Early motoneuron formation in the cervical spinal cord of the mouse: An electron microscopic, serial section analysis

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Abstract
Cells of the ventrolateral region of the cervical spinal cord were reconstructed from serial thin sections in order to provide direct evidence concerning the early stages of motoneuron differentiation. Analysis of 321 sections from the spinal cord of a mouse embryo on the tenth day of gestation (E10) was performed. At E10 the general appearance of the ventrolateral region is that of a pseudostratified columnar epithelium. It consists primarily of radially oriented ventricular cells, while the remaining cells are either rounded mitotic cells (located at the neurocoel) or cells differentiating into either accessory root or ventral root motoneurons. The following early stages of motoneuron differentiation have been distinguished: anaxonic primitive bipolar cells (stage 1); primitive bipolar cells with an axon (stage 2); unipolar cells (stage 3); and secondary bipolar cells (stage 4). The earliest morphological change in the differentiation of both types of motoneurons appears to be the withdrawal of the apical process from the neurocoel and the migration of the Golgi apparatus, centrioles and cilium along the apical process toward the nucleus (anaxonic primitive bipolar cell). The nucleus then appears to migrate peripherally and the basal process (which never completely withdraws) to transform into an axon (primitive bipolar cell with an axon). Subsequent stages have only been identified in accessory root motoneurons; it appears that ventral root motoneurons are not as advanced in their differentiation at E10. In the unipolar stage the apical process has completely withdrawn, and the nucleus appears paler and rounder than that of earlier cells. Finally, in the secondary bipolar stage the cell is oriented dorsoventrally with an axon directed dorsally and a dendrite ventrally from the opposite pole of the cell. During differentiation of accessory root motoneurons, the Golgi apparatus, centrioles and cilium are found first in the apical process, then at the base of the axon, and finally at the base of the dendrite. The translocation of these organelles occurs secondary to the initial formation of first the axonal and then the dendritic process.