A mosaic embryo is characterised by containing two different cell lines at the same time, i.e. normal cells and cells with chromosomal alterations. The main cause is an error in the distribution of chromosomes in the cells of the embryo as it divides during development.
An embryo should contain 23 pairs of chromosomes, half of which come from the male gamete and half from the female gamete. In this way, the karyotype of each individual is made up of 46 chromosomes.
When we carry out a pre-implantation genetic diagnosis to find out the genetic information of the embryos and thus find out if there is any type of chromosomal anomaly, the information that we can obtain is as follows:
Euploid normal embryo, with 23 pairs of chromosomes in each cell; 22 pairs of autosomal chromosomes and one pair of sex chromosomes (XX or XY).
Aneuploid the number of chromosomes deviates from normal by addition or subtraction of chromosomes, chromosome pairs or chromosome fragments. In other words, it is an embryo that contains more or less information than normal. If the cell has more than three chromosomes with different types of aneuploidy (monosomies, nullisomies, trisomies or tetrasomies), it is considered complex aneuploid. If the gain or loss is of a variable length fragment of a chromosome, it is considered segmental aneuploidy.
Mosaic presence in the same embryo of cell lines with different karyotypes.
In an egg donation cycle, considering that the male gamete does not have any type of chromosomal abnormality (FISH in normal spermatozoa), we must take into account that the percentage of aneuploidy in egg donation is below 20%, as the donated eggs come from young, healthy women and therefore their eggs are of high quality. This implies a higher probability of implantation and pregnancy rate.
Regarding mosaicism, ovodonation does not exclude the presence of embryos with some type of chromosomal alteration, therefore, mosaicism can also be present in embryos from donor eggs, although in a much lower percentage.
For this reason, there is no indication for PGT-A in egg donation, unless there is a male factor such as an altered FISH, which may increase the probability of finding embryos with chromosomal alterations.