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In vitro fertilization (IVF) and embryonic development of mature and meiotically arrested porcine oocytes were compared in this study. After in vitro maturation (IVM) for 48 h of cumulus-oocyte complexes, 75.4% (n = 442) of them extruded a visible polar body (PB). Most oocytes with a polar body (PB+ group) were found to be at metaphase II (M-II) stage (91.4%). Most oocytes without a visible polar body (PB− group, n = 144) appeared to be arrested at the germinal vesicle (GV) (41.6%) and first meiotic metaphase (M-I) (34.0%) stages. After IVF of oocytes (the day of IVF = Day 0), there was no significant difference between PB+ and PB− groups in rates of sperm penetration, monospermy, and oocyte activation after the penetration. Embryonic development was assessed by staining with 1% orcein. On Day 2, although there was no difference between the embryo cleavage in PB+ (n = 447) and PB− (n = 217) groups (47.0% and 35.9%, respectively), PB+ embryos had more cells than the PB− embryos (3.37 and 2.81 cells, respectively) (P &lt; 0.05; ANOVA). On Day 4, the cleavage rate of PB+ embryos was higher than that of PB− embryos (45.4% and 24.3%, respectively), and PB+ embryos had more cells than the PB− embryos (8.26 and 6.0 cells, respectively) (P &lt; 0.05; ANOVA). On Day 6, a significantly higher number of PB+ embryos developed to the blastocyst stage than that of the PB− embryos (34.6% and 20.7%, respectively) (P &lt; 0.05). However, by subtracting the GV oocytes from the PB− group, there was no difference in blastocyst rates between the M-I arrested and M-II oocytes (35.3% and 34.6%, respectively). The number of blastomer nuclei in embryos obtained from the PB+ group (52.0) was significantly higher than that of the PB− group (29.1); however, the proportion of inner cell mass and trophectoderm cells in PB+ and PB− blastocysts did not differ significantly (1:1.9 and 1:2.2, respectively) (P &lt; 0.05). Chromosome analysis revealed that PB+ blastocysts had significantly more diploid blastomeres (69.7%) than PB− blastocysts (44.0%), whereas PB− blastocysts had significantly more triploid cells (34.0%) compared with PB+ oocytes (8.4%)(P &lt; 0.05; χ2 test). These results indicate that porcine oocytes arrested at the M-I stage undergo cytoplasmic maturation during culture and have the same ability to develop to blastocysts after IVF as M-II oocytes but with a lower cell number; the latter might be caused by the slower embryonic development.

299 IN VITRO DEVELOPMENT OF IMMATURE PORCINE OOCYTES FERTILIZED IN VITRO TO THE BLASTOCYST STAGE