Reproduction in Domestic Animals

Farm Animals Embryonic Stem Cells 197that it may be possible to circumvent this limitation byco-transplanting SSCs together with the supportingSertoli cells (Shinohara et al. 2003).Although the ambitious goal to obtain mature spermof one species in the testis of a different one has not beenreached in domestic species, transplants of SSCsbetween different individuals of the same species hasbeen successful in pigs and goats (Honaramooz et al.2002, 2003a; b). Through this intriguing technique, ithas even been possible to transplant SSCs obtained froma transgenic buck and to transmit to the progeny thetransgene through the sperm ejaculated by a normalbuck who received the transgenic SSCs (Honaramoozet al. 2003b). These data support the vision that SSCscan represent the ideal way to propagate transgenesthrough the sperm, making the ideal substitute of ESCsfor this purpose.Multipotent germ stem cellsIndeed, this could be just the beginning of a moreextensive use of SSCs as suggested by a recent findingobtained during the prolonged culture of these cellsin vitro which was originally aimed at their transformation.It has been noticed that among mouse post-natalSSCs cultured in vitro for an extended period of time,ESC-like cells appeared upon treating the culture with amixture of cytokines and growth factors (Kanatsu-Shinohara et al. 2004). These cells have been namedmultipotent germ stem cells (mGSCs) as they arecompletely different from SSCs. In fact, whereas SSCscan give rise only to spermatozoa, mGSCs show all theproperties typical of mESCs including pluripotency andthe ability to generate germ-line chimeras (Shinoharaand Kanatsu-Shinohara 2007).Recently, the isolation of mGSCs has been describedalso from adult mouse testis (Guan et al. 2006) furtherenhancing the potential utility of these cells as a viablealternative to ESCs both as a way to genetically modifythe germ line and a source of undifferentiated cells fortherapeutic purposes.Extensive efforts have been spent in the attempt toobtain similar cell lines in other species and in human inparticular, but, at present, no positive results have beenannounced. Work on farm animals has lead to theidentification of germ cell-specific markers in sheep(Rodriguez-Sosa et al. 2006) and pig (Luo et al. 2006;Goel et al. 2007) which should be instrumental inreaching an highly purified cell population.ConclusionsThe long quest for farm animal ESCs has not lead yet tothe derivation of a bona fide ESC line, convincinglydisplaying all the properties of analogous cells in mouse.Many reports have described cell lines in domesticspecies, which presented several important featurestypical of ESCs. Such features may be sufficient in orderto use these cells as tools for the genetic manipulation astheir self-renewal may be more prolonged than that ofnormal primary cultures, thus enabling the possibility totransform and select the cells to be used as nucleusdonors in cell transfer experiments. An alternative use offarm animal ESCs is as an excellent experimental modelin pre-clinical trial assessing the feasibility of cell therapybecause of the closer morphological and physiologicalresemblance to humans of species like the pig whencompared with the mouse.However, the persistent lack of standard methods forthe derivation, maintenance and characterization ofESCs in domestic species stimulates the search foralternatives.Embryonic germ cells may represent such an alternative.Indeed, these cells showed a higher plasticity thanESCs as they were able to contribute to embryonicdevelopment forming chimeric newborns. However, asfor ESCs, standardization is still far away and efficiencyis very low.Recent results indicated SSCs as possible tools forgerm-line genetic modifications with some proof ofprinciple results already achieved. But, a real breakthough could arise from the isolation of mGSCs,virtually equivalent to ESCs, from newborn and adultmouse testis. 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