The kinetics of oocyte activation and polar body formation in bovine embryo clones.

The kinetics of polar body formation were examined in parthenogenetically activated, in vitro matured and aged bovine oocytes. Subsequently, the presence or absence of polar body formation was determined in bovine embryo clones. Polar body formation, defined as telophase II, occurred by 1 (13/40, 43%) and 2 h (15/21, 71%) postparthenogenetic activation of metaphase II stage oocytes. Parthenogenetically activated oocytes readily formed pronuclei by 4 h. Some oocytes had chromatin in a highly condensed state at 1, 2, and 4 h postactivation (13/72, 18%). These oocytes often (10/13, 77%) appeared to be "self-enucleated," as the condensed chromatin was found in a membrane-bound extrusion. The phenomenon was most prevalent when oocytes were handled at room temperature (25-27 degrees C). Nuclear transfer procedures were established to bring about synchronous blastomere fusion and oocyte activation conditions. Synchronous conditions were achieved only when oocytes were handled and manipulated at 37-39 degrees C. Embryo clones examined 2 h postfusion did not form a polar body. Conversely, nucleate demi-oocyte controls were at the late telophase II stage of meiosis. The results are discussed in relation to cell cycle effects on bovine nuclear transfer.