Cleavage rate of mouse embryos in vivo and in vitro

Mouse embryos (Q strain) developing in vivo from the 2-cell to the blastocyst stage showed a constant cell doubling time of about 10 h. Embryos cultured in vitro over the same period showed a rate of cleavage which was initially almost as great as in the reproductive tract, but subsequently declined to give a doubling time of about 24 h. Addition of oestrogen to the culture medium increased the diameter of blastocysts but did not increase cell number.