Fig. 4.

A model for regulation of the kinetics of rhodopsin expression. Retinal progenitor cells (white) divide to produce a combination of mitotic and postmitotic progeny. Cells fated to become rod cells (rod precursors; light gray) are postmitotic. After a delay, the cells begin to express rhodopsin and form outer segments (dark gray). For cells born before E19 (A, B), the onset of rhodopsin appears synchronous. Thus, early-born rod precursors wait longer than later-born rod precursors before expressing rhodopsin. For cells born on or after E19 (C), there is a fixed lag of, on average, ∼6 d, although the first rhodopsin-positive cells appear 48–72 hr after the terminal mitosis. The regulation of rhodopsin expression can be thought of as having two phases. The first phase lasts up to E19 in the rat and appears to involve a mechanism of counting that does not depend on cell division and is capable of keeping a postmitotic cell (A) and a mitotic cell (B) in synchrony. The second phase lasts ∼6 d and begins either on E19 (for the early cells) or after the cell has undergone its terminal mitosis. Heterochronic experiments suggest that the control of timing is cell intrinsic.