We have recently reported the use of sequential simplex optimization methods to design a medium (SOM) that overcomes the block to development beyond two cells which occurs in vitro in embryos from an outbred strain of mouse. We have examined this medium and several others for their ability to foster development of CF1 female x B6D2F1 male mouse embryos through the blastocyst stage. A modification of medium SOM, designated KSOM, with an increased concentration of K+ (2.5 mM), also supports growth beyond the two-cell block; compared to other media tested it produces a higher rate of compaction (100%), provides a larger yield of blastocysts (88%), and stimulates an increased rate of cell division of the trophoblast cells. The total cell number of KSOM-cultured blastocysts (44 +/- 12; n = 30) indicates that 5-6 cell divisions are possible when embryos are cultured for 96 h from pronuclear zygotes. This is a significant improvement in performance over that of other defined media for the culture of zygotes to blastocysts.
