Germ cell transplantation in pigs--advances and applications.

Transplantation of germ cells from fertile donor mice to the testes of infertile recipient mice results in donor-derived spermatogenesis and transmission of the donor haplotype to offspring of recipient animals. In the pig, germ cells can be transplanted to a recipient testis by ultrasound-guided cannulation of the rete testis with delivery of cells by gravity flow. It is important to note that germ cell transplantation was successful between unrelated, immuno-competent pigs, whereas transplantation in rodents requires syngeneic or immuno-compromised recipients. Efficiency of colonization of the recipient testis by donor-derived germ cells can be improved by pretreatment of the recipient animal to deplete endogenous germ cells. Genetic manipulation of isolated germ line stem cells and subsequent transplantation will result in production of transgenic sperm. Transgenesis through the male germ line has tremendous potential in species like pigs 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 developmental abnormalities associated with somatic cell nuclear transfer and reprogramming. Viral transduction of germ cells prior to transplantation has been used to generate transgenic rodents and has also shown early promising results in pigs. Current research is directed toward improving protocols for isolation and culture of porcine male germ cells to increase efficiency of transgene transmission and to allow for gene targeting prior to germ cell transplantation. It is expected that germ cell transplantation will then provide a viable alternate approach to generate germ line transgenic pigs.