The effect of delay on the expression of the t6/t6 genotype.

In vivo, t6/t6 embryos are developmentally arrested between gestation days 5.5 (short-egg-cylinder stage) and 6.75 (long-egg-cylinder stage). In the present series of studies we used both in vivo and in vitro blastocyst delay followed by in vitro outgrowth to determine whether the t6/t6 lethality is time- or stage-specific. The results show that the t6/t6 genome is expressed differently in vivo and in vitro and that the in vitro expression of the homozygous t6 genome differs with different methods of effecting developmental delay. Although delay increases the life span of t6/t6 embryos it does not alter the stage of lethality. One method used to effect delay (ovariectomy) causes the t6/t6 embryos to remain as blastocysts for a significantly longer period of time than their wild-type littermates when placed into outgrowth medium. This distinction provides a unique method for obtaining a sample composed entirely of t6/t6 embryos at a stage prior to the lethal period.