Nature and nurture in the differentiation of retinal photoreceptors and neurons.

This article reviews recent studies using a novel experimental system in which undifferentiated precursor cells from the 8-day chick embryo retina are grown in low density, clump-free, dissociated cell culture. The cultures initially consist of a morphologically homogeneous population of isolated process-free, round cells. Analysis of the cultures by phase contrast light microscopy, scanning and transmission electron microscopy, immunocytochemistry and autoradiography, shows that during the first week in vitro some precursor cells acquire a well differentiated photoreceptor phenotype, while others develop as neurons. Given that these divergent differentiation pathways are followed by cells developing in a homogeneous microenvironment in the absence of intercellular contacts, the evidence suggests that precursor cells present in the 8-day chick embryo retina are already preprogrammed to undergo an extensive series of chemical and structural modifications necessary to differentiate as either neurons or photoreceptors.