EMBRYO CLONING IN CATTLE - THE USE OF INVITRO MATURED OOCYTES

In vitro matured (IVM) bovine oocytes were examined to determine their potential viability in embryo cloning. Activation competence, as moni tored by pronuclear formation, increased with oocyte age. Oocytes read ily 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 oocy tes 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 d erived improved development of nuclear transfer embryos (polar body po sitive 25% versus polar body negative 10%, P < 0.05; large follicle oo cytes 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 fr equency of embryos produced, pregnancies confirmed between days 50 and 60 of gestation, or the number of calves born. We conclude that selec ted IVM oocytes are equivalent to in vivo matured oocytes when used fo r bovine embryo cloning.