Fine structure of the retinal epithelium of the tiger salamander (Ambystoma tigrinum).

The retinal pigment epithelium (RPE) of the tiger salamander (Ambystoma tigrinum) consists of a single layer of large squamous-shaped cells. These RPE cells are but minimally infolded basally (sclerally) but show numerous apical (vitreal) processes that interdigitate with the photoreceptor cells. A series of apically-located tight junctions joins the RPE cells to form part of the blood-ocular barrier. Internally the RPE nucleus is large, vesicular and flattened. Smooth endoplasmic reticulum predominates in these cells with only occasional small profiles of rough endoplasmic reticulum present. Polysomes and basally-located mitochondria are plentiful. Golgi zones, lipid droplets, lysosome-like bodies and phagosomes of outer segment material are also present. Myeloid bodies which may show ribosomes on their surface are also numerous. The round to oval melanosomes undergo retinomotor movements and in the light-adapted state reach almost to the external limiting membrane. Bruch's membrane or complexus basalis shows a pentalaminate structure with the central elastic lamina poorly represented. The choriocapillaris is a single layer of large anastomosing capillaries which are but minimally fenestrated.