Migration and differentiation of neurons in the retina and optic tectum of the chick.

Migration and differentiation of immature neurons in the retina and optic tectum were studied using retrograde transport of DiI and immunocytochemistry. The results demonstrate that many of these cells migrate via a method of perikaryal translocation. DiI was applied to the optic nerves or tecta in fixed chick embryos (Embryonic Days 4-10); 1-2 months later the tissues were dissected and examined as whole mounts or vibratome sections using fluorescent and confocal microscopy. In both the retina and the optic tectum many labeled cells have a bipolar shape with leading and trailing processes contacting the pial and ventricular surfaces, respectively. Axons grow from the leading processes before somata reach their final locations and dendrites sometimes begin to grow prior to retraction of trailing processes. Immunocytochemical studies using a monoclonal antibody (TUJ1) specific for postmitotic neurons show a similar pattern. These results indicate that neuronal migration occurs via more than one mode. In thin tissues like the retina and newly forming optic tectum, many postmitotic neurons migrate by translocating their somata while retaining connections with the pial and/or ventricular surfaces. In thicker, more complex tissues like the maturing optic tectum, cerebral cortex, and cerebellum, most young neurons appear to detach from the surfaces and migrate along processes of radial glia or other cell types.