A detailed analysis of early events during in-vitro fertilization of bovine follicular oocytes

Bovine follicular oocytes collected at slaughter were matured and fertilized in vitro with in-vitro capacitated spermatozoa. Analysis of 621 penetrated ova fixed at various times after in-vitro insemination led to definition of 6 stages of early development. A time sequence for sperm penetration, sperm head decondensation, male pronucleus formation, the activation of second meiotic division, female chromosome decondensation and pronucleus development was established. First sperm penetration into the ooplasm was recorded 6 h after insemination; 1-2 h was required for the sperm head to decondense and another 4-6 h to develop into the opposing pronucleus stage. Synkaryosis and first cleavage occurred 28 h after fertilization. Examination of the early stages revealed four types of abnormalities, i.e. polyspermy, polygyny, asynchrony between male and female pronucleus development, and preactivation of cytokinesis.