In vitro studies on shedding and phagocytosis of rod outer segments in the rat retina: effect of oxygen concentration.

The processes of outer segment shedding and phagocytosis were found to be significantly enhanced in explants of rat retina and pigment epithelium (PE) newly placed in culture. The number of phagosomes reached a maximum at 2 hr of incubation and then rapidly decreased, as is observed in rats entrained in a normal light-dark schedule when the lights are turned on in the morning. This pattern was not qualitatively affected by the time of day the tissues were excised or by the presence of a high concentration of melatonin in the culture medium. The shedding peak in vitro was observed only when the retina-PE unit was incubated in an atmosphere composed of 45% oxygen, 5% CO2, and 50% air. When the atmosphere contained lower (95% air and 5% CO2) or higher (95% O2 and 5% CO2) oxygen, PE cells contained only a few scattered phagosomes. These functional differences may be caused by different arrangement of PE cell microvilli at a "moderate" (45%) O2 concentration vs. "low" or "high" O2 concentrations. A balanced O2 level thus appears to be critical for phagocytosis in explant culture.