A new membrane antigen revealed by monoclonal antibodies is associated with motoneuron axonal pathways

Chick embryonic motoneurons selectively grow out from the spinal cord as the first step of their selective axonal growth. In order to detect the molecules responsible for motoneuron outgrowth from the cord, we produced and immunohistochemically screened many monoclonal antibodies (MAbs) against cord and somite. We found that two of them, called M7412 and M7902, selectively bound to the cell surface of the anterior half of the sclerotome, where motoneurons selectively extend their axons. Immunohistochemistry and immunoblot results were identical for these antibodies and the antigen was called M7412 antigen. Although neural crest cells also migrate into the anterior half of the sclerotome, the expression of M7412 antigen by sclerotome cells was independent of the neural crest, because neural crest removal did not affect the appearance of the antigen. Furthermore, MAb M7412 bound to the mesenchymal cells along presumptive major nerve trunks in the limb and to the structures surrounding myotubes in muscles during the formation of intramuscular nerve branches. These results suggest that M7412 antigen might be a substrate for general, but not specific, growth of motoneuron axons. If this is the case, we must also infer that some molecule inhibitory for motoneuron growth is localized in the posterior half of sclerotome, because at upper cervical levels the M7412 antigen was also expressed intensely in the posterior sclerotome, whereas motoneurons still grew only into the anterior half. The M7412 antigen was transiently expressed in such various tissues as somite; muscles; blood vessels; spinal cord cells, especially motoneurons innervating the limb; and dorsal root and other peripheral ganglion cells. The M7412 antigenic molecule was extractable with NP40 from a membrane fraction of whole chick embryos and its molecular weight was estimated to be 70 kDa from immunoblot analysis. Thus, our monoclonal antibodies have revealed a new membrane-associated molecule which is likely to play a role in cell-cell interactions during development of motoneurons.