Phagosome movement and the diurnal pattern of phagocytosis in the tapetal retinal pigment epithelium of the opossum.

The diurnal pattern of phagocytosis and the movement of phagosomes was studied in the tapetal retinal pigment epithelium (RPE) of the opossum, Didelphis virginiana. The opossum was chosen because of its rod-dominated retina and large tapetal RPE cells (up to 100 micrometers in height), which are packed with reflective granules and contain little melanin. Thus phagosomes and their passage from apical to basal cell border were easily seen. Opossums were maintained on a 12 hr light/12 hr dark cycle and were sacrificed during the day and night. Phagosomes were consulted by light microscopy in sections extending 2 mm along the eye's vertical meridian. The diurnal pattern of rod phagocytosis was generally similar to that reported for other species, although an elevated phagosome content in two animals that in some cases there might be a smaller nighttime peak in addition to the large burst occurring after light onset. To determine the spatial distribution of phagosomes at different times, the RPE cells were divided into apical, middle, and basal thirds, and the phagosomes in each region were counted. A large number of phagosomes was observed in the basal RPE 1 to 2 hr after light onset, after which the number declined, reaching a low level late in the light period. In contrast, there were few phagosomes in the apical and mid-RPE even after light onset. This suggested that phagosomes remained in the apical and mid-RPE for only a short time. To examine its effect on phagosome movement, colchicine was injected intravitreally prior to the burst of phagocytosis. Colchicine blocked the movement of phagosomes producing a row of phagosomes along the apical margin of the RPE. These results suggest that there is a rapid, microtubule-mediated movement of phagosomes from apical to basal border.