Some animals have the ability to generate large numbers of oocytes throughout life. This raises the question whether persistent adult germline stem cell populations drive continuous oogenesis and whether they are capable of mounting a regenerative response after injury. Here we demonstrate the presence of adult oogonial stem cells (OSCs) in the adult axolotl salamander ovary and show that ovarian injury induces OSC activation and functional regeneration of the ovaries to reproductive capability. Cells that have morphological similarities to germ cells were identified in the developing and adult ovaries via histological analysis. Genes involved in germ cell maintenance including Vasa, Oct4, Sox2, Nanog, Bmp15, Piwil1, Piwil2, Dazl, and Lhx8 were expressed in the presumptive OSCs. Colocalization of Vasa protein with H3 mitotic marker showed that both oogonial and spermatogonial adult stem cells were mitotically active. Providing evidence of stemness and viability of adult OSCs, enhanced green fluorescent protein (EGFP) adult OSCs grafted into white juvenile host gonads gave rise to EGFP OSCs, and oocytes. Last, the axolotl ovaries completely regenerated after partial ovariectomy injury. During regeneration, OSC activation resulted in rapid differentiation into new oocytes, which was demonstrated by Vasa 1 /BrdU 1 coexpression. Furthermore, follicle cell proliferation promoted follicle maturation during ovarian regeneration. Overall, these results show that adult oogenesis occurs via proliferation of endogenous OSCs in a tetrapod and mediates ovarian regeneration. This study lays the foundations to elucidate mechanisms of ovarian regeneration that will assist regenerative medicine in treating premature ovarian failure and reduced fertility. STEM CELLS 2017;35:236-247 SIGNIFICANCE STATEMENTStem cells that orchestrate ovarian regeneration have never been studied in the axolotl. This animal can regenerate many tissues perfectly, thus studying the underlying mechanisms of ovarian regeneration in this organism has great potential for translational outcomes. Many women suffer from premature ovarian failure and reduced fertility. With recent understanding of oogonial stem cells in vertebrates, inducing ovarian regeneration by therapeutic means is a desired application. Deciphering initiation and progression of ovarian regeneration via oogonial stem cells, in an animal that shows conservation of genes utilized in mammalian oogenesis, has great potential impact in the field of regenerative medicine.

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