Retinal pigment epithelial fine structure in the American crow (Corvus brachyrhynchos).

The fine structure of the retinal epithelial (RPE) region has been investigated by light and electron microscopy in the American crow (Corvus brachyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous deep basal (scleral) infoldings and plentiful apical (vitreal) microvillar processes which surround photoreceptor inner and outer segments. The RPE cells are joined laterally by a series of tight junctions (Verhoeff's membrane) located in the mid to basal region. Within the epithelial cells, smooth endoplasmic reticulum is very abundant while rough endoplasmic reticulum (RER) is scarce. Mitochondria of various shapes are abundant basally while polysomes are plentiful and widespread. In the light-adapted state RPE nuclei are large and vesicular and basally located while the melanosomes of these cells are predominantly located with the apical processes indicating photomechanical movements. Myeloid bodies are large and numerous and often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) is typical of avian species in that it is pentalaminate and the lamina densa is displaced near the choriocapillaris. The endothelium of the choriocapillaris is thin facing Bruch's membrane but is only moderately fenestrated. Some of these fenestrations display a double-layered diaphragm while the majority show the more typical single-layered diaphragm noted in most species.