Origin of trisomy: no evidence to support the ovarian mosaicism theory.

OBJECTIVE: Trisomy is the most common type of chromosome abnormality, affecting 4% of clinically recognised pregnancies, of which, trisomies 16, 21 and 22 are the most prevalent. It has been suggested that a large proportion of maternally derived trisomic pregnancies, specifically trisomy 21, are the result of low-level ovarian mosaicism. In this study, we aimed to reproduce these previously published results on trisomy 21 and investigate the other common maternally derived trisomies (i.e. trisomies 16 and 22) by determining chromosome copy number in fetal ovarian and control skin cells. METHODS: Ovarian and control skin tissue was collected from eight karyotypically normal female fetuses of between 10 and 14 weeks gestation, which were terminated for social reasons. Tissues were dissociated and fluorescence in situ hybridisation was performed with break-apart probes: CBFß (16q22), RUNX1 (21q22) and EWSR1 (22q12). RESULTS: A small number of trisomic cells, 13 out of 51,146 cells examined (0.025%), were identified in both ovarian and control skin samples. Only three of these trisomic cells were present in the fetal ovarian tissue. CONCLUSION: This study found no evidence of fetal ovarian mosaicism for trisomies 16, 21 and 22.

Cytogenetic analysis of human blastocysts.

UNLABELLED: The human blastocyst is key to understanding the aetiology of constitutional chromosome abnormalities in our species. OBJECTIVES: To investigate the range and incidence of chromosome abnormalities in a large series of human blastocysts, using classic cytogenetic techniques. METHODS: Using thymidine, cell division is synchronized in spare five-to-eight-day-old human blastocysts generated by IVF. A simple acetic acid disaggregation step produces discrete metaphases for G-band analysis. Subsequent FISH analysis of both metaphase and interphase nuclei allows further exploration of an abnormality detected by G-banding, including the investigation of any mosaicism. RESULTS: A total of 438 blastocysts have been prepared. Where analysis was possible, 3% appeared polyploid (mainly tetraploid), 29% were diploid : tetraploid mosaics and 68% were uniformly diploid. Abnormalities observed include triploidy, trisomy 16, trisomy 2, trisomy for unidentifiable D-group chromosome, mosaic trisomy 3, and mosaic trisomy 3 and trisomy 7. CONCLUSION: Comparison of results with existing data from both first trimester pregnancies and cleavage stage embryos suggests significant loss of haploid and monosomic embryos, as well as loss of some trisomies, prior to the blastocyst stage. It appears that the general range and incidence of most main groups of constitutional abnormalities observed in the first trimester (including mosaic forms) are in place by the blastocyst stage.