Cone photoreceptor shedding in the tree shrew (Tupaia belangerii).

Tree shrews were sacrificed at various times during a 12 h light-12 h dark cycle and the retinal pigment epithelium (RPE) was examined for phagosomes. Analysis of photoreceptor densities showed that the tree-shrew retina consists of approximately 96% cone photoreceptors. Therefore, phagosomes in the RPE were assumed to be mostly those of cones. A peak in the number of RPE phagosomes was found about one hour after the onset of light. The number of phagosomes/mm RPE during the light cycle varied from 17.02 at the peak to 2.49 ten hours after light onset. During the dark cycle, values ranged from 0.10 to 0.61 phagosomes/mm RPE. Size profiles of phagosomes showed that large phagosomes peak in number 1/2 h after light onset, while smaller sizes peak at about 1 h after light onset. This may indicate that maximal shedding and phagocytotic activity occurs sometime before the peak in the total number of phagosomes is reached. Statistical corrections for phagosome size, section thickness and phagosomal degradation time were applied to the data in order to assess outer segment renewal time for tree shrew cones.