Summary. Preliminary observations showed that one-cell embryos from random-bred MF1 mice avoid cleavage arrest at the two-cell stage (' in vitro two-cell block') when cultured in modified M16 culture medium containing lactate and pyruvate but lacking glucose. The roles of lactate, pyruvate and glucose during preimplantation development of embryos from random-bred mice in vitro were therefore examined. When all three substrates were present continuously during culture, one-cell embryos arrested at the two- to four-cell stages. Improved development to the morula stage after 96 h in culture was obtained in media containing pyruvate alone, lactate and pyruvate, pyruvate and glucose, lactate pyruvate and glucose for the first 24 h, and medium containing lactate and pyruvate for the remaining 72 h. In a second experiment, embryos were cultured in medium containing pyruvate alone, lactate and pyruvate or pyruvate and glucose for the first 24 h, and lactate plus pyruvate medium for the second 24 h. Subsequent transfer to medium containing lactate, pyruvate and glucose supported the morula to blastocyst transition. These results show that developmental arrest in vitro can be overcome by changing the combination of energy substrates at different stages of preimplantation development.

Keywords: preimplantation; embryo; blastocyst; two-cell block; culture; mouse