Cytogenetic analysis of giant oocytes and zygotes to assess their relevance for the development of digynic triploidy

BACKGROUND: In order to assess the role of binucleate giant oocytes for generating digynic triploidy, we studied their frequency, maturation patterns and chromosomal complements at metaphase II (MII) or after fertilization. METHODS: Uncleaved, giant zygotes were incubated with podophyllotoxin and vinblastine, treated with hypotonic solution and fixed by a gradual fixation method. Giant MII oocytes were directly subjected to hypotonic treatment. The chromosomes were stained with Giemsa. RESULTS: A total of 7065 oocytes were collected during the study period, of which 18 (0.26%) were classified as giant cells. When considering only those patients in whom giant cells were identified (among other normal sized cells) a giant cell frequency of 18/237 (7.6%) was found. Nine cells underwent a union of the nuclei during maturation to MII and four of them became fertilized showing two pronuclei. Seven oocytes maintained the binucleate state to MII and one of them was fertilized showing three pronuclei. Ten unfertilized cells were available for cytogenetic analysis and proved to be diploid. All five giant zygotes revealed triploidy. CONCLUSIONS: The data suggest that giant oocytes may play an important, yet underestimated role in causing digynic triploidy. We recommend the exclusion of giant oocytes from IVF trials and that giant cells should be discarded, even if they carry the regular number of two pronuclei.