Structural and numerical chromosomal abnormalities are common in early developing embryos, and these abnormalities may cause spontaneous abortions and implantation failure. The reproductive risk of carriers with structural chromosomal abnormalities depends on the breakpoint positions, the segregation patterns and the sex of the carrier. These carriers have a lower chance of producing normal or balanced gametes due to abnormal segregation of chromosomes at meiosis leading to repeated spontaneous abortions and infertility. Preimplantation genetic diagnosis (PGD) is offered to couples who have already been diagnosed with a single gene disorder or a chromosome imbalance to select an embryo free from the mutation or an embryo with a balanced karyotype prior to implantation and pregnancy. PGS is applied to patients experiencing repeated implantation failures or spontaneous abortions with normal karyotypes. Translocations are the most common type of structural chromosome rearrangement. Both reciprocal and Robertsonian translocations are phenotypically normal. PGD for translocations was initially performed by fluorescence in situ hybridization (FISH) at cleavage stage embryos. However, with the recent developments, many centers have opted for the use of array comparative genomic hybridization (aCGH), single-nucleotide polymorphism (SNP) arrays and next generation sequencing (NGS).
Part of the book: Chromosomal Abnormalities
Preimplantation embryo development follows a series of critical events. Remarkable epigenetic modifications and reprogramming of gene expression occur to activate the embryonic genome. In the early stages of preimplantation embryo development, maternal mRNAs direct embryonic development. Throughout early embryonic development, a differential methylation pattern is maintained although some show stage‐specific changes. Recent studies have shown that differential demethylation process results in differential parental gene expression in the early developing embryos that may have an impact on the correct development. In the recent years, noncoding RNAs, long noncoding RNAs (lncRNA) and short of mRNAs and therefore their role in preimplantation development has gained significance.
Part of the book: Embryo Cleavage