The initial development of motor neurons in the neural tube of rat embryos.

The time of origin of motor neurons and their distribution in the spinal cord was studied in rat embryos by combining whole-embryo culture and the Islet-1 immunostaining technique. Cells immunostained for Islet-1 appeared in the trunk neural tube by 27 hours (corresponding to E10.625) in culture of E9.5 embryos, at which time the cell number of the neural tube in a transverse section was about 200. When the neural tube retarded developmentally by lithium treatment, the time of appearance of the motor neurons was delayed to 33 hours in culture (corresponding to E10.875), but the cell number of the neural tube was about 200. After the initial appearance of motor neurons in the ventral aspect of the neural tube, they distributed in a group in the periphery of the basal plate by 48 hours (corresponding to E11.5) in culture, although in the retarded neural tube the number of motor neurons was small and they did not form a cluster. The percentage of Islet-1-positive cells at the point of the same cell number of the trunk neural tube in the transverse section was higher in the retarded embryos than in controls. These results suggest that motor neurons begin to appear when the cell number of the neural tube in the transverse section becomes about 200 and their initial development is more stable than overall neural tube development.