The fine structure of the tapetum of the cat eye has been investigated by electron microscopy. The tapetum is made up of modified choroidal cells, seen as polygonal plates grouped around penetrating blood vessels which terminate in the anastomosing capillary network of the choriocapillaris. The tapetal cells are rectangular in cross-section, set in regular brick-like rows, and attain a depth of some thirty-five cell layers in the central region. This number is gradually reduced peripherally, and is replaced at the margin of the tapetum by normal choroidal tissue. The individual cells are packed with long slender rods 0.1 µ by 4 to 5 µ. The rods are packed in groups and with their long axes oriented roughly parallel to the plane of the retinal surface. Each cell contains several such groups. Cells at the periphery or in the outer layers of the tapetum are frequently seen to contain both tapetal rods and melanin granules, the latter typical of the choroidal melanocytes. Also melanocyte granules may have intermediate shapes. These observations plus the similar density of the two inclusions lead to the belief that the tapetal rods may be melanin derivatives.
A fibrous connective tissue layer lies between the tapetum and the retina. The subretinal capillary network, the choriocapillaris, rests on this layer and is covered by the basement membrane of the retinal epithelium. The cytoplasm of the retinal epithelium exhibits marked absorptive modifications where it comes in contact with the vessels of the choriocapillaris. This fibrous layer and the basement membrane of the retinal epithelium apparently comprise the structural elements of Bruch's membrane.