In vivo and in vitro development of S- and M-cones in rat retina.

Purpose: Organ cultures of the rodent retina could provide a powerful tool in the study of cone development and differentiation. Previous attempts, however, have failed to show M-cone development in organ cultures of the mouse and rat retina. This study mimicked the in vivo dynamics of S- and M-cone development in a culturing approach for the postnatal rat retina.

Methods: Retinas of Brown Norway rats were collected at different developmental ages (postnatal day [P]0-P270) to study cone development in vivo. For culturing, the retinas were prepared from P0 to P2 animals and allowed to develop in organ culture for 2 to 15 days. Subsequently, opsin expression was analyzed immunohistochemically and morphometrically.

Results: In control retinas, S-opsin was already expressed at birth, whereas M-opsin was detected after P4. The maximum density of S-opsin-positive cones was reached at P10 (∼17,000 cells/mm(2)) and of M-opsin-positive cones, at P12 (∼14,000 cells/mm(2)). The number of both cone types decreased gradually thereafter to ∼1,000 S-opsin cones/mm(2) and ∼4,000 M-opsin cones/mm(2) in the adult. In culture, both cone types developed with dynamics of appearance comparable to those in vivo, with a peak density of ∼12,300 cones/mm(2) for S-opsin and ∼7,500 cones/mm(2) for M-opsin labeling.

Conclusions: These results in rat retina showed for the first time that cone development and expression dynamics can be mimicked in organ culture. With this experimental approach, it will be possible to evaluate aspects of cone development under controlled experimental conditions and to elucidate factors crucial for proper cone differentiation.