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 the cell fates in the retina, including, for example, photoreceptors. Further, there is a conserved order of histogenesis in the verte- brate retina, suggesting that a temporal mechanism in- teracts in the control of cellular determination. The temporal mechanism involved could result from differ- ent inducing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regulate the competence of cells to respond to them. To differen- tiate between these mechanisms, cells from young em- bryonic retinas were dissociated and grown together with those from older embryos, and the timing of pho- toreceptor determination assayed. Young cells appeared uninfluenced by older cells, expressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the hetero- chronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos into older hosts. Even the graft cells at the immediate margin of the transplant failed to express photoreceptor markers ear- lier than normal, despite their being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to photoreceptor-inducing cues by a mechanism that runs on a cell autonomous schedule, and that the conserved order of histogenesis is based in part on this competence clock. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: