Transplantation of primordial germ cells (PGCs), which are the progenitor cells of gametes, is a powerful tool for generation of transgenic chickens. However, the frequencies of transgene integration into the genome of purified PGCs still remain low. An in vitro culture system enabling chicken PGCs to propagate efficiently would be useful for efficient transgenesis of PGCs. In the present study, we optimized the culture conditions for chicken PGCs to enhance the proliferation and evaluated the germline transmission of cultured PGCs that proliferated for long periods of time. PGC-like cells (PGC-LCs), that have remarkably similar morphological characteristics to intact PGCs, could be derived by cultivation of blood containing PGCs obtained from 2.5-day-old chicken embryos according to the protocol of van de Lavoir et al. (2006). We determined which feeder cells and which growth factors were required to improve proliferation of PGC-LCs. Male PGC-LCs survival and proliferation were enhanced during culture in the basic medium containing either basic fibroblast growth factor (bFGF) alone or both bFGF and stem cell factor (SCF) on a feeder of buffalo rat liver (BRL) cells. Male PGC-LCs could be propagated in defined culture condition for extended periods. These cells expressed the germline-specific protein Vasa and undifferentiated cell marker stage-specific embryonic antigen-1 (SSEA-1) and pluripotency genes Nanog and PouV. Furthermore, Male PGC-LCs cultured for 225 d could migrate toward and colonize within recipient gonads and transmit to the next generation following transplantation. We succeeded in produce 3 offspring originating from long-term cultured PGC-LCs from a germline chimeric rooster (6%). The present study represents valuable steps toward defining a culture condition enabling PGCLCs to propagate efficiently for long periods in vitro with maintenance of their commitment to the germline.
An in vitro culture system of chicken primordial germ cells (PGCs) has been recently developed, but the growth factor involved in the proliferation of PGCs is largely unknown. In the present study, we investigated the growth effects of chicken stem cell factor (chSCF) on the in vitro proliferation of chicken PGCs. We established two feeder cell lines (buffalo rat liver cells; BRL cells) that stably express the putative secreted form of chSCF (chSCF1-BRL) and membrane bound form of chSCF (chSCF2-BRL). Cultured PGC lines were incubated on chSCF1 or chSCF2-BRL feeder cells with fibroblast growth factor 2 (FGF2), and growth effects of each chSCF isoform were investigated. The in vitro proliferation rate of the PGCs cultured on chSCF2-BRL at 20 days of culture was more than threefold higher than those cultured on chSCF1-BRL cells and more than fivefold higher than those cultured on normal BRL cells. Thus, use of chSCF2-BRL feeder layer was effective for in vitro proliferation of chicken PGCs. However, the acceleration of PGC proliferation on chSCF2-BRL was not observed without FGF2, suggesting that chSCF2 would act as a proliferation co-factor of FGF2. We transferred the PGCs cultured on chSCF2-BRL cells to recipient embryos, generated germline chimeric chickens and assessed the germline competency of cultured PGCs by progeny test. Donor-derived progenies were obtained, and the frequency of germline transmission was 3.39%. The results of this study demonstrate that chSCF2 induces hyperproliferation of chicken PGCs retaining germline competency in vitro in cooperation with FGF2.
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