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 retinoic acid (RA) on embryos of the Japanese medaka, Oryzias latipes. Furthermore, as not much is known about early medaka development, a morphological description of control embryos is given and compared with well studied early cyprinid development. In medaka embryos epiboly and convergence 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 that stage converging cells are already located relatively dorsally. RA treatment leads, in a stage- and dose-dependent way, to a shortening of the embryonic axis. This may be caused by a complete or partial failure of extension movements which may occur during convergence. Convergence and gastrulation appear to be, at least partly, unaffected by RA since the embryo proper is located dorsally and consists of two separate germ layers. Observation of medaka embryos until 4 days post fertilization (dpf) revealed that RA treatment caused poorly developed anterior structures (eyes, brain, heart, blood vessels). However, pectoral fins and melanin granules were always present.