MOSAICISM IN THE INNER CELL MASS OF HUMAN BLASTOCYSTS

Although mosaicism was shown to be a normal feature in cleaving embryo s, its consequences for the late preimplantation stages are unknown. W e performed blastocyst immunosurgery, followed by fluorescent in-situ hybridization (FISH), to determine the number of cells and degree of m osaicism in the inner cell mass (ICM) of human blastocysts. Of 47 ICM samples analysed, 20 had aneuploid cells, and two also had a few tetra ploid cells. The average degree of aneuploidy in the ICM was similar t o the overall blastocyst mosaicism, suggesting that there is probably no selection for euploid ICM, The lower degree of blastocyst mosaicism , compared with the cleavage-stage embryos, may be due to a mechanism of selection against the embryos with high frequency of mosaicism, lea ding to elimination of these embryos prior to blastocyst formation.