Phagosome Number and Distribution in Retinal Pigment Epithelial Cells of Vitiligo Mutant Mice

The vitiligo mutant mouse has a disorder affecting the interaction of retinal pigment epithelium (RPE) and photoreceptor cells of the neural retina. Among the phenotypic features are patches of hyper- and hypopigmentation in the embryonic RPE, increased RPE cell production neonatally, and a later onset of progressive photoreceptor cell degeneration that continues for more than one year until all photoreceptor cells are gone. Failure of RPE microvilli to intertwine with rod outer segments (ROS) at any age, the accumulation of ROS membranous fragments in the subretinal space, and a relatively early retinal separation from the RPE suggested analysis of whether RPE phagocytosis might be impaired. Post-natal day 23 (P23) and P36 mutant and congenic control wild-type mice were kept in darkness overnight and eyes were examined by light and transmission electron microscopy 0.5 hr before, 1.5 hr after and 10.5 hr after lights turned on at 0700 hr. At these ages ROS have not yet degenerated, though they are shorter than normal and somewhat misoriented. The number of phagosomes per RPE cell was markedly reduced in mutants compared to controls at both ages and all time points. Nonetheless, the highest counts were obtained 1.5 hr after the lights turned on in mutant and control specimens. In the mutant eyes, the proportion of phagosomes in the microvillous zone of the RPE cells was consistently lower than in any other cellular compartment. Phagosome distribution in the apical and basal zones of the RPE cell cytoplasm was within normal limits. Macrophage-like cells become numerous in the subretinal space at older ages, but were already present at P23 and P36, and contained phagosomes in their cytoplasm. The hypothesis is proposed that binding of ROS to RPE cells might be defective in vitiligo mice, in contrast to the rdy rat, where the work of others indicates that binding is normal and the subsequent ingestion of phagosomes is impaired.