Fine structure of the retinal epithelium, Bruch's membrane, and choriocapillaris of the pigeon (Columba livia)

Abstract

As part of a comparative morphological study, the fine structure of the retinal pigment epithelium, the choriocapillaris, and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the pigeon (Columba livia). In this species the retinal pigment epithelium consists of a single layer of cuboidal cells, which show numerous very deep basal (scleral) infoldings and extensive apical (vitreal) processes enclosing photoreceptor outer segments. These epithelial cells are joined laterally by prominent basally located tight junctions. Internally, smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Polysomes are abundant as are mitochondria, which often display a ring-shaped structure. The cell nucleus is large and vesicular. Melanosomes are plentiful in both the apical region of the cell body and within the apical process in the light-adapted condition. Myeloid bodies are large and numerous and very often have ribosomes on their outer border. Bruch's membrane shows a pentalaminate structure but with only a poorly represented central elastic lamina. Profiles of the choriocapillaris are relatively small and not overly abundant. The endothelium of the choriocapillaris, while extremely thin facing the retinal epithelium, is but minimally fenestrated.