Human Reproduction, Vol. 15, No. 11, 2404-2410,
November 2000
© 2000 European Society of Human Reproduction and Embryology
Spontaneous blastomere fusion after freezing and thawing of early human embryos leads to polyploidy and chromosomal mosaicism*
1 Success Through Assisted Reproductive Technologies (START), 2 University Health Network and Department of Laboratory Medicine and Pathobiology, and Medical Biophysics, Faculty of Medicine, University of Toronto, and 3 Department of Medicine, Immunology and Obstetrics and Gynecology, University of Toronto, Toronto, Ontario, Canada
The incidence of blastomere fusion after cryopreservation of early human embryos (day 2 and day 3) was investigated using the standard propanediol technique. The process of fusion was observed in all developmental stages (from 2 to 10 cells) and the frequency of this event was 4.6% in day 2 (41/889) and 1.5% in day 3 (10/646) embryos that survived the thawing (embryos with 50–100% intact cells). Fusion of two, and occasionally of several, blastomeres resulted in the formation of multinucleated hybrid cells, which clearly indicated that the ploidy of these newly created cells had been altered. This event, depending on the number of fused cells per embryo, transformed the embryos into either entirely polyploid embryos (complete fusion at 2- or 3-cell stage) or into mosaics being a mixture of polyploid and normal cells. Chromosomal preparations of embryos affected by blastomere fusion indicated the presence of tetraploid mitotic plates. Also, fluorescence in-situ hybridization (FISH) analysis using DNA probes targeting unique sequences on chromosomes 9, 15, 17 and 22 indicated the existence of tetraploid and diploid fluorescence signals in the interphase nuclei within mosaics. Therefore, observations on live and fixed embryos suggested that tetraploid (4n) or hexaploid (6n) and tetraploid–diploid or more complex aberrations of ploidy might be formed as a consequence of blastomere fusion. Furthermore, this demonstrates that freezing and thawing may induce numerical chromosomal changes in human embryos.
Key words: cell fusion/chromosomal aberrations/cryopreservation/mosaicism/polyploidy
* Presented at the 2000 Canadian Fertility and Andrology Society (CFAS) Annual Meeting, St. John's Newfoundland, Canada, September 14–16, 2000.
4 To whom correspondence should be addressed at: Success Through Assisted Reproductive Technologies (START), 655 Bay Street, 18th Floor, PO Box 4, Toronto, Ontario, M5G 2K4, Canada. E-mail: hbalakier{at}startclinic.com