THE ORIGIN AND EFFICIENT DERIVATION OF EMBRYONIC STEM-CELLS IN THE MOUSE

By explanting tissues isolated microsurgically from implanting strain 129 mouse blastocysts individually on STO feeder cells we have establi shed that embryonic stem (ES) cells originate from the epiblast (primi tive ectoderm), Isolated early epiblasts yielded ES cell lines at a su bstantially higher frequency than intact blastocysts regardless of whe ther they were explanted whole or as strictly single-cell suspensions, When explanted from delayed-implanting 129 blastocysts, epiblasts gav e lines consistently in 100% of cases, If primary embryonic fibroblast s rather than STO cells were used as feeders, germline-competent ES ce ll lines were obtained readily from epiblasts of delayed-implanting bl astocysts of several hitherto refractory strains, particularly when re combinant leukemia inhibitory factor was included in the medium during the initial period of culture, Because lines were obtained from the n onpermissive CBA/Ca strain at a rate of up to 56%, this approach to th e derivation of germline-competent ES cell lines may not only prove ge neric for the mouse but also worth pursuing in other species of mammal .