Seminars

The reproductive capacity of animals is determined, in large part, by the numbers of eggs a female can produce during her lifetime. In mammals, such as humans, new eggs are only produced in the embryonic ovary, and no new eggs are produced in adults. By contrast, egg production in other vertebrates, such as fishes, appears to not be limited to a discrete developmental stage, but instead occurs throughout juvenile and adult life. Despite the fact this is a fundamental process, relatively little is known about the mechanism responsible for regulating the numbers of eggs produced in any non-mammalian vertebrate. I will present evidence that new eggs in adult zebrafish are continually produced from “egg” stem cells, called oogonial stem cells. Stem cells are characterized as cells that have the ability to both proliferate and to produce progenitor cells that can differentiate. In the zebrafish ovary we can readily identify both mitotic germ cells and germ cells that are actively entering meiosis. Interestingly, both pre- and early post-meiotic germ cells localize to a discrete zone on the surface of the ovary that we have termed the germinal zone, which we hypothesize is the oogonial stem cell niche. To further define the oogonial stem cells, we have identified two germ cell-specific promoters that allow us to track early stage germ cells: the /ziwi/ promoter drives expression in germ cells at all stages, with highest levels in pre- and early post-meiotic cells, while the /dmc1/ promoter drives expression in only pre- and early post-meiotic cells. Using these transgenics, we have developed both mosaic analysis and cell transplantation assays for identifying oogonial stem cells. Results from our transplantation analysis further support the existence of oogonial stem cells, and surprisingly revealed that these cells are developmentally plastic as they can readily be reprogrammed into spermatogonial stem cells capable of producing mature sperm when transplanted into a male recipient. Cell signaling is known to play an important role in regulating stem cell maintenance, including germline stem cells. In zebrafish, however, the signaling mechanism that regulates germline stem cells is not known. We have found that mutations in the Fgf ligand encoded by /ika/fgf24/ affect germ cell maintenance during larval stages. /fgf24/ mutants have normal numbers of germ cells at 24 hpf, but most lack germ cells at 20 dpf and thus develop into sterile adults. To determine if Fgf signal reception is required cell autonomously in germ cells, we have produced transgenic animals that express a dominant-negative Fgf receptor (dnFgfr) in these cells. We find that we can readily detect expression of the /dnfgfr/:GFP fusion protein in oocytes, whereas we only rarely detect its expression in oogonia. These results suggest that Fgf signaling is required for maintaining pre-meiotic germ cells in zebrafish larvae and juvenile ovaries.