The large question motivating this specific project is to understand how developmental processes responsible for patterning the dorso-ventral axis of the vertebrate neural tube intersect with those regulating proliferation and neurogenesis. In the spinal cord neuron production and differentiation follows a ventral to dorsal gradient with motoneuron generation from basal plate precursors preceding differentiation of dorsal interneurons from alar precursor cells. Preliminary results suggest differences in cell generation time of the spinal cord neurepithelium that correlate with both developmental stage and position on the dorso-ventral axis. Experiments are proposed to further establish these differences and to explore regulation of these events at both embryological and molecular levels. The applicant proposes to test whether signals from the paraxial mesoderm and/or the ectodermal epithelium differentially influence cell generation time and neurogenesis in the dorsal and ventral regions of the spinal cord using experimental embryological methods. Cumulative BrdU labeling and immunocytochemistry will be used to estimate changes in cell cycle parameters and neuronal differentiation in normal and surgically manipulated embryos. Possible effects of surgical manipulations on cell death will be addressed using TUNEL assays. Molecular regulation of neurogenesis in the spinal cord will be studied by evaluating the expression patterns of four Id family members using in situ hybridization. Id family members are known to be negative regulators of the positive acting basic helix-loop-helix transcription factors, and they function in other systems as inhibitors of cell differentiation. Id proteins have also been implicated in cell cycle control and they are target genes of molecules involved in specifying the axes of the spinal cord. The applicant proposes to test the hypothesis that one or more of these genes is expressed in proliferating spinal cord neural epithelium, and that expression is down-regulated when cells withdraw from the cell cycle and begin to differentiate. The results of the initial studies will be used to guide future investigations on the patterned regulation of neurogenesis and its cessation, events that are critical for normal nervous system development. Deficiencies in neurogenesis may be underlying causes of human congenital brain abnormalities and/or mental retardation.