Human Reproduction, Vol. 15, No. 8, 1876-1877,
August 2000
© 2000 European Society of Human Reproduction and Embryology
Letters to the editor |
Hypotriploid tripronuclear oocytes with two polar bodies obtained after ICSI: is irregular chromatid segregation involved?
1 Centre for Reproductive Medicine, Department of Gynecology and Obstetrics, University of Ulm, Prittwitzstrasse 43, 89075 Ulm/Donau, 2 Laboratory for Medical Genetics, Friedenstrasse 1, 89073 Ulm/Donau, Germany
Dear Sir,
A peculiarity of intracytoplasmic sperm injection (ICSI) is the occurrence of abnormally fertilized oocytes with three pronuclei (PN) and one (= the first) polar body (PB). It is generally accepted that these cells are digynic in their origin, the supernumerary PN being due to the non-extrusion of the second PB (Grossmann et al., 1997
). Provided that the participating male and female gametes carry a normal, haploid chromosome set (n = 23), the pronuclear chromosomal constitution of such an oocyte will be n/n/n with a total chromosome count of 3n = 69. Previous cytogenetic studies have already revealed an interesting phenomenon, i.e. an irregular chromosome segregation between the two maternal complements which results in a pronuclear chromosome pattern of n/n – x/n + x but once again in a total count of 3n = 69 (Macas et al., 1996; Rosenbusch and Sterzik, 1996).
An abnormal appearance of 3-PN can also be found in oocytes characterized by the regular extrusion of a haploid second PB. Macas et al. (1996) described three cells with a pronuclear chromosome pattern of n/n – x/x and suggested that some chromosomes may have separated from the maternal complement at late anaphase or early telophase of second meiotic division, leading to the formation of an additional PN. Of note, corresponding tripronuclear oocytes should reveal a total chromosome count of 2n = 46. We have also studied three abnormally fertilized oocytes with 3-PN and 2-PB. At first sight, their origin could have been explained by the mechanism described above and thus, a diploid state had to be expected. However, these oocytes turned out to be hypotriploid. The first zygote possessed a single metaphase and could be karyotyped (56,XXX,-A,-B,-2D,-4E,-3F,-2G) whereas chromosome clumping and overlapping did not allow a precise analysis in the second case. This cell was judged to have 56 or 57 chromosomes. The last oocyte showed two separate metaphases and the following karyotype: 23,Y / 36,X,-3A,-2B,-C,-D,-2F,-G. The latter example indicates that only the maternal contribution had been affected by chromosome loss. Our cytogenetic studies are based on a special technique designed to prevent artificial loss of chromosomes and chromosomal fragments by trying to keep the cytoplasm intact during fixation (Mikamo and Kamiguchi, 1983). Those cases with excessive chromosome scattering due to presumed cell rupture are excluded from analysis. The above mentioned results were surprising in view of the fact that there were no signs of oocyte damage. The cytoplasm appeared intact and rigorous screening of the slides did not yield any indications for scattered chromosomes. Moreover, comparable examples of severe hypotriploidy could not be detected in previous studies of multipronuclear zygotes (Rosenbusch et al., 1997, 1998). Together with the data established in our laboratory for haploid spermatozoa (Rosenbusch and Sterzik, 1994) and oocytes (Kamiguchi et al., 1993), this seems to rule out a contribution of extremely hypohaploid germ cells. As a possible explanation for our findings we therefore surmise an incomplete chromatid segregation into the second PB. The proposed mechanism would comprise an inconspicuous chromatid separation within the maternal complement and the regular formation of one male and one female PN. However, part of the maternal chromatids migrating towards the periphery for being extruded in the second PB would lag behind and cause the appearance of an additional female PN. Briefly, an affected oocyte can be characterized by a second PB chromosome constitution of n-x, a pronuclear chromosome pattern of n/n/x and a total chromosome count of 3n = <69.
It is reasonable to relate the presumed cytogenetic irregularities to adverse effects of ICSI on the microtubular system of the oocyte, as discussed by Macas et al. (1996). However, a precise assessment of such phenomena is difficult in view of the fact that the participating gametes themselves can carry numerical abnormalities. Moreover, combined effects on chromosome distribution are conceivable, e.g. an irregular chromatid segregation between oocyte and second PB followed by an incomplete chromatid migration into the second PB. In conclusion, it would be of interest to simultaneously analyse both the pronuclear as well as the second PB chromosome complements in future studies.
Notes
3 To whom correspondence should be addressed 
References
Grossmann, M., Calafell, J.M., Brandy, N. et al. (1997) Origin of tripronucleate zygotes after intracytoplasmic sperm injection. Hum. Reprod., 12, 2762–2765.
Kamiguchi, Y., Rosenbusch, B., Sterzik, K. and Mikamo, K. (1993) Chromosomal analysis of unfertilized human oocytes prepared by a gradual fixation-air drying method. Hum. Genet., 90, 533–541.[Web of Science][Medline]
Macas, E., Imthurn, B., Roselli, M. and Keller, P.J. (1996) The chromosomal complements of multipronuclear human zygotes resulting from intracytoplasmic sperm injection. Hum. Reprod., 11, 2496–2501.
Mikamo, K. and Kamiguchi, Y. (1983) A new assessment system for chromosomal mutagenicity using oocytes and early zygotes of the chinese hamster. In Ishihara, T. and Sasaki, M.S. (eds) Radiation-induced chromosome damage in man. Alan R. Liss, Inc., New York, 411–432.
Rosenbusch, B. and Sterzik, K. (1994) Cytogenetics of human spermatozoa. The frequency of various chromosome aberrations and their relationship to clinical and biological parameters. Arch. Gynecol. Obstet., 255, 81–89.[Web of Science][Medline]
Rosenbusch, B. and Sterzik, K. (1996) Irregular chromosome segregation following ICSI? [letter] Hum. Reprod., 11, 2337–2338.
Rosenbusch, B., Schneider, M. and Sterzik, K. (1997) The chromosomal constitution of multipronuclear zygotes resulting from in-vitro fertilization. Hum. Reprod., 12, 2257–2262.
Rosenbusch, B., Schneider, M. and Sterzik, K. (1998) Chromosomal analysis of multipronuclear zygotes obtained after partial zona dissection of the oocytes. Mol. Hum. Reprod., 4, 1065–1070.
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