EFFECTS OF RETINOIC ACID ON EPIBOLY, CONVERGENCE AND GASTRULATION OF EMBRYOS OF THE MEDAKA, ORYZIAS-LATIPES (TELEOSTEI, CYPRINODONTIDAE)

The present study reports the early effect of exogenously applied reti noic acid (RA) on embryos of the Japanese medaka, Oryzias latipes. Fur thermore, as not much is known about early medaka development, a morph ological description of control embryos is given and compared with wel l studied early cyprinid development. In medaka embryos epiboly and co nvergence roughly start at the same stage. Gastrulation, resulting in an outer epiblast and an inner hypoblast layer, is observed from about 25% epiboly onwards and is restricted to the dorsal side, since at th at stage converging cells are already located relatively dorsally. RA treatment leads, in a stage- and dose-dependent way, to a shortening o f the embryonic axis. This may be caused by a complete or partial fail ure of extension movements which may occur during convergence. Converg ence and gastrulation appear to be, at least partly, unaffected by RA since the embryo proper is located dorsally and consists of two separa te germ layers. Observation of medaka embryos until 4 days post fertil ization (dpf) revealed that RA treatment caused poorly developed anter ior structures (eyes, brain, heart, blood vessels). However, pectoral fins and melanin granules were always present.