Embryo cloning in cattle: the use of in vitro matured oocytes

In vitro matured (IVM) bovine oocytes were examined to determine their potential viability in embryo cloning. Activation competence, as monitored by pronuclear formation, increased with oocyte age. Oocytes readily formed a pronucleus when challenged with an electrical pulse 30 h after the onset of maturation. Developmental competence of IVM oocytes tended to increase with oocyte age (P = 0.079). Selection of IVM oocytes on the basis of the presence of a polar body 24 h after the onset of maturation and the size of the follicle from which the oocyte was derived improved development of nuclear transfer embryos (polar body positive 25% versus polar body negative 10%, P < 0.05; large follicle oocytes 31% versus small follicle oocytes 14%, P < 0.05). When selected, IVM oocytes were compared with in vivo matured oocytes recovered from superovulated cows and heifers; no difference was detected for the frequency of embryos produced, pregnancies confirmed between days 50 and 60 of gestation, or the number of calves born. We conclude that selected IVM oocytes are equivalent to in vivo matured oocytes when used for bovine embryo cloning.