Ultrastructural analysis of colloidal gold immunocytochemical staining and immunofluorescence microscopy has been used to study the presence of neural cell adhesion molecule (NCAM) on the surface of neuronal growth cones. The studies were carried out with cultures of rat hypothalamic and ventral mesencephalic cells, using morphology and expression of tyrosine hydroxylase, neurofilaments, and glial fibrillary acidic protein as differential markers for neurons and glia. NCAM was found on all plasmalemmal surfaces of neurons including perikarya and neurites. The density of NCAM varied for different neurons growing in the same culture dish, and neurons had at least 25 times more colloidal gold particles on their plasmalemmal membranes than astroglia. Of particular interest in the present study was a strong labeling for NCAM on all parts of neuritic growth cones, including the lamellar and filopodial processes that extend from the tip of the axon. The density of NCAM was similar on different filopodia of the same growth cone. Therefore, in situations where homophilic (NCAM-NCAM) binding might contribute to axon pathfinding, a choice in direction is more likely to reflect differences in the NCAM content of the environment, rather than the distribution of NCAM within a growth cone. On the other hand, the variation in NCAM levels between single neurons in culture was significant and could provide a basis for selective responses of growing neurites.

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