Development in vitro of mouse embryos from the two-cell egg stage to the early somite stage.

About 1-3 % of mouse blastocysts, which had initially been cultured from the two-cell stage in chemically defined medium or about 3-5 % of blastocysts which were explanted from the uterus, developed to the early somite stage when cultured in vitro on collagen. Twocell eggs were initially cultivated in chemically defined medium to the blastocyst stage. Blastocysts were then transferred to Eagle's minimal essential medium (MEM) plus 10 % heat-inactivated calf serum. Two barriers to further development were overcome. First, the formation of endoderm and ectoderm from the inner cell mass immediately after attachment to collagen. Second, formation of the embryo proper from the embryonic region. Both barriers were overcome by using heat-inactivated human cord serum after the blastocysts hatched from the zona pellucida and attached to collagen. After attachment, embryos were cultured in MEM plus 20 % heat-inactivated human cord serum which was changed daily until early somite stages. Apparently normal healthy development in vitro occurred, as judged by light and electron microscopic examination.