BLUE-CONE HORIZONTAL CELLS IN THE RETINAE OF HORSES AND OTHER EQUIDAE

The morphology of horizontal cells chiefly of the horse, but also of a sses, mules, and a zebra, has been examined by Lucifer yellow injectio ns into lightly fixed retinae and by immunocytochemistry. In common wi th other mammals, equids have a B-type horizontal cell, i.e., a cell w ith dendrites synapsing with cones and possessing a single axon synaps ing with rods. Most mammalian retinae have a further type of horizonta l cell, the A-type, also synapsing with cones but without an axon, The second type of horizontal cell in equids also has no axon; otherwise, it is most unusual. Compared with other mammalian A-type cells, it ha s a very large dendritic field, both absolutely and relative to the de ndritic fields of B-type cells, The dendrites are fine and sparsely br anching. Their most striking feature is that they bear a low density o f irregularly spaced synaptic terminal aggregates, suggesting their co ne contacts are selective. Immunolabeling of S (blue)-cones in horse r etina showed that they comprise, depending on retinal location, 10-25% of the cone population. For a single horse A-type cell, it is shown t hat 44 of its 45 terminal aggregates are congruent with the pedicles o f S-cones. Immunostaining with a calbindin antibody demonstrated that each type of horizontal cell forms an independent regular mosaic. The density ratio of B- to A-type cells varied between 5 and 10. This is t he first demonstration in a mammalian retina of a horizontal cell type with a direct input exclusively from S-cones.