Relationship between Distribution of Tapetum Fibrosum and Retinal Pigment Epithelium in the Sheep Eye

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The tapetum lucidum is a light-reflective device improving visual sensitivity in mesopic vision. There have been few studies on tapetal distribution and its relationship with degree of pigmentation in the retinal pigment epithelium (RPE). In the present study, sheep's eyes were used for macroscopical observation of the tapetum, and then histological sections of the posterior eyecups were made to analyze the distribution of tapetal thickness and degree of pigmentation in the RPE. Macroscopically, with available light, the tapetum was located in the dorsal eye fundus to the optic disc and showed an L-shape with the horizontally elongated nasal part and the dorsally expanded temporal part. In photographs with a flash, the tapetal area expanded and showed a more triangular shape. The tapetum histologically consisted of layers of dense collagen fibers and was thicker in the temporal part than in the nasal part. The maximum tapetal thickness was approximately 70 μm. The histological tapetal area was similar to the tapetal shape with a flash light. The pigmentation of the RPE was divided into three types, nonpigmented, transitional, and pigmented areas. The nonpigmented area was similar to the tapetal shape with available light. It is suggested that approximately 55% of the histological tapetal area is covered with the nonpigmented area and is functional under a natural light condition. The functional tapetal area was similar to the L-shaped high-density area of retinal ganglion cells.