A comparison of degree of cortical granule exocytosis in zona-free pig oocytes induced by different artificial stimulators

Abstract

Cortical granule (CG) exocytosis occurs when the first sperm fertilizes an oocyte to modify the extracellular environment and zona pellucida of the oocyte thereby preventing polyspermy. Apart from sperm penetration, many artificial stimulators have been reported to induce CG exocytosis in mammalian oocytes, including pig oocytes. However, little detailed classification of the degree of exocytosis after the stimulation have been done, especially in zona-free oocytes. This study is, therefore, aimed to investigate the degree of exocytosis in zona-free pig oocytes after the stimulation with artificial stimulators. Oocytes were matured in vitro for 48 hr. Zona-free matured oocytes were then stimulated with each stimulator, including sperm insemination as a positive control group. The degree of exocytosis was monitored at 5 min, 6,12 and 24 hr after stimulation. The results showed that calcium ionophore A23187 induced 100% of the oocytes to release the granules, and induced the maximum percentage of oocytes exhibiting type I CG exocytosis (67.8%) at 24 hr after stimulation. Apart from oocytes cultured alone, the highest percentage of oocytes that did not release their cortical granules was seen in the oocytes treated with puromycin at 6 hr after treatment (31.2%). Of the stimulators calcium ionophore A23187 seems to be the most effective at inducing cortical release, while puromycin and sulphadiazine have very little effect on CG exocytosis. The potentials of artificial stimulators on the release of cortical granules may lead to their application in preventing polyspermy. However, further studies are needed to investigate whether or not the release of cortical granules in zona-free pig oocytes induced by artificial stimulators, such as calcium ionophore A23187, can prevent polyspermy in vitro.