The time of origin, development, and morphology of neurons originating before the establishment of the cortical plate (preplate cells) were studied in the telencephalic vesicle of the rat from the embryonic day 10 (E10) to E15. The first postmitotic neurons settle superficial to the ventricular zone in the preplate at E12. We have observed mitotic figures in the preplate at E12 and E13, and regarded them as a possible source of cortical neurons outside the ventricular zone. It is suggested that these cells may correspond to Cajal-Retzius cells. The appearance of cells before the formation of the cortical plate was studied in animals injected with tritiated thymidine at E12 and E13 and correlated with observations made using an antibody to neuron specific class III β-tubulin that becomes detectable immediately after the last mitotic division. Immunoreactive cells detected with this antibody concentrate at E12 immediately below the pial surface constituting the preplate. Radially oriented cells, most probably subplate neurons, were also present spanning the full thickness of the neuroepithelium, suggesting that the preplate contains different cell classes. We present a working hypothesis that explains the sequence of developmental events during the early phases of cortical histogenesis.

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