Relationship between the size of perivitelline space and the incidence of polyspermy in porcine oocytes

The size of each part of porcine oocytes cultured in NCSU37 medium, NCSU37 medium whose NaCl concentration was lowered from 108.76 to 61.6 mM and NCSU37 medium containing 0.1% tunicamycin, an inhibitor of glycoprotein synthesis, was measured to investigate how the size of the perivitelline space changed as oocytes matured, and the reason(s) the size of the perivitelline space changed. The incidence of polyspermy in porcine oocytes after insemination was also observed, in order to ascertain a relationship between the size of the perivitelline space and the incidence of polyspermy. The size of perivitelline space enlarged as the time of culture was prolonged, even in the use of either medium examined. The perivitelline space in oocytes cultured in NCSU37 medium for 44 hrs was 3.9μm, but it was a significantly larger in oocytes cultured in NCSU37 medium with decreased concentration of NaCl (6.4μm) and a significantly smaller in oocytes cultured in NCSU37 medium containing tunicamycin (2.4μm). From the measurements of each part of oocytes during culture, it was suggested that the enlargement in the perivitelline space following treatment with low NaCl was attributed to reduction of the oocyte cytoplasm, while the decrease in the perivitelline space following treatment with tunicamycin was attributed to expansion of the oocyte cytoplasm. Oocytes cultured for 44 hrs in NCSU37 medium, NCSU37 medium with decreased concentration of NaCl and NCSU37 medium containing tunicamycin showed the maturation rate of 88.8, 93.7 and 87.2%, respectively, and the fertilization rate after insemination of 83.6, 87.0 and 88.2%, respectively. There were no differences in either the maturation rate or the fertilization rate seen among these three culture media. On the other hand, the incidence of polyspermy of oocytes matured in NCSU37 medium was 52.1%, which was significantly higher than 30.0% of oocytes matured in NCSU37 medium with decreased concentration of NaCl, and was significantly lower than 71.7% of oocytes matured in NCSU37 medium containing tunicamycin. From these findings, it was clarified that the higher incidence of polyspermy is closely related to the smaller perivitelline space in porcine oocytes.