Influences of the neural cell adhesion molecule on axon growth and guidance.

The neural cell adhesion molecule (NCAM) has been shown to be a ligand in the formation of cell-cell bonds. This molecule is present on essentially the entire surface of differentiated nerve cells, including the cell body, neurite shaft, and growth cone. In mediating membrane-membrane adhesion, NCAM appears to ligate with itself, and one of its most obvious functions is in the self-association of nerve fibers to form fascicles. In most cases fasciculation occurs by the successive elongation of axons along other axons and, therefore, is likely to represent a growth cone-neurite shaft interaction as well as a shaft-to-shaft adhesion. Competition between neurite shafts and the surrounding substrate for growth cone adhesion probably represents a major factor in the branching of nerve bundles. In addition to neurons, NCAM appears on some glial and muscle cells. Recent experiments suggest that this molecule is involved in growth cone guidance along adhesive pathways on glial precursors in the vertebrate central nervous system, and in the initial interaction of axons with muscle prior to synaptogenesis.