The pigmented epithelium sustains cell growth and tissue differentiation of chicken retinal explants in vitro

Embryonic retinae from 5-6-day-old chicks (E5-E6) were cut into stripes either in close contact with (RPE stripes) or in absence of the neighboring retinal pigmented epithelium (R stripes). The stripes were explanted and cultivated in vitro for up to 6 days, during which time they show the following differences in their characteristics of growth and differentiation. Compared with R stripes, RPE stripes morphologically showed a significant increase in size during the first 2 days in culture. Using E5 tissue, this is also demonstrated by a higher rate of cell proliferation (as measured by uptake of radioactive thymidine as well as by DNA contents). In contrast, R stripes after two days in culture show a much stronger neurite growth. After longer periods of culturing (5-6 days) we can show by cholinesterase histochemistry (AChE and BChE) and by PNA-lectin binding that the RPE stripes have started to form all major layers of the in vivo retina, whereas R stripes remain unstratified and start to degenerate earlier. We conclude that the pigment epithelium might exert a specific stimulus on growth and tissue differentiation of the neural retina not only during in vitro, but possibly also during in vivo development. The in vitro methods introduced here could become useful model systems to further investigate the significance of the RPE for developmental, regenerative and even adult processes of the neural retina. Their future applicability in ophthalmologic research is briefly discussed.