Chromosomally abnormal cells are not selected for the extra‐embryonic compartment of the human preimplantation embryo at the blastocyst stage

Human Reproduction, Dec 2003

BACKGROUND: Although well defined for embryos at cleavage stages, the occurrence and frequency of chromosomal aberrations in human blastocysts is relatively unknown. It has been reported that only one in four blastocysts is comprised totally of chromosomally normal cells. One of the selection mechanisms for the embryo proper to become free of these chromosomally abnormal cells would be to sequester them to the extra‐embryonic compartment during development. The study aim was to investigate whether such a mechanism of selection exists in human preimplantation embryos. METHODS: Inner cell mass (ICM)/trophectoderm (TE) differentiation was performed, followed by fluorescence in‐situ hybridization (FISH), to study the chromosomal distribution in both populations of cells. RESULTS: Of the 94 successfully analysed blastocysts, 68.8 ± 1.5% of all analysable nuclei per blastocyst showed a disomic chromosomal content. Only 22.6% of blastocysts analysed were classified as normal. Of the embryos classified as abnormal at the blastocyst stage, 11.9% showed a simple mosaic pattern and 32.1% a complex mosaic pattern. An equally large group of blastocysts showed either a chaotic pattern (16.7%), or the chromosomal pattern could not be classified. The average degree of normal cells in the ICM (67.9%) was similar to the degree observed in the TE (69.5%). CONCLUSIONS: These findings indicate that chromosomally abnormal cells are not preferentially segregating to the extra‐embryonic compartment of the human preimplantation embryo at the blastocyst stage. Hence, other mechanisms should be responsible for an absence of chromosomally abnormal cells in the embryo proper at later stages of development. One possible mechanism might be the elimination of the chromosomally abnormal cells by selective cell death activation.

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Chromosomally abnormal cells are not selected for the extra‐embryonic compartment of the human preimplantation embryo at the blastocyst stage

Human Reproduction Chromosomally abnormal cells are not selected for the extra-embryonic compartment of the human preimplantation embryo at the blastocyst stage Josien G.Derhaag 1 2 3 Edith Coonen 1 2 3 Marijke Bras 1 2 3 J.Marij Bergers Janssen 1 2 3 Rosie Ignoul-Vanvuchelen 1 2 3 Joep P.M.Geraedts 0 2 3 Johannes L.H.Evers 1 2 3 John C.M.Dumoulin 1 2 3 0 Department of Molecular Cell Biology and Genetics, Maastricht University , Maastricht , The Netherlands 1 Department of Obstetrics and Gynaecology, University Hospital Maastricht 2 Maastricht , P.O. Box 5800, 6202 AZ Maastricht , The Netherlands 3 Research Institute Growth and Development (GROW), Maastricht University BACKGROUND: Although well dened for embryos at cleavage stages, the occurrence and frequency of chromosomal aberrations in human blastocysts is relatively unknown. It has been reported that only one in four blastocysts is comprised totally of chromosomally normal cells. One of the selection mechanisms for the embryo proper to become free of these chromosomally abnormal cells would be to sequester them to the extra-embryonic compartment during development. The study aim was to investigate whether such a mechanism of selection exists in human preimplantation embryos. METHODS: Inner cell mass (ICM)/trophectoderm (TE) differentiation was performed, followed by uorescence in-situ hybridization (FISH), to study the chromosomal distribution in both populations of cells. RESULTS: Of the 94 successfully analysed blastocysts, 68.8 6 1.5% of all analysable nuclei per blastocyst showed a disomic chromosomal content. Only 22.6% of blastocysts analysed were classied as normal. Of the embryos classied as abnormal at the blastocyst stage, 11.9% showed a simple mosaic pattern and 32.1% a complex mosaic pattern. An equally large group of blastocysts showed either a chaotic pattern (16.7%), or the chromosomal pattern could not be classied. The average degree of normal cells in the ICM (67.9%) was similar to the degree observed in the TE (69.5%). CONCLUSIONS: These ndings indicate that chromosomally abnormal cells are not preferentially segregating to the extra-embryonic compartment of the human preimplantation embryo at the blastocyst stage. Hence, other mechanisms should be responsible for an absence of chromosomally abnormal cells in the embryo proper at later stages of development. One possible mechanism might be the elimination of the chromosomally abnormal cells by selective cell death activation. chromosomal mosaicism/uorescence in-situ hybridization/human blastocyst/inner cell mass/trophectoderm Introduction In human preimplantation embryosespecially those with abnormal morphologychromosomal mosaicism has been shown to be a frequent feature (Bongso et al., 1991; Pellestor et al., 1994a; b). Reported proportions of chromosomally abnormal preimplantation embryos ranged between 30 and 70% in embryos at day 2 or 3 of development (Munne et al., 1993; 1997; Delhanty et al., 1997; Laverge et al., 1997). Data appear to depend on the number of probes applied simultaneously, the type of probes used, the embryo morphology, embryo development, and the presence of multinucleated blastomeres. Numerical chromosome abnormalities often originate during male or female meiosis, but they may also arise at the time of fertilization or during further post-zygotic development to the blastocyst stage. Reports of the presence of chromosomal abnormalities in embryos at later stages of development are relatively rare. At the cleavage stages, 29% of morphologically normal human embryos have been shown to be chromosomally abnormal (Munne et al., 1995), while in 10-day-old pig embryos (Long and Williams, 1982), in 13- to 14-day-old sheep blastocysts (Murray et al., 1986) and in bovine blastocysts obtained after IVF (Iwasaki et al., 1992) polyploidy and in particular mixoploidy (i.e. mosaicism of diploid and polyploid cells) has been reported. In an early study of human blastocysts (Benkhalifa et al., 1993), chromosomal mosaicism was reported to be present in 29% of cases, and it was noted later that the percentage of embryos showing chromosomal mosaicism increased to almost 100% at the blastocyst stage. The proportion of abnormal cells per embryo was 16% (Bielanska et al., 2000; 2002; Leonetti et al., 2000; Ruangvutilert et al., 2000). Data acquired by others (Sandalinas et al., 2001) when investigating 54 blastocysts showed that, during development up to the blastocyst stage, there was no denite selection against most of chromosomal abnormalities which occurred at cleavage stages. Another group (Clouston et al., 2002) examined human blastocysts using classical cytogenetic techniques and reported that, when compared with cleavage-stage embryos, there was a decrease in embryos which revealed a haploid or monosomic chromosomal content, as well as some trisomies. Studies applying the comparative genome hybridization technique reported that 25% (Voullaire et al., 2000) and in some (...truncated)


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Josien G. Derhaag, Edith Coonen, Marijke Bras, J.Marij Bergers Janssen, Rosie Ignoul‐Vanvuchelen, Joep P.M. Geraedts, Johannes L.H. Evers, John C.M. Dumoulin. Chromosomally abnormal cells are not selected for the extra‐embryonic compartment of the human preimplantation embryo at the blastocyst stage, Human Reproduction, 2003, pp. 2565-2574, 18/12, DOI: 10.1093/humrep/deg485