African catfish (Clarias gariepinus): Gonadotrophin and androgen levels in plasma. J Endocrinol 140(2): 265–273. 35. Good-Avila SV, Yegorov S, Harron S, Bogerd J, Glen P, et al. (2009) Relaxin gene family in teleosts: Phylogeny, syntenic mapping, selective constraint, and expression analysis. BMC Evol Biol 9: 293. 10.1186/1471-2148-9-293. 36. Zou S, Kamei H, Modi Z, Duan C (2009) Zebrafish IGF genes: Gene duplication, conservation and divergence, and novel roles in midline and notochord development. PLoS One 4(9): e7026. 10.1371/journal.pone. 0007026. 37. de Waal PP, Wang DS, Nijenhuis WA, Schulz RW, Bogerd J (2008) Functional characterization and expression analysis of the androgen receptor in zebrafish (Danio rerio) testis. Reproduction 136(2): 225–234. 10.1530/REP-08-0055. 38. Leal MC, de Waal PP, Garc´ıa-Lo´pez A, Chen SX, Bogerd J, et al. (2009) Zebrafish primary testis tissue culture: An approach to study testis function ex vivo. Gen Comp Endocrinol 162(2): 134–138. 10.1016/j.ygcen.2009.03.003. 39. Glauche I, Moore K, Thielecke L, Horn K, Loeffler M, et al. (2009) Stem cell proliferation and quiescence—two sides of the same coin. PLoS Comput Biol 5(7): e1000447. 10.1371/journal.pcbi.1000447. 40. Schulze C (1979) Morphological characteristics of the spermatogonial stem cells in man. Cell Tissue Res 198(2): 191–199. 41. Schulze C (1988) Response of the human testis to long-term estrogen treatment: Morphology of sertoli cells, leydig cells and spermatogonial stem cells. Cell Tissue Res 251(1): 31–43. 42. Nakamura S, Kobayashi K, Nishimura T, Higashijima S, Tanaka M (2010) Identification of germline stem cells in the ovary of the teleost medaka. Science 328(5985): 1561–1563. 10.1126/science.1185473. 43. Cairns J (1975) Mutation selection and the natural history of cancer. Nature 255(5505): 197–200. 44. Rando TA (2007) The immortal strand hypothesis: Segregation and recon- struction. Cell 129(7): 1239–1243. 10.1016/j.cell.2007.06.019. 45. Conboy MJ, Karasov AO, Rando TA (2007) High incidence of non-random template strand segregation and asymmetric fate determination in dividing stem cells and their progeny. PLoS Biol 5(5): e102. 10.1371/journal.pbio.0050102. 46. Szotek PP, Chang HL, Brennand K, Fujino A, Pieretti-Vanmarcke R, et al. (2008) Normal ovarian surface epithelial label-retaining cells exhibit stem/ progenitor cell characteristics. Proc Natl Acad Sci U S A 105(34): 12469–12473. 10.1073/pnas.0805012105. 47. Chan RW, Gargett CE (2006) Identification of label-retaining cells in mouse endometrium. Stem Cells 24(6): 1529–1538. 10.1634/stemcells.2005-0411. 48. Kuwahara R, Kofman AV, Landis CS, Swenson ES, Barendswaard E, et al. (2008) The hepatic stem cell niche: Identification by label-retaining cell assay. Hepatology 47(6): 1994–2002. 10.1002/hep.22218. 49. Zhang Z, Shao S, Meistrich ML (2007) The radiation-induced block in spermatogonial differentiation is due to damage to the somatic environment, not the germ cells. J Cell Physiol 211(1): 149–158. 10.1002/jcp.20910. 50. Miura T, Yamauchi K, Takahashi H, Nagahama Y (1991) Hormonal induction of all stages of spermatogenesis in vitro in the male Japanese eel (Anguilla japonica). Proc Natl Acad Sci U S A 88(13): 5774–5778. 51. de Waal PP, Leal MC, Garc´ıa-Lo´pez A, Liarte S, de Jonge H, et al. (2009) Oestrogen-induced androgen insufficiency results in a reduction of proliferation and differentiation of spermatogonia in the zebrafish testis. J Endocrinol 202(2): 287–297. 