Survival rate to term of chimeric morulae produced by aggregation of five to nine embryos in the mouse, Mus musculus.

Chimeric morulae from five to nine embryos were produced by aggregation after removal of the zonae pellucidae and brief incubation in phytohemagglutinin-P. After 28 hr of in vitro culture, these aggregates were transferred to the left uterine horn of pseudopregnant recipient female mice. Each recipient also received control embryos in the right uterine horn. Genetic markers were included so that aggregate-derived offspring could be distinguished from control offspring. Aggregate-derived embryos survived to term, but at a much reduced rate. In the 25 recipients that produced litters, 6.2% (N=211) of the aggregated embryos developed into liveborn young. Survival-to-term for control embryos was 38.8% (N=387). Survival-to-term was independent of the number of embryos aggregated. The aggregate-derived embryos were apparently undergoing size regulation following implantation.