In vitro studies on neural specificity.

The migration of the retinal ganglionic axons through the optic nerve, over the tectal surface, and into the tectum to synapse correctly with brain neurons has been one of the most studied paradigms of neural specificity. Through the use of dissected tecta from the developing chick embryo, dissociated retinal cells can make the same choices in vitro that the ganglion cell termini make in vivo. That is, cells from the dorsal retina prefer to adhere to ventral tectum fragments and vice versa, in accord with the final map of the retina on the tectum. This assay has been used to analyze the biochemical components on the cell and tectal surfaces that might account for the recognition observed. Also, the assay has made it clear that virtually all the retinal cells can distinguish between dorsal and ventral tectal fragments, even though the cells of the retina that normally synapse with the tectum make up not more than 5% of the total population of the retina. One reason for this may be that, although only the retinal ganglion cells send their processes into the brain, these processes use all of the retina initially to get to the fundus of the retina, where the optic nerve begins. The vast majority of the retinal cells may possess surface recognition molecules not for finding the tectum but for serving as substrates for the few retinal cells whose axons must first leave the retina before finding the correct tectal locus.