EVALUATION OF DEVELOPMENTAL COMPETENCE, NUCLEAR AND OOPLASMIC MATURATION OF CALF OOCYTES

In this study we evaluated nuclear and ooplasmic maturation of prepube ral calf oocytes to determine a possible cause for their low developme ntal competency. Calf oocytes resumed meiosis and arrested at the MII stage at rates similar to that of adult animals; however, zygotes deri ved from calf oocytes cleaved and developed at significantly lower rat es. Ooplasmic maturation was assessed during oocyte maturation and fer tilization. Transmission electron microscopy revealed that a majority of calf oocytes exhibited some delay in organelle migration and redist ribution following maturation. Immunofluorescence microscopy showed th at following IVF, a higher percentage of calf oocytes had abnormal chr omatin and microtubule configurations than those of adult cattle. Thes e anomalies were characterized by delayed formation of sperm aster and asynchronous pronuclear formation. Microfluorometry was used to chara cterize the Ca2+ responses of calf oocytes to the addition of agonists or after IVF. The addition of thimerosal demonstrated the presence of Ca2+ stores in calf oocytes. Injection of near threshold concentratio ns of inositol 1,4,5-trisphosphate (InsP(3)), used to test the sensiti vity of the InsP(3)R, released significantly less Ca2+ in calf than in cow oocytes, whereas higher concentrations of InsP(3) (500 mu M) rele ased maximal [Ca2+](i) in both oocytes. These results suggested that t he Ca2+ content of intracellular stores was similar, but the sensitivi ty of the InsP(3)R may be different. Following insemination, calf oocy tes exhibiting [Ca2+](i) oscillations displayed comparable amplitude a nd intervals to cow oocytes; however, a significantly higher number of fertilized calf oocytes failed to show oscillations. Our findings sug gest that the low developmental competence of calf oocytes can be attr ibuted, at least in part, to incomplete or delayed ooplasmic maturatio n. (C) 1996 Wiley-Liss, Inc.