Effects of alteration in the light cycle on outer segment shedding in the fetal retina

Shedding of outer segment discs and their phagocytosis by retinal pigment epithelial cells (RPE) in the guinea pig starts to occur during in utero development. The number of phagosomes in the fetal RPE varies in a cyclical manner somewhat analogous to that observed in mature animals. The present study addresses the question of whether the cycles in fetus and mother are independent or linked. Pregnant guinea pigs were subjected to 10 days of alteration of their lighting schedules beginning on the 46th day of gestation. Dams and fetuses were sacrificed on day 56, and large phagosomes in the RPE of each were counted. A 10-hour phase advance was employed, ie, from 0800 (lights on): 2000 (light off) in controls, to 2200 (on): 1000 (off) in experimental animals. Counts of large phagosomes in the posterior retina were compared in both control and experimental nulliparous, gravid, and fetal animals. A significant burst of shedding occurred within one hour after light onset in the experimental groups comprised of either nulliparous or pregnant animals. The latter also showed a smaller rise in phagosome numbers at the original time of onset of illumination. In contrast, no shift in the time of maximum shedding activity was observed in fetuses in the experimental regime. Their activity was similar to that present in the original 0800:2000 cycle. Whereas maternal RPE shedding cycles can be reentrained in a 10-day period, those in the 46- to 56-day fetal retina are refractory to significant change. Based on this finding, it is likely that the shedding cycles in the mother and fetus at this age are under independent control, and, therefore, that shedding in the fetus is not mediated by maternal borne factors.