The tumor suppressor protein p53 is considered a guardian of genome integrity, regulating the induction of apoptosis and cell cycle arrest in response to irradiation to block the transmission of teratogenic mutations to progeny cells. We examined the function of p53 in highly radiosensitive tissues, the developing brain and mature testis, using a small fish model, medaka (Oryzias latipes). Medaka offer advantages as a vertebrate model system, as the transparency and small size of the embryos enables clear detection of apoptotic cells in the developing brain. In addition, the simple architecture of medaka testes enables more precise identification of the differentiating spermatogenic stages compared with mammals. We found that in irradiated p53-deficient embryonic brain, diminished induction of apoptosis facilitated tissue regeneration earlier compared to irradiated wild-type embryos, which remained structural abnormalities in the retina at hatching. Moreover, the prominent delay in apoptotic induction in irradiated p53-deficient testes could induce transient mis-differentiation during spermatogenesis, such as the formation of ovum-like cells (testis-ova). However, all testis-ova cells were eliminated via p53-independent apoptosis, and spermatogenesis was completely restored within 1 month after irradiation. Collectively, these data indicate that p53 is not indispensable for the restoration of irradiation-induced damaged tissues.