THE MORPHOLOGY AND DISTRIBUTION OF HORIZONTAL CELLS IN THE RETINA OF A NEW-WORLD MONKEY, THE MARMOSET CALLITHRIX-JACCHUS - A COMPARISON WITH MACAQUE MONKEY

The morphology and distribution of horizontal cells was studied in the retina of a New World monkey, the marmoset, Callithrix jacchus, and c ompared with that of the Old World macaque monkey. Horizontal cells in macaque and marmoset were either labelled with the carbocyanine dye, DiI, and then photoconverted, or were labelled by intracellular inject ion with Neurobiotin. The marmoset has two types of horizontal cell, H 1 and H2, which have dendritic and axonal morphology similar to their counterparts in Old World monkeys and human. The dendritic-field size of both cell types increases with distance from the fovea. Both types make contact with the vast majority of the cones within their dendriti c field. The dendrites of H1 cells in marmoset contact almost twice as many cones as H1 cells in macaque at an equivalent eccentricity. With increasing distance from the fovea, H1 cells make contact with more c ones but have, on average, fewer terminal knobs inserted in each cone. The increase in dendritic-field area of H1 cells is balanced by a dec rease in spatial density (from 4500 cells/mm(2) at 25 deg eccentricity to 1000 cells/mm(2) in far peripheral retina), so coverage of the ret ina remains fairly constant, between 5 and 8. Overall, the results sho w that the qualitative morphological properties, as well as quantitati ve population properties of horizontal cells, are common to both New W orld and Old World primates.