Fine structure of the retinal pigment epithelium in the mud minnow (Umbra limi)

Abstract

The fine structure of the retinal pigment epithelial layer and associated regions has been studied by electron microscopy in the adult mud minnow Umbra limi. The pigment epithelium is composed of a single layer of large columnar cells. Each epithelial cell has abundant small mitochondria, much smooth endoplasmic reticulum (often in highly organized arrays), myeloid bodies, phagosomes and pigment granules. Rough endoplasmic reticulum and ribosomes are scarce. The scleral or basal border of the epithelial cells is but minimally infolded whereas the vitreal or apical surface displays numerous elongated processes which surround the inner and outer segments of the photoreceptors. Unattached, presumably phagocytic cells are a constant feature both between the retinal epithelial cells and within Bruch's membrane. Bruch's membrane lacks a central elastic layer and is composed only of three layers. The endothelial wall of the choriocapillaris bordering Bruch's membrane is typically very thin with a few fenestrations. This region of the mud minnow eye is morphologically similar to that described in other teleost species but differs from that described in most mammals.