Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 288–294 (2008); doi: 10.1111/j.1439-0531.2008.01176.xISSN 0936-6768Male Germ Cell TransplantationI DobrinskiSchool of Veterinary Medicine, Center for Animal Transgenesis and Germ Cell Research, University of Pennsylvania, Kennett Square, PA, USAContentsTransplantation of male germ line stem cells from a donoranimal to the testes of an infertile recipient was first describedin 1994. Donor germ cells colonize the recipient’s testis andproduce donor-derived sperm, such that the recipient male candistribute the genetic material of the germ cell donor. Germcell transplantation represents a functional reconstitutionassay for male germ line stem cells and as such has vastlyincreased our ability to study the biology of stem cells in thetestis and define phenotypes of infertility. First developed inrodents, the technique has now been used in a number ofanimal species, including domestic mammals, chicken and fish.There are three major applications for this technology inanimals: first, to study fundamental aspects of male germ linestem cell biology and male fertility; second, to preserve thereproductive potential of genetically valuable individuals bymale germ cell transplantation within or between species;third, to produce transgenic sperm by genetic manipulation ofisolated germ line stem cells and subsequent transplantation.Transgenesis through the male germ line has tremendouspotential in species in which embryonic stem cells are notavailable and somatic cell nuclear transfer has limited success.Therefore, transplantation of male germ cells is a uniquelyvaluable approach for the study, preservation and manipulationof male fertility in animals.IntroductionMale fertility requires efficient production of spermatozoathroughout the adult life of the male. Spermatogenesisis characterized by sequential steps of cellproliferation and differentiation resulting in the productionof virtually unlimited numbers of spermatozoa(Russell et al. 1990). The foundation of this system is themale germ line stem cell, which has the unique potentialfor both self-renewal and production of differentiateddaughter cells that ultimately form spermatozoa (Huckins1971; Clermont 1972; Meistrich and van Beek1993a). The male germ line stem cell is the only cell in anadult body that divides and can contribute genes tosubsequent generations, making it an obvious target forgenetic manipulations (see below). Because stem cellsare ultimately defined by function, unequivocal identificationdepends on an assay to demonstrate thepotential to reconstitute the appropriate body system.For male germ line stem cells, this assay wasestablished in 1994, when it was demonstrated thattransplantation of germ cells from fertile donor mice tothe testes of infertile recipient mice resulted in donorderivedspermatogenesis and sperm production by therecipient animal (Brinster and Zimmermann 1994). Theuse of donor males carrying a marker gene allowed foridentification of donor-derived spermatogenesis in therecipient mouse testis and proved that the donorhaplotype is passed on to the offspring by recipientanimals (Brinster and Avarbock 1994). In 13 years sincethe initial report, the technique has found widespreaduse in rodents, and more recently, also in non-rodentanimals (see below). Some of the most importantmilestones are summarized in Table 1.Cross-Species Transplantation of Male GermCellsIn 1996, production of rat sperm in mouse testes wasachieved following cross-species (xenogeneic) spermatogonialtransplantation from rats to mice (Clouthieret al. 1996) and was subsequently successful from miceto rats (Ogawa et al. 1999b; Zhang et al. 2003). Thiswork illustrated that the cell cycle during spermatogenesisis controlled by the germ cell, not the Sertolicell (Franca et al. 1998). Recently, it was confirmedthat rat sperm produced in a host mouse testis arecapable of supporting normal development whenintroduced into rat oocytes by ICSI (Shinohara et al.2006). Hamster spermatogenesis also occurred successfullyin the mouse host (Ogawa et al. 1999a); yet, withincreasing phylogenetic distance between donor andrecipient species, meiotic differentiation could no longerbe achieved in the mouse testis. Transplantation ofgerm cells from donors ranging from rabbits and dogs,to pigs and bulls, and ultimately non-human primatesand humans, resulted in colonization of the mousetestis, but spermatogenesis became arrested at the stageof spermatogonial expansion (Dobrinski et al. 1999,2000; Nagano et al. 2001b, 2002a). It appears that theinitial steps of germ cell recognition by the Sertoli cells,localization to the basement membrane and initiationof spermatogonial proliferation are conserved betweenevolutionary divergent species. Yet, the testicularenvironment (Sertoli cells and paracrine factors) ofthe recipient mouse appears to be unable to supportspermatogenic differentiation and meiosis from donorspecies other than rodents. This incompatibility couldtheoretically be addressed by co-transplantation ofgerm cells and donor Sertoli cells to the mouse testis(Shinohara et al. 2003), and complete spermatogenesisfrom different mammalian species in a mouse host wasachieved by testis tissue transplantation (Honaramoozet al. 2002b). Although xenogeneic spermatogonialtransplantation to rodent testes did not result inspermatogenesis from donor species other thanrodents, it nonetheless provides a bioassay for stemcell potential of germ cells isolated from other species(Dobrinski et al. 1999, 2000; Izadyar et al. 2003a).Recently, an elegant study demonstrated spermatogenesisand fertility after transplantation of primordialgerm cells between rainbow trout and salmon,Ó 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag