The development of the cervical spinal cord of the mouse embryo. I. A Golgi analysis of ventral root neuron differentiation

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Abstract
No previous Golgi studies have described early neuron differentiation in mammals near the time of neural tube closure. By leaving embryos in utero and varying the impregnation times, the cells of the cervical spinal cord of mice of 8–11 days of gestation (E8–E11, where E0 = day vaginal plug observed) were stained and analyzed in the present study. The early stages of ventral root motoneuron differentiation described earlier (Wentworth and Hinds, '78), using serial section electron microscopy, have been confirmed in this study and include preaxonic neuroblasts (stage 1–detached from the ventrical but still attached to the basal lamina), bipolar neuroblasts (stage 2), and unipolar neuroblasts (stage 3). Later stages of differentiation described in this study include young neurons with dendrites: secondary bipolar (stage 4) and multipolar neurons (stage 5). At E8 the neural tube is just closing and consists of undifferentiated ventricular cells. On E9 a few ventral root neuroblasts are differentiating in the cervical spinal cord. The majority of these cells are in the bipolar and unipolar neuroblast stages, although the most advanced cells observed had reached the secondary bipolar neuron stage of differentiation. By E10 a large cluster of ventral root motoneurons is differentiating in the ventrolateral cord. Some cells have reached the bipolar stage, a few cells are becoming multipolar, and a subdivision into dorsolateral and vrntromedial subgroups is beginning. By E11 the medial and lateral subdivisions are more obvious, and the most advanced cells are well‐differentiated multipolar neurons oriented either mediolaterally or dorsoventrally, although cells in the earlier unipolar and secondary bipolar stages are still observed.