A molecular approach to retinotectal specificity.

An assay is described to examine the hypothesis that retinal neurons adhere preferentially to that part of the optic tectum near theri normal synaptic termini. The method measures the adherence of isotopically labelled cell bodies from either the dorsal or ventral half of the neural retina of chick embryos to dorsal and ventral tectum halves. When a labelled cell suspension is prepared from a dorsal half-retina, more cells adhere to the ventral half of the tectum. When the cells are from the ventral part of the retina, more bind to the dorsal half of the tectum. This preferential adhesion mimics the retinotectal projection found in vivo and supports an interpretation of neuronal specificity dependent on cell surface adhesive properties. Molecular mokels are presented that utilize glycosyltransferases and their substrates as the basis for adhesive recognition. Two of these models suggest that quantitative changes in the distribution of transferases and their substrates determine retinotectal specificity. The third proposes qualitative variations in these molecules across the retina and tectum.