Fine structure of the retinal epithelium and retinal tapetum lucidum of the goldeye Hiodon alosoides.

The fine structure of the retinal epithelium of the goldeye (Hiodon alosoides) has been investigated with electron microscopy in both the light- and dark-adapted condition. The retinal epithelial cells are extremely tall columnar cells. They form a single layer abutting on Bruch's membrane basally (sclerally) and interdigitating with bundles of photoreceptor cells apically (vitreally). The retinal epithelial cells and the photoreceptor bundles are arranged in a hexagonal fashion so that each epithelial cell normally contacts three photoreceptor bundles and three other epithelial cells. In the light-adapted state, melanosomes are sparsely but evenly distributed throughout the cell body except for the apices, which contain more melanosomes. In dark-adaptation, melanosomes become very concentrated in the cell apex between photoreceptor bundles, with some accumulation of melanosomes also noted basally. The predominant feature of these epithelial cells is the presence of numerous reflective bodies which act as a retinal tapetum lucidum. During light-adaptation these reflective bodies are scattered throughout the cell but in dark-adaptation they accumulate centrally. In the body of the epithelial cell these reflective particles are diffusely arranged, whereas they are regularly arranged where epithelial cells ensheath photoreceptor bundles. Circadian changes are also noted in the morphology of the myeloid bodies and mitochondria and in the location of the epithelial cell nucleus. Wandering phagocytes are present at the retinal epithelial-photoreceptor interface. Bruch's membrane is thick and trilaminate and the choriocapillaris endothelium is thin and sparsely fenestrated.