Trisomy/tetrasomy 21 mosaicism in CVS: interpretation of cytogenetic discrepancies between placental and fetal chromosome complements

Trisomy/tetrasomy 21 mosaicism was found in chorionic villi (semidirect pre paration) obtained from a 40 year old pregnant woman. Since both cell lines were abnormal, the couple elected for pregnancy termination. Placenta and fetal tissue samples were obtained for cytogenetic study. Long term culture d villi showed a non-mosaic trisomy 21 karyotype, while other tissues showe d either a normal karyotype or normal/trisomy21 mosaicism. These discrepanc ies could be explained by a modified "bottle neck" embryogenic model with a trisomic zygote and a non-disjunction event taking place in one of the fir st divisions. Our case emphasises the need for confirmatory studies in othe r tissues when mosaicism is encountered in chorionic villi, even if all cel l lines are abnormal.