Some mosaic types increase with maternal age.

Objective: This study evaluated mosaicism in a large series of cleavage-stage human embryos analysed by fluorescence insitu hybridization. Materials/Methods: Only embryos with at least three cells analysed were included (n = 1235) of which 556 were mosaics. These embryos were all analysed al least with probes for chromosomes XY, 13, 15, 16, 18, 21, 22 and either 1 or 17. Results: The most common types of mosaicism were chaotic (48%), diploid/polyploid (26%), and those caused by mitotic non-disjunction (25%). The number of abnormal cells per embryo ranged from 44% in diploid/polyploid to 84% in chaotic mosaics. Chromosome 16 was most commonly involved in mitotic non-disjunction mosaics. Although overall mosaicism did not increase with maternal age, the average maternal age of the embryos that had mosaics caused by mitotic non-disjunction was significantly higher than that for normal or other mosaic embryos (P < 0.1001). Conclusions: During cleavage-stage, the embryonic genome is not yet fully activated and consequently the mRNA and protein pools are still similar to those found in the oocyte. It is therefore proposed that the malfunctioning of the meiosis apparatus, which is similar to the mitotic one. may either cause meiotic errors or mitotic non-disjunction at cleavage-stage embryo development. [ABSTRACT FROM AUTHOR]