The development of the cervical spinal cord of the mouse embryo. II. A Golgi analysis of sensory, commissural, and association cell differentiation

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Abstract
The present report describes for the first time the early differentiation of dorsal root ganglion cells and internuncial cells in the spinal cord of mammals using the rapid Golgi technique; embryos of 9–11 days gestation (E9–E11, where E0 = day vaginal plug observed) were used. On E9 dorsal root ganglion cells are relatively undifferentiated and are just beginning to take on a bipolar appearance. A few commissural neurons are differentiating in the alar and basal plates, but their axons have not yet crossed the ventral commissure. Occasionally an ipsilateral funicular or association cell is seen in the alar plate. By E10 bipolar dorsal root ganglion cells send processes both centrally into the spinal cord and peripherally to join the ventral root as the spinal nerve. The relatively few association neurons are intermingled among commissural cells except that they are not found in the region of the ventral horn. The axon of many commissural neurons crosses the ventral commissure. On E11 a thin lateral funiculus is seen as well as the oval bundle of His (presumptive dorsal funiculus). An occasional dorsal root ganglion cell has attained the unipolar stage of differentiation. The commissural and association neurons are still intermingled along the outer regions of the mantle layer, especially in the alar plate region. Both commissural and association cells start to differentiate within the ventricular layer as radially oriented, bipolar neuroblasts. As the cell somas reach the outer region of the ventricular layer the cells become unipolar. The cells reorient more dorsoventrally and a primary dendrite grows first from the pole opposite the axon. This sequence of cell differentiation is in general agreement with what is seen in the differentiation of ventral root motoneurons.