Electron microscopic observations on the retinal pigment epithelium and choroid in the domestic pig (Sus scrofa)

Summary The morphology of the retinal pigment epithelium (RPE) and adjacent choroid has been investigated by electron microscopy in the domestic pig. The RPE consists of a single layer of cells which are columnar posteriorly but become cuboidal and even squamous moving peripherally in the fundus. The cells of the RPE layer regardless of location display basal (scleral) infoldings and apical (vitreal) processes and are joined laterally by junctional complexes. Throughout the retina the epithelial cells are rich in smooth endoplasmic reticulum and mitochondria but less so in rough endoplasmic reticulum and polysomes. The epithelial nucleus is vesicular and basally located. In the superior fundus an area of the RPE is very lightly pigmented and richer in lysosomes than is this layer in the inferior and peripheral fundus. The choroid overlying this area is also lightly pigmented and contains much collagen in a lamellar arrangement. This region may represent a vestigial tapetum fibrosum. Bruch's membrane is slightly thicker posteriorly but is everywhere seen to have a pentalaminate substructure. The choriocapillaris is a single layer of large capillaries which show numerous fenestrations facing the RPE. In the superior fundus the choriocapillaris is also highly fenestrated facing the choroid.