Retinal regeneration after injury induced by gamma-ray irradiation during early embryogenesis in medaka,

Zebrafish, a small fish model, exhibits a multipotent ability for retinal regeneration after damage throughout its lifetime. Compared with zebrafish, birds and mammals exhibit such a regenerative capacity only during the embryonic period, and this capacity decreases with age. In medaka, another small fish model that has also been used extensively in biological research, the retina's inner nuclear layer (INL) failed to regenerate after injury in the hatchling at eight days postfertilization (dpf). We characterized the regenerative process of the embryonic retina when the retinal injury occurred during the early embryonic period in medaka. We employed a 10 Gy dose of gamma-ray irradiation to initiate retinal injury in medaka embryos at 3 dpf and performed histopathological analyses up to 21 dpf. One day after irradiation, numerous apoptotic neurons were observed in the INL; however, these neurons were rarely observed in the ciliary marginal zone and the photoreceptor layer. Numerous pyknotic cells were clustered in the irradiated retina until two days after irradiation. These disappeared four days after irradiation, but the abnormal bridging structures between the INL and ganglion cell layer (GCL) were present until 11 days after irradiation, and the neural layers were completely regenerated 18 days after irradiation. After gamma-ray irradiation, the spindle-like Müller glial cells in the INL became rounder but did not lose their ability to express SOX2. Irradiated retina at 3 dpf of medaka embryos could be completely regenerated at 18 days after irradiation (21 dpf), although the abnormal layer structures bridging the INL and GCL were transiently formed in the retinas of all the irradiated embryos. Four days after irradiation, embryonic medaka Müller glia were reduced in number but maintained SOX2 expression as in nonirradiated embryos. This finding contrasts with previous reports that 8 dpf medaka larvae could not fully regenerate damaged retinas because of loss of SOX2 expression.