10.1677/JOE-09-0050. SSC Niche and SSC In Zebrafish 52. Miura T, Miura C, Ohta T, Nader MR, Todo T, et al. (1999) Estradiol-17beta stimulates the renewal of spermatogonial stem cells in males. Biochem Biophys Res Commun 264(1): 230–234. 10.1006/bbrc.1999.1494. 53. Butler JM, Kobayashi H, Rafii S (2010) Instructive role of the vascular niche in promoting tumour growth and tissue repair by angiocrine factors. Nat Rev Cancer 10(2): 138–146. 10.1038/nrc2791. 54. Nikolova G, Strilic B, Lammert E (2007) The vascular niche and its basement membrane. Trends Cell Biol 17(1): 19–25. 10.1016/j.tcb.2006.11.005. 55. Miura T, Ohta T, Miura CI, Yamauchi K (2003) Complementary deoxyr- ibonucleic acid cloning of spermatogonial stem cell renewal factor. Endocrinol- ogy 144(12): 5504–5510. 10.1210/en.2003-0800. 56. Miura C, Kuwahara R, Miura T (2007) Transfer of spermatogenesis-related cDNAs into eel testis germ-somatic cell coculture pellets by electroporation: Methods for analysis of gene function. Mol Reprod Dev 74(4): 420–427. 10.1002/mrd.20653. 57. van Keulen CJ, de Rooij DG (1974) The recovery from various gradations of cell loss in the mouse seminiferous epithelium and its implications for the spermatogonial stem cell renewal theory. Cell Tissue Kinet 7(6): 549–558. 58. Kanatsu-Shinohara M, Ogonuki N, Miki H, Inoue K, Morimoto H, et al. (2009) Genetic influences in mouse spermatogonial stem cell self-renewal. J Reprod Dev;10.1262/jrd.09-153N. 59. Nakagawa T, Nabeshima Y, Yoshida S (2007) Functional identification of the actual and potential stem cell compartments in mouse spermatogenesis. Dev Cell 12(2): 195–206. 10.1016/j.devcel.2007.01.002. 60. O’Shaughnessy PJ, Hu L, Baker PJ (2008) Effect of germ cell depletion on levels of specific mRNA transcripts in mouse Sertoli cells and Leydig cells. Reproduction (135): 839–850. 61. Gomes WR, Hall RW, Jain SK, Boots LR (1973) Serum gonadotropin and testosterone levels during loss and recovery of spermatogenesis in rats. Endocrinology 93(4): 800–809. 62. Morris ID, Bardin CW, Musto NA, Thau RB, Gunsalus GL (1987) Evidence suggesting that germ cells influence the bidirectional secretion of androgen binding protein by the seminiferous epithelium demonstrated by selective impairment of spermatogenesis with busulphan. Int J Androl 10(5): 691–700. 63. Ohta T, Miyake H, Miura C, Kamei H, Aida K, et al. (2007) Follicle-stimulating hormone induces spermatogenesis mediated by androgen production in Japanese eel, Anguilla japonica. Biol Reprod 77(6): 970–977. 10.1095/biolre- prod.107.062299. 64. Garc´ıa-Lo´pez A, Bogerd J, Granneman JC, van Dijk W, Trant JM, et al. (2009) Leydig cells express follicle-stimulating hormone receptors in African catfish. Endocrinology 150(1): 357–365. 10.1210/en.2008-0447. 65. Miura T, Miura C, Konda Y, Yamauchi K (2002) Spermatogenesis-preventing substance in Japanese eel. Development 129(11): 2689–2697. 66. Li Y, Oral O, Abe K, Eto K, Abe S (2008) The roles of pericystic cells and Sertoli cells in spermatogonial proliferation stimulated by some growth factors in organ culture of newt (Cynops pyrrhogaster) testis. Gen Comp Endocrinol 159(1): 80–87. 10.1016/j.ygcen.2008.07.023. 67. Honaramooz A, Behboodi E, Megee SO, Overton SA, Galantino-Homer H, et al. (2003) Fertility and germline transmission of donor haplotype following germ cell transplantation in immunocompetent goats. Biol Reprod 69(4): 1260–1264. 10.1095/biolreprod.103.018788. 68. Neumann JC, Dovey JS, Chandler GL, Carbajal L, Amatruda JF (2009) Identification of a heritable model of testicular germ cell tumor in the zebrafish. Zebrafish 6(4): 319–327. 10.1089/zeb.2009.0613. PLoS ONE | www.plosone.org 16 September 2010 | Volume 5 | Issue 9 | e12808