Elevated protein tyrosine phosphorylation in the optic tract of the chick embryo

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of protein tyrosine kinase activity in the optic tract of the developing chick by immunoperoxidase staining. In the stage 34 (day 8) chick embryo, phosphotyrosine-modified proteins were abundant within outgrowing neuronal processes in the optic nerve head and nerve fiber layer of the retina, and in the developing stratum opticum at the surface of the optic tectum. These sites corresponded to regions where migrating growth cones and fasciculating bundles of some, but not all, retinal ganglion cell axons were located. Phosphotyrosine-modified proteins were also abundant in and highly restricted to the process-rich layers of the embryonic optic tectum. Phosphotyrosine immunoreactivity decreased dramatically in the corresponding regions of the optic tract of the adult chicken, indicating that protein tyrosine phosphorylation occurred principally in developing, rather than mature, neuronal processes. These findings are in accord with the idea that protein tyrosine phosphorylation may be important in cell-cell or cell-substratum interactions of ganglion cell axons.