The effect of the incubation temperature on the cleavage rate of mouse embryos in vitro.

We have studied the effect of an elevated incubation temperature on the in vitro cleavage rate of one- and two-cell mouse embryos. Two-cell embryos demonstrated a significantly higher rate of cleavage when incubated at 39 degrees C as compared to 37 or 41 degrees C. When recovered at the one-cell stage, the difference in cleavage rate between the groups incubated at 37 and 39 degrees C did not reach significance. While the morphology of the embryos incubated at 37 degrees C did not differ from that of the embryos incubated at 39 degrees C in either group, a significantly higher rate of degeneration was noted in the group of two-cell embryos incubated at 41 degrees C. These findings may apply to the human in vitro fertilization and embryo transfer system (IVF-ET), where the existing "lag" between embryo and endometrium could be narrowed if embryo cleavage rates could be accelerated. Further documentation of the normality of these "advanced" embryos is required.