Premature chromosome condensation after in-vitro fertilization.

During an in-vitro fertilization programme, 320 inseminated oocytes showing neither formation of pronuclei nor cell cleavage were studied cytogenetically. Fourteen of 17 oocytes exhibiting no extrusion of a polar body were characterized by an approximately diploid set of metaphase II chromosomes with four of these oocytes also showing an additional set of prematurely condensed sperm chromosomes of the G1-phase (G1-PCC). These chromosomes were single chromatids. Among 211 oocytes characterized by polar body extrusion, the same type of chromosomes were found in 22, along with metaphase II chromosomes in the haploid range. This phenomenon can be explained by the permanent arrest of the oocytes at metaphase II after sperm penetration, which allows the presence of cytoplasmic chromosome condensing factors to remain, leading to the induction of PCC in the sperm nucleus. In single cases, PCC was also observed in fertilized eggs that had formed two or three pronuclei. In these cases, PCC resulted either from distinct pronuclear asynchrony or interchromosomal asynchrony within the chromosome set.