CONFINED PLACENTAL MOSAICISM FOR TRISOMY-2, TRISOMY-3, TRISOMY-7, TRISOMY-8, TRISOMY-9, TRISOMY-16, AND TRISOMY-22 - THEIR INCIDENCE, LIKELY ORIGINS, AND MECHANISMS FOR CELL LINEAGE COMPARTMENTALIZATION

Analysis of confined placental mosaicism (CPM) for trisomies 2, 3, 7, 8, 9, 16, and 22, in diagnostic chorionic villus sampling procedures, demonstrates apparent incidences of CPM for individual trisomies of be tween 9 and 91 cases per 100 000 pregnancies, with trisomy 7 being the most common. More detailed analysis of the percentage of aneuploid ce lls present, and the distribution of abnormality between the cytotroph oblast and extra-embryonic mesoderm cell lineages, shows a highly spec ific pattern for each chromosome. Theoretical considerations, in conju nction with direct observations, indicate that the overriding influenc e on the patterns of cell distribution seen in CPM is the distribution of aneuploid cells laid down during blastogenesis. This in turn refle cts closely the origin of mosaicism from either correction of a trisom ic conception or post-fertilization somatic error. The pattern of aneu ploid cells for each trisomy, as seen at the end of the first trimeste r and later in pregnancy, can therefore be used to predict the relativ e contribution of meiotic and mitotic errors to CPM, and hence the lik ely incidences of uniparental disomy from this source, upd(16)mat bein g the most common (1 in 10 000 continuing pregnancies). In addition, C PM for trisomies 2, 3, and 8 shows strong evidence of a non-random dis tribution of aneuploid cells between the different extra-embryonic cel l lineages. Analysis of comparable data from spontaneous abortion mate rial repeats this non-random pattern for trisomies 2 and 3, and sugges ts that a similar phenomenon may also be occurring for trisomy 22. A n on-random distribution could be attributable to selection for or again st, or intolerance of, particular trisomic cells in certain lineages, but is more probably a result of either cell lineage-specific non-disj unction or consistent uneven compartmentalization of aneuploid cells d uring blastocyst development.