Extrinsic cues and intrinsic competence act in concert for cell fate determination in the developing vertebrate retina. However, what controls competence and how precise is the control are largely unknown. We studied the regulation of competence by examining the order in which individual retinal progenitor cells (RPCs) generate daughters. Experiments were performed in Xenopus laevis, whose full complement of retinal cells is formed in 2 days. We lineage-labeled RPCs at the optic vesicle stage. Subsequently we administered a cell cycle marker, 5-bromodeoxyuridine (BrdU) at early, middle or late periods of retinogenesis. Under these conditions, and in this animal,BrdU is not cleared by the time of analysis, allowing cumulative labeling. All retinal cell types were generated throughout nearly the entire retinogenesis period. When we examined the order that individual RPCs generated daughters,we discovered a regular and consistent sequence according to phenotype: RGC,Ho, CPr, RPr, Am, BP, MG. The precision of the order between the clones supports a model in which RPCs proceed through stepwise changes in competence to make each cell type, and do so unidirectionally. Because every cell type can be generated simultaneously within the same retinal environment, the change in RPC competence is likely to be autonomous.
Supplementary material
We thank R. Dorsky and W. Harris for their involvement at the inception of this study and for advice during execution and analysis. We thank R. Molday for the rhodopsin antibody and Sylvia Evans for the pCS2-ITR vector and gfp3 cDNA. We are appreciative of members of the Rapaport lab for technical support, and personnel at the NEI Core Grant for Vision Science (P30 EY12598-02) for confocal training and use. D.H.R. was funded by NEIEY11875. Deposited in PMC for release after 12 months.
