What is Mosaicism?

Mosaicism is a term that's related to, in our field, the level of chromosomes in each individual cell. So when I talk about a cell having a chromosome problem or whether it's chromosomally normal, that generally talks about the embryo as a whole. The issue of mosaicism is that it could be, and it is the case in some embryos, that one cell is not the same as another cell. So we know for example in pregnancy, that we have something called confined placental mosaicism. In other words there is mosaicism in this growing foetus placenta structure, but the mosaicism that exists exists only in the placenta. It's confined to the placenta. When I say mosaicism in this context, I mean that the chromosome nature of some cells is not exactly the same as other cells. So therefore, if we look at this what I call foetal placental context, and go back a few days or weeks, we end up with an embryo. We might end up with an embryo with 30 cells say, that's going to one day become a tiny growing baby, the foetus, and its own placenta. But of those 30 cells, let's say some of them, two or three, are going to be in the placenta with a chromosome makeup that's slightly different to the other 12 cells that's going to be the placenta. It's therefore a mosaic. It's got some cells different to another, making up the whole embryo, mainly confined to the placental part. So you can get an absolutely normal foetus, a normal baby, where the placenta has abnormal chromosomes. That's not uncommon in natural reproduction, and we have to look out for that in our embryo status too, because that's just the stage proceeding the foetus.