Fine structure of the retinal epithelium (RPE) of the emu (Dromaius novaehollandiae)

As part of an ongoing comparative fine structural study, the retinal pigment epithelium (RPE), choriocapillaris and complexus basalis (Bruch's membrane) in the emu (Dromaius novaehollandiae) have been investigated by light and electron microscopy. The RPE consists of a single layer of cuboidal cells joined basally by a series of tight junctions. Basally (sclerally) the RPE cells display numerous deep infoldings while apically (vitreally) plentiful microvillar processes interdigitate with the photoreceptor outer segments. Internally the epithelial cells show a large vesicular nucleus, plentiful smooth endoplasmic reticulum (SER) and numerous polysomes, but very little rough endoplasmic reticulum (RER). Numerous pleomorphic mitochondria are predominantly basally located. In the light-adapted specimens studied, the melanosomes of the RPE are almost exclusively located within the apical processes of these cells. Phagosomes and lysosome-like bodies are also present, as are myeloid bodies which often display ribosomes on their outer surface. Bruch's membrane (complexus basalis) is typical of avian species in that it is pentalaminate and the central lamina densa is displaced towards the choriocapillaris. The choricocapillaris endothelium is attenuated but only minimally fenestrated facing Bruch's membrane. Most of these fenestrations show a single-layered diaphragm but fenestrations with a double-layered diaphragm are not uncommon.