DIFFERENTIATION OF PHOTORECEPTORS, GLIA, AND NEURONS IN THE RETINA OFTHE CICHLID FISH AEQUIDENS PULCHER - AN IMMUNOCYTOCHEMICAL STUDY

Light-microscopic immunocytochemistry was carried out to investigate t he developmental dynamics of several neurochemical markers in the reti na of blue acara (Aequidens pulcher). As a rule, double-label experime nts were performed in order to determine the absolute and relative tim ing of the appearance of these markers. The diameter of eye-ball (from 0.6 to 1.2 mm) and the body length (from 4.6 to 9.4 mm) enlarged in p arallel during the observation period of 2 to 9 days after spawning (d ay 2-9); hatching took place usually on day 2. Immunoreactive prolifer ating cell nuclear antigen (ir-PCNA) was present in all neuroblasts (t he embryonic homogeneous cell stage; day 1.0-2.0), but was lost progre ssively in a center-to-periphery and apparent proximal-to-distal seque nce as the cells and layers differentiated. In late larvae and juvenil es, ir-PCNA was confined to a ring of dividing neuroblasts at the reti nal margin and to a population of scattered rod precursors in the oute r nuclear layer. Immunoreactive structures of representative antigens progressively appeared after ir-PCNA had decayed. Around hatching, at the synaptic separation stage (day 2.0-2.5), luteinizing hormone-relea sing hormone-ir centrifugal fibers, visinin-ir cones, glial fibrillary acidic protein-ir structures and gamma-aminobutyric acid-ir cell bodi es appeared, which were followed by the emergence of rhodopsin-ir rods and tyrosine hydroxylase-ir interplexiform cells (on day 2.5-3.0) and serotonin-, neuropeptide Y- and substance P-ir amacrine cells (on day 3.0-4.0). The results indicate that photoreceptor cells, and especial ly rods start to differentiate at an earlier stage of retinogenesis th an has previously been proposed. In addition, an extraretinal tissue i n the brain identified as the prospective pineal organ was found to be visinin- and rhodopsin-immunoreactive on day 1.5-2.0 before these pho toreceptor-specific antigens became positive in the retina.