Retinal regeneration after injury induced by gamma-ray irradiation during early embryogenesis in medaka, <i>Oryzias latipes</i>

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.斑马鱼（Zebrafish）作为小型鱼类模型，在其整个生命周期中，视网膜损伤后均具备视网膜再生的多能能力。与斑马鱼相比，鸟类与哺乳动物仅在胚胎阶段具备此类再生能力，且该能力随年龄增长逐渐衰退。青鳉（medaka）是另一类被广泛应用于生物研究的小型鱼类模型，其受精后8天（days postfertilization, dpf）的孵化幼体视网膜内核层（inner nuclear layer, INL）在损伤后无法完成再生。本研究针对青鳉早期胚胎期发生视网膜损伤时的胚胎视网膜再生过程进行了系统表征。我们采用10戈瑞剂量的γ射线照射，于青鳉胚胎受精后3天（3 dpf）诱导视网膜损伤，并直至受精后21天（21 dpf）开展组织病理学分析。照射后1天，内核层中可见大量凋亡神经元，但睫状缘区与光感受器层中极少观测到此类神经元。直至照射后2天，受照射视网膜内仍存在大量聚集的固缩细胞；该类细胞于照射后4天消失，但内核层与神经节细胞层（ganglion cell layer, GCL）间的异常桥接结构持续至照射后11天，而视网膜神经层于照射后18天完全再生。γ射线照射后，内核层中的纺锤形穆勒胶质细胞（Müller glial cells）形态变得更趋圆形，但并未丧失SOX2的表达能力。尽管所有受照射胚胎的视网膜中均一过性形成了内核层与神经节细胞层间的异常层状结构，但受精后3天的青鳉胚胎受照射视网膜可于照射后18天（即21 dpf）完成完全修复。照射后4天，胚胎青鳉的穆勒胶质细胞数量有所减少，但仍与未照射胚胎一样维持SOX2的表达。该发现与既往研究报道相悖：既往研究指出，受精后8天的青鳉幼体因SOX2表达缺失，无法完全修复受损视网膜。