Cytoplasmic maturation for activation of pig follicular oocytes cultured and arrested at metaphase I

A large population (62-90%) of pig follicular oocytes can mature to metapha se II after culture for 48 h. However, a proportion (6-22%) remain in an im mature stage at metaphase I (metaphase I-arrested). The main objective of t his study was to determine whether the cytoplasm of metaphase I-arrested pi g oocytes is capable of being activated by sperm penetration or parthenogen etic stimulation. After culture for 48 h, oocytes without a polar body (73% were shown to be at metaphase I after staining) and those with a polar bod y (94% were at metaphase II) were fertilized in vitro. A total of 69% and 6 2% of the oocytes were activated to form a female pronucleus, respectively, and the rate of polar body extrusion induced by fertilization in the activ ated oocytes was 90% (the first polar body) and 95% (the second polar body) , respectively. When oocytes without and with a polar body were stimulated with an electric pulse, 53% and 81% of the oocytes were activated, respecti vely. The rate of polar body extrusion in the activated oocytes was 73% (th e first polar body) and 79% (the second polar body), respectively. In contr ast, young metaphase I oocytes cultured for 24 h had low (6%) or zero activ ation rate after in vitro fertilization or electric pulse stimulation. Howe ver, about one-third of the young metaphase I oocytes penetrated by spermat ozoa after in vitro fertilization responded to electric pulse 12 h after in semination, and almost all (93%) were activated when they were stimulated 2 4 h after insemination. Patterns of poly-peptide synthesis and histone H1 k inase activity were similar in metaphase I-arrested and metaphase II oocyte s, and were characterized by increase in a 25 kDa polypeptide and by decrea se in kinase activity. Although the first step of meiotic division is impai red, these results indicate that metaphase I-arrested oocytes are mature cy toplasmically.