Horizontal cell differentiation in the retina of the Brazilian opossum, Monodelphis domestica

Abstract Differentiation of many diverse neuronal phenotypes is an essential part of nervous system development. We have studied the differentiation of horizontal cells, one of the basic neuronal types in the vertebrate retina, in a small, easily maintained marsupial by immunocytochemistry using antineurofilament and antivimentin antibodies. At birth the retina consists of proliferating neural epithelial cells, with a few early ganglion cells. Horizontal cells were first detected in 12-day-old pups; somas were within the epithelial neuroblastic layer and processes extended radially. By 19 days there were tangentially oriented dendrites and a few longer processes, the beginning of the outer plexiform (first synaptic) layer. By the time of eye opening (about 34 days) the basic histological organization of the mature retina was established. In the mature retina and during development, horizontal cell neurites in the outer plexiform layer, as well as ganglion cell axons, reacted strongly with several antineurofilament antibodies and with antivimentin; horizontal cell somas were detected only with one antineurofilament antibody. Only one population was detected, which we identify as the short-axon subtype, by comparison with horizontal cells in other marsupials and in eutherian mammals. This is the first description of the putative absence in a marsupial of one of the two horizontal cell subtypes found in most amniotes, including mammals so far studied, except murid rodents, which have only the short-axon subtype. Absence of one subtype in Monodelphis supports the hypothesis that the short-axon cell is the basic conserved phenotype of this class and suggests that experimental analysis of differentiation of horizontal cells in Monodelphis and murid rodents, compared to marsupials and eutherian mammals which have the basic two subtypes, can help elucidate mechanisms for controlling differentiation of specific cellular phenotypes and the variations in neurons within and among species.