Retinal pigment epithelial fine structure in the nighthawk (Chordeiles minor).

The retinal pigment epithelial (RPE) layer of a nocturnally active bird, the nighthawk (Chordeiles minor), has been investigated by electron microscopy. This layer is composed of a single layer of low cuboidal cells joined laterally by tight junctions. Basally (sclerally), the epithelial cells display deep infoldings while apically (vitreally) numerous processes enclose photoreceptor outer segments. Internally, the nucleus is large, vesicular and basally located. Smooth endoplasmic reticulum and mitochondria are abundant. Rough endoplasmic reticulum (RER) is scarce but numerous polysomes are present. Myeloid bodies are widespread and often appear to be continuous with the RER membranes. Phagosomes of outer segment material are present but not numerous in the light-adapted condition. Large spindle-shaped melanosomes as well as numerous rounded tapetal spheres are present in the apical region and apical processes of the epithelial cells. The composition of the reflective tapetal spheres is unknown. Bruch's membrane (complexus basalis) in most locations displays a pentalaminate appearance while in some regions it is trilaminate. The endothelium of the choriocapillaris facing Bruch's membrane is extremely thin but only sparsely fenestrated.