Topological distribution of different forms of neural cell adhesion molecule in the developing chick visual system

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Abstract
The cell adhesion molecule isolated from neural tissue (N-CAM) is a membrane glycoprotein which is directly involved in calcium-independent adhesion among nerve cells and their processes (for review see refs 1,2). N-CAM has an unusual carbohydrate moiety containing a large and variable amount of sialic acid, the variation reflecting both the type of tissue and its developmental age3,4. N-CAM is believed to be a ligand in the formation of cell-cell bonds5 and a decrease in sialic acid content from 30% to 10% is associated with a marked enhancement of the molecule's binding activity6–8. Antibodies to N-CAM block its function and inhibit or alter bundling of nerve fibres9, retinal cell development10–12 and nerve-muscle interaction13,14. Here we use micro-gel elec-trophoresis15 to compare N-CAM from several parts of the developing chick visual system. The results indicate that N-CAM from the retina of 5–10-day-old embryos already exists in a relatively sialic acid-poor form, whereas the tectum and optic nerve beyond the eye contain sialic acid-rich N-CAM until much later in development. These studies suggest that the perikaryon and proximal axon shaft of retinoganglion cells have N-CAM with a lower sialic acid content than the distal portion of the axons, and that resulting differences in neurite adhesivity may be an important factor in the formation of the optic system.

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