Retinal pigment epithelial fine structure in the Australian Galah (Eolophus roseicapillus) (Aves)

As part of a comparative morphological study, the fine structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in the galah (Eolophus roseicapillus), an Australian cockatoo. The RPE consists of a single layer of low cuboidal cells joined basally by a series of zonulae occludentes. Basally (sclerally) the retinal epithelial cells display numerous deep infoldings while apically (vitreally) microvillar processes interdigitate with photoreceptor outer segments. Internally the RPE cells show a large vesicular nucleus, plentiful smooth endoplasmic reticulum (SER) and numerous polysomes but very little rough endoplasmic reticulum (RER). Numerous mitochondria are located basally. In the light-adapted specimens studied, the melanosomes of the RPE are almost exclusively located within the apical processes indicating that retinomotor movement of this pigment probably occurs. Phagosomes and lysosome-like bodies are present as are myeloid bodies which may show ribosomes on their outer surface. The choriocapillaris endothelium is thin but only minimally fenestrated facing Bruch's membrane. Most fenestrations of the choriocapillaris display a single-layered diaphragm while the remainder have a double-layered diaphragm. Bruch's membrane (complexus basalis) is typical of avian species in that it is pentalaminate with the central lamina densa displaced towards the choriocapillaris.