Time course of opsin expression in developing rod photoreceptors.

We have investigated the time course of rod photoreceptor determination in the goldfish retina. Rod precursor cells located in the outer nuclear layer of the mature retina continuously generate rod photoreceptors. In this study, we asked when rod precursor cells begin to express opsin, which would signal their commitment to the rod pathway of differentiation. There are three possibilities: a rod precursor could express opsin while still mitotic, at or shortly after the terminal mitosis but before differentiation, or during differentiation. We used immunocytochemistry with antibodies against bromodeoxyuridine, BrdU (a thymidine analogue) and against opsin to determine when during the mitotic history of a cell the expression of opsin first occurred, taking a double labelled cell to be evidence of commitment to the rod cell fate. We found that the first double labelled cells appeared at 4 days after BrdU injection. The number of double labelled cells increased to peak at 10 days, and then fell. These results support the hypothesis that dividing rod precursor cells are probably multipotent stem cells not committed to the rod cell fate.