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

Abstract

Purpose

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.

Methods

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.

Results

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.

Conclusions

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.

Keywords:

• Medaka
• radiation
• retinal regeneration
• Müller glia
• SOX2

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan to TY (16K00541).

Author contributions

Conceptualization, TY; data curation, TY; formal analysis, TY; funding acquisition, TY; investigation, TY, TN, KH, IM, KN, KI, and HS; project administration, TY; supervision, TY; validation, TY; visualization, TY; writing – original draft, TY; writing – review and editing, TY, KN, KI, SO, and HM.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan to TY (16K00541).

Notes on contributors

Takako Yasuda

Takako Yasuda, Ph.D., is a researcher at the Department of Chemical and Biological Sciences, Japan Women’s University, Tokyo, Japan.

Takuya Nakazawa

Takuya Nakazawa, M.Sc., was a student at the Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.

Kei Hirakawa

Kei Hirakawa, M.Sc., was a student at the Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.

Ikumi Matsumoto

Ikumi Matsumoto, was a technical assistant at the Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.

Kento Nagata

Kento Nagata, Ph.D., is a researcher at the Department of Radiation Effects Research, Institute for Radiological Science, National Institutes for Quantum Science and Technology, Chiba, Japan.

Shunta Mori

Shunta Mori, M.Sc., was a student at the Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.

Kento Igarashi

Kento Igarashi, Ph.D., is an assistant professor at the Department of Applied pharmacology, Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima, Japan.

Hiroshi Sagara

Hiroshi Sagara, Ph.D., is an assistant professor of Medical Proteomics Laboratory, Institute of Medical Science, The University of Tokyo.

Shoji Oda

Shoji Oda, Ph.D., is an associate professor of Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.

Hiroshi Mitani

Hiroshi Mitani, Ph.D., is an emeritus professor of Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan.