LACTOSERIES CARBOHYDRATE EPITOPES IN THE VERTEBRATE RETINA

The distribution of N-acetyl-lactosamine (NALA), a cell-surface carboh ydrate epitope of the lactoseries, has been studied in the retina of r epresentative species of all vertebrate classes by Light microscope im munohistochemistry. In only some species of different classes (fish, a mphibia and mammals) was NALA expression detected, and in these animal s the distribution showed profound interspecies variability. In fishes and amphibia in which NALA was present, patterns ranged from single i mmunopositive cells to homogeneous labelling of cell layers. In mammal s, NALA was found only in retinas that are cone dominated (tree squirr el and primates). In the tree squirrel, there was a dense cellular sta ining of the photoreceptor cell layer; whereas in primates, the carboh ydrate epitope occurred only on some photoreceptor cells. From these r eceptor cells, positive axons could be traced to the inner plexiform l ayer. In spite of the profound interspecies differences, NALA is not r andomly expressed, as its exclusive expression in mammals with cone-do minated vision indicates. The suggestion of a functional relevance for NALA glycosylation of retinal cells is supported by the labelling pat tern for HNK-1 in these species, which was different from the pattern found in rod-dominated mammalian retinas.