MIGRATING NEURONS IN THE DEVELOPING CEREBRAL-CORTEX OF THE MOUSE SENDCALLOSAL AXONS

The presence of migrating callosal neurons during the development of t he murine cerebral cortex was studied using biocytin and the lipophili c dye, -dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine cyanine perc hlorate as retrograde tracers. After injections of biocytin in the pre sumptive somatosensory cortex of newborn mice which were analysed one day later, many anterogradely labelled fibres coursed towards the cont ralateral hemisphere through the corpus callosum. Retrogradely labelle d callosal cells were also observed. Most callosal neurons corresponde d to immature pyramidal cells. In addition, a few biocytin-labelled ca llosal neurons displayed extremely fusiform shapes, vertical orientati on and a short, single process emerging from the apical side of the pe rikaryon. At the electron microscopic level, these cells had features identical to those described for migrating callosal neurons. Twenty-fo ur hours after birth, these migrating neurons were almost exclusively observed in the upper, dense aspect of the cortical plate (presumptive layers II-III) and only very exceptionally in the infragranular layer s. No retrogradely labelled cell resembling migrating neurons were not iced after injections on postnatal days 2 or 5. To study migrating cal losal neurons at embryonic stages, crystals of the lipophilic dye were injected in the corpus callosum or the contralateral white matter in embryos aged 17, 18 and 19 days, corresponding to the initial developm ent of the corpus callosum in mice. Whereas callosal migrating neurons were not detected at embryonic days 17 and 18, injections of the lipo philic dye on embryonic day 19 revealed the presence of labelled migra ting neurons in the infragranular layers. To corroborate further that these cells are migrating neurons, [H-3]thymidine was administered on embryonic days 16 and 17, and labelled mice were injected with biocyti n on embryonic day 19 or the first postnatal day. Retrogradely labelle d callosal neurons resembling migrating neurons were autoradiographica lly labelled. These results indicate that the specification of certain neuronal types and the emergence of their cell type-specific characte ristics occur shortly after postmitotic neurons leave the ventricular zone, before being positioned within the cortical plate.