Retinal photoreceptor and related gene expression in normal and clinostat-treated fish embryos

Medaka fish had performed mating behavior successfully in space for the first time among vertebrate, and the eggs which were laid in space developed normally, and hatched during the space travel of 15 days aboard the space shuttle in the second International Microgravity Laboratory (IML-2) mission in 1994 (Ijiri 1994). But there has been few studies whether microgravity affects the development of rather complex tissues in this fish. Investigating this problem, we focused on the organogenetic events in the retina in developing Medaka under normal and simulated microgravity conditions (by a three-dimensional clinostat, 3D-clinostat). Our results showed that both normal and 3D-clinostat-treated Medaka embryos developed on almost equal time course. Moreover, we investigated the development of the retina in normal and 3D-clinostat-treated embryos, but there were no differences in organogenesis of their retina. Lamination of retina occurred almost at equal timing and the expressions of opsin genes in the 3D-clinostat-treated group also began almost at the same time as control. Our observations suggest that there were no definite effects of simulated microgravity on the organizations of a complex tissue such as retina in developing fish embryos.