Long-term proliferation of mouse primordial germ cells in culture

Primordial germ cells (PGCs) are first identifiable as a population of about eight alkaline phosphatase-positive cells in the 7.0 days postcoitum mouse embryo¹. During the next 6 days of development they proliferate to give rise to the 25,000 cells that will establish the meiotic population ². Steel factor is required for PGC survival both in vivo³ and in vitro^4,5 and together with leukaemia inhibitory factor stimulates PGC proliferation in vitro⁶. In feeder-dependent culture, PGCs will proliferate for up to 7 days, but their numbers eventually decline and their proliferative capacity is only a fraction of that seen in vivo ^6,7. Here we report a further factor that stimulates PGC proliferation in vitro, basic fibroblast growth factor (bFGF). Furthermore, bFGF, in the presence of steel factor and leukaemia inhibitory factor, stimulates long-term proliferation of PGCs, leading to the derivation of large colonies of cells. These embryonic germ cells resemble embryonic stem cells, pluripotent cells derived from preimplantation embryos, or feeder-dependent embryonal carcinoma cells, pluripotent stem cells of PGC-derived tumours (teratomas and teratocarcinomas)⁸. To our knowledge, these results provide the first system for long-term culture of PGCs.