Expression of a neurofilament protein by the precursors of a subpopulation of ventral spinal cord neurons.

The expression of neurofilament proteins (NF-H, NF-M, and NF-L) in replicating neuroepithelial cells and postmitotic neuroblasts in the embryonic chick trunk neural tube was examined by immunohistochemistry. Anti-NF-M, in particular, resulted in bright staining of some mitotic cells, which were found to be strictly localized to a midventral and an extreme dorsal position in the neural tube. Those in the midventral position were observed with greatest frequency during Days 3 and 4 of incubation and became increasingly rare thereafter. During the same period of time, and in the same small ventral region, NF-M-positive interphase cells, presumably migrating postmitotic neuroblasts, were also present. In contrast, NF-L-positive mitotic cells were rarely seen. NF-L-positive migrating and differentiating neuroblasts were observed throughout the ventral half of the neural tube except in the midventral area containing NF-M-positive mitotic cells and NF-M-positive migrating neuroblasts. These results, together with known temporal and spatial patterns of neurogenesis in the spinal cord, suggest that the expression of NF-L and NF-M, in the form recognized by our antibodies, may not be initiated coordinately, or even in the same sequence, in different types of neuroblasts, and that only the immediate precursors of a specific subpopulation of ventral spinal cord neurons begin expressing NF-M in the terminal cell cycle. In addition, the NF-M-positive mitotic cells, when observed in anaphase and telophase, had NF-M-positive material associated with both emerging daughter cells and the migrating neuroblasts were frequently found in closely associated pairs, consistent with the suggestion that these precursor cells undergo a symmetrical terminal division to yield two daughter postmitotic neuroblasts.