The specificity of retinotectal connections studied by retinal grafts onto the optic tectum in chick embryos

Abstract

Pieces of retina from known quadrants of 4-day chick embryo eyes were grafted onto the surface of the optic tectum of 6- or 7-day embryos (the contralateral eye of the hosts having been excised in an earlier operation to prevent the normal complement of optic fibers from reaching the tectum). The embryos with grafts were incubated 5 days, then prepared for histological study. Examination of serial sections stained with Protargol revealed that the retinal grafts had differentiated normally and produced fibers which passed onto the surface of the tectum as a single bundle, or as parallel fascicles. These coursed for varying distances on the surface membrane above the tectum, ending in many cases by piercing the membrane and entering the superficial tectal layers. The location of the graft on the tectal surface and the course and length of the fiber bundles were recorded, and the results grouped according to the quadrant of origin of the retinal graft. Fibers from a particular quadrant of retina preferentially grew toward specific areas of the tectum (dorsal retina to lateroventral tectum; ventral retina to dorsal tectum; nasal retina to posterodorsal tectum; temporal retina to anterodorsal tectum). These findings correlated with the known retinotopic distribution of normal optic fiber projections onto the tectum. Possible mechanisms mediating the specific guidance of fibers are discussed.