An in vitro model of proliferation and differentiation of the chick retina: coaggregates of retinal and pigment epithelial cells.

Pigment epithelial (PE) cells exert a pronounced organizing effect when added to embryonic day (E) 5-6 chick retinal cells in a reaggregation system such that after a period of 14-21 d of culture, the main layers of an intact E10-E14 retina are reconstructed (Vollmer et al., 1984). In the present study we investigated the time course of the formation of retina-like structures in retinal-pigment epithelial aggregates, in particular, the fate of the PE cells and their influence on processes of differentiation within the aggregates. PE cells first form a core in the center of the aggregates and migrate to the periphery at later stages. The PE core affects the organization of proliferation and differentiation, phenomena that were monitored using 3H-thymidine autoradiography and AChE histochemistry, respectively. A double-staining procedure combining both techniques on the same section is described. Soon after aggregation, a matrix zone and a zone of differentiated cells are formed. At later steps, proliferation becomes gradually restricted to a narrow band within the aggregates comparable to the in vivo situation. The spatiotemporal pattern of withdrawal from mitosis resembles that of the in vivo retina. Proliferation in aggregates is sustained over a longer period with PE, as compared with aggregates formed by retinal cells alone. AChE staining in aggregates in the presence of PE shows a layered appearance, while there is only crude sorting-out of labeled and unlabeled cells in aggregates composed of retinal cells only. The basis of PE cell action as well as the relevance of this in vitro system for understanding normal eye development are briefly discussed.