period of cortical development and tended to exhibit radial (inside-to-outside) and tangential (ventral-to-dorsal and rostral-to-caudal) neurogenetic gradients. Our findings indicate that laminar and tangential locations of cortical neurons are not established solely by a combination of mechanisms for the inside-out movement of newly generated neurons in each cortical area and for the broad tangential neurogenetic gradients. The results of this study suggest a distinct way of cortical development in which neurons with a common molecular phenotype are generated concurrently and migrate toward their eventual positions, which are not necessarily located in a single lamina. In addition, our results suggest some kind of tangential heterogeneity in the mechanism involved in neocortical histogenesis, supporting the concept of early regional specification within the neocortex.