THE DEVELOPMENTAL COMPETENCE OF BOVINE EMBRYOS WITH REDUCED CELL NUMBERS

Early cleavage stage bovine embryos can now be readily produced and maintained in vitro; therefore, their usefulness for producing identical calves and providing biopsies for sexing was investigated and compared with a conventional method of embryo splitting. Up to half the total number of blastomeres can be removed from 2- to 16-cell embryos without adversely affecting their subsequent development into blastocysts. Two- and 4-cell embryos needed to be maintained at 38-degrees-C during manipulation because of their sensitivity to cooling. Until the time of hatching, cell numbers in such blastocysts were reduced in proportion to the numbers of cells originally removed. The transfer of two demi-embryos produced from bisected blastocysts resulted in pregnancy rates similar to that obtained with whole embryos and there were no differences between cell numbers in, or survival of, demi-embryos produced in vitro or in vivo. Multiple calves also have been produced from single blastomeres isolated from 4-cell embryos produced in vitro. Seventy three percent of the 4-cell embryos manipulated produced either 3 or 4 blastocysts. Pregnancy rates resulting from transferred pairs of quarter embryos were lower than from demi-embryos, but there are some indications that success rates may be increased if the smaller embryos are co-transferred with fresh trophoblastic vesicles.