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Germ cell transplantation and testicular xenografting in mammals
Ina Dobrinski
Center for Animal Transgenesis and Germ Cell Research, University of Pennsylvania,
School of Veterinary Medicine, Kennett Square, PA 19348, USA
Male reproductive efficiency relies on efficient sperm production throughout the adult
life of the male. At the foundation of this system is the male germline stem cell which has
the unique potential for both self-renewal and production of differentiated daughter cells
that ultimately form spermatozoa. The male germline stem cell is unique in that it is the
only cell in an adult body that divides and can contribute genes to subsequent
generations, making it an obvious target for genetic manipulations. Transplantation of
germline stem cells from fertile donor mice to the testes of infertile recipient mice results
in donor-derived spermatogenesis and transmission of the donor’s genetic material to the
offspring of the recipients. More recently, germ cell transplantation was also successful in
pigs, goats, cattle and primates. Importantly, germ cell transplantation was feasible
between unrelated, immuno-competent donors and recipients in larger mammals, whereas
transplantation in rodents requires genetically matched or immuno-compromised
recipients. Transplantation of transfected germ cells has been investigated as an alternate
means to generate transgenic animals through manipulation of the male germ line. Some
success has been reported in generating transgenic mice and rats by retroviral or lentiviral
transduction of germ cells prior to transplantation. Recently, we could demonstrate stable
transduction of mouse and goat germ cells with transgene transmission to the next
generation by using a recombinant adeno-associated virus vector that integrates into nonreplicating cells. Transgenesis through the male germline has tremendous potential in
species where embryonic stem cell technology is not available and current options to
generate transgenic animals are inefficient. Introduction of a genetic change prior to
fertilization will circumvent problems associated with manipulation of early embryos and
nuclear reprogramming. In addition, if a genetic modification is introduced into male
germline stem cells prior to transplantation to the testes of a prepubertal recipient male,
time required until transgenic sperm can be harvested is significantly shorter than when
transgenic founder animals are generated by somatic cell nuclear transfer. Germ cell
transplantation therefore represents an approach to germline manipulation that is
potentially more efficient than currently existing strategies.
Mammalian spermatogenesis is a complex process that has not been completely
recapitulated in vitro. As an alternate approach to germ cell transplantation that is more
easily applicable to diverse mammalian species, ectopic grafting of testis tissue to
immunodeficient mice serves as an in vivo culture system for the propagation of male
germ cells. Xenografted testis tissue from immature males of different mammalian
species produced functional sperm. In xenografts of immature testis tissue, maturation of
the testicular somatic cells is significantly accelerated by exposure to the endocrine
environment of the adult mouse host whereas the length of the spermatogenic cycle is
unchanged. Global gene expression profile is very similar between testis tissue xenografts
and donor testis in situ. However, efficiency of spermatogenesis in testis xenografts
appears to be dependent on donor age and species. Accessibility of the tissue in the
mouse host allows manipulation of spermatogenesis in a controlled manner that is often
not feasible in the donor species, especially in humans. Testis tissue grafting also is a new
option for male germline preservation. As a source of male gametes even from immature
gonads, grafting of fresh or preserved testis tissue offers an invaluable tool for the
conservation of fertility. Therefore, transplantation of testis tissue from diverse donor
species, including primates, into a mouse host represents a unique model for the study,
preservation and manipulation of male fertility in mammalian species.