Cellular competence plays a role in photoreceptor differentiation in the developing Xenopus retina

Factors in the environment appear to be responsible for inducing many of th e cell fates in the retina, including, for example, photoreceptors. Further , there is a conserved order of histogenesis in the vertebrate retina, sugg esting that a temporal mechanism interacts in the control of cellular deter mination. The temporal mechanism involved could result from different induc ing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regu late the competence of cells to respond to them. To differentiate between t hese mechanisms, cells from young embryonic retinas were dissociated and gr own together with those from older embryos, and the timing of photoreceptor determination assayed. Young cells appeared uninfluenced by older cells, e xpressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the heterochronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos int o older hosts. Even the graft cells at the immediate margin of the transpla nt failed to express photoreceptor markers earlier than normal, despite the ir being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to p hotoreceptor-inducing cues by a mechanism that runs on a cell autonomous sc hedule, and that the conserved order of histogenesis is based in part on th is competence clock. (C) 2001 John Wiley & Sons, Inc.