Ventricular mixing ; its functional implication and regulatory mechanisms

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Intermixing behavior of ventricular zone (VZ) cells were examined in the slice culture of the neural tube. Slices were prepared from the E10.5 Olig2-CreER: Z/EG double heterozygous fetus that had been treated with tamoxifen at E9.5 to induce Cre-mediated recombination. Slices were maintained for up to 16 hours. GFP+ Olig2 lineage cells in the pMN domain were focused and images were captured every 10 minutes. We observed 105 GFP+ cells in the midline region of the cultured slices. Among them, 45 cells relocated over 2-3 rows of cells during the time window. Thus, nearly half of observed VZ cells underwent ventricular mixing.We next examined whether ventricular mixing occurred in the spinal cord and hindbrain in vivo. Olig2-CreER: Z/EG double heterozygous fetus treated with tamoxifen at E9.5 was analyzed at E10.5 or at adult stage. In caudal hindbrain of the adult double heterozygous animal, Olig2 lineage cells differentiated into not only somatomotor neurons in the hypoglossal nucleus but also visceromotor and serotonergic neurons, the latter of which have been reported to be derived from more ventral Nkx2.2 domain. Twenty-four hours after the tamoxifen treatment, some of Olig2 lineage cells relocated down to the Nkx2.2 domain and expressed Nkx2.2 in the VZ, and a limited number down to the floor plate. Furthermore, some of Olig2 lineage cells in the parenchymal region expressed Phox2b in the hindbrain and Sim 1 in the spinal cord; Phox2b is a critical transcription factor for visceromotor neuron differentiation and Sim 1 is a marker transcription factor for V3 interneurons. The present results elucidated that Olig2 lineage cells crossed the domain border by ventricular mixing, and change their phenotype at the progenitor stages, probably depending of the new positional information.