Timing of sequential changes in chromosome configurations during the second meiotic division and cytoplasmic events of pig oocytes matured and fertilized in vitro

Abstract

The present study determined the specifics of early events of fertilization of pig oocytes matured in vitro (IVM) and subsequently fertilized in vitro (IVF). The IVF system used for sperm capacitation, time of fertilization and fixation of eggs at 1–3 h intervals during the 30 h period after insemination precisely measured the timing of each critical stage of fertilization with precision. The first evidence of sperm penetration was observed 3 h after insemination. Attachment of the spermatozoa in the ooplasm caused resumption of meiosis (A 2/T 2 stage of second meiotic division), and was commonly observed from 4 to 5 h after insemination. The sperm heads of penetrating spermatozoa started to decondense, tails became detached and completion of the second meiosis (second polar body extrusion) occurred from 6 to 9 h. Further decondensation of the sperm head and female chromosomes took place from 9 to 12 h. Opposing and apposing pronuclei were frequently observed from 12 to 18 h and 18 to 24 h after insemination, respectively. Syngamy and prophase stage of first mitotic division, including first cleavage were observed from 24 to 30 h.