What is the meaning of placental mosaicism seen in chorionic villus sampling?

what is the meaning of placental mosaicism seen in chorionic villus sampling?

How can we get 2 cell lines.

Aren’t the chorionic villi made of fetal tissue only?

Confined Placental Mosaicism (CPM) is when there are two different cell lines in the chorionic villus sample. Commonly, the abnormal chromosomes are seen in the placental cells while the fetal chromosomes are normal. The mosaicism tends to be confined within the trophoblast because of two mechanisms, postzygotic nondisjunction within the placenta or trisomic rescue in the fetus. Trisomic rescue is when there is preferential allocation of trisomic cells to the compartment of the placenta in order to “rescue” the fetus.
Due to the discrepancy, whether the fetus has an anomaly or not cannot be confirmed from CVS. Thus, follow-up procedures such as amniocentesis or fetal blood sampling may be needed to confirm the diagnosis. Thank you.