Kinetics of the migration of neurons to rat somatosensory cortex

The laminar location of a neuron in the mature cortex is defined by early events in its ontogeny. In the present study, quantitative [(3)H]thymidine ([(3)H]dT) autoradiography was used to define some of these early events. Four indices were calculated: a proliferation index (indicative of the fraction of cells that was cycling), the leaving fraction (the fraction of cells that permanently left the cycling population and migrated to cortex, the release time (the time post-mitotic cells remained in the proliferative zone(s) before initiating their migrations), and the rate of migration. The proliferation index was relatively high on G13 and progressively declined to a nadir on G21. In contrast, the leaving fraction was lowest on G13 and on G21. The release time for a particular subpopulation did not vary with the time of origin. On the other hand, the release time for the earliest generated cells was significantly shorter than it was for the remaining population of cells labeled by a particular injection of [(3)H]dT. The mean rate of migration was affected by neither the time of origin nor the timing of the onset of migration. Thus, once a cell becomes permanently post-mitotic, the behavior of the young neuron (as defined by its release time and rate of migration) is highly ordered. It is the time of origin, as determined by the desynchrony of the cycling activity of proliferating cells, that determines the ultimate disposition of a cortical neuron.