Morphology of the tapetum fibrosum in the eye of the domestic sheep

The fine structure of the tapetum lucidum of the domestic sheep was studied by both light and electron microscopy. In this species the reflective layer is a tapetum fibrosum situated in the choroid and restricted to the superior fundus. The tapetum is composed of a large array of fine extracellular collagen fibrils partially separated into lamellae 2–5 μm thick by fibrocytes. The collagen fibrils display the normal cross striations of native collagen, are 0.2 μm in diameter, and are arranged in a hexagonal pattern with a space of 0.2–0.25 μm between fibrils. The diameter and spacing of these fibrils is consistent with the constructive interference of reflected light. A vascular lamella adjacent to the retinal epithelium contains the choriocapillaris, pericytes, and irregularly arranged collagen fibrils. Over the tapetum the retinal epithelium is nonpigmented while in nontapetal regions it is pigmented. Bruch's membrane is pentalaminate in both tapetal and nontapetal regions.