Cytogenetics of Placenta—A Workshop Report

Abstract

This workshop was introduced by a historical overview of placental cytogenetics presented by Dr G. Simoni. He provided evidence that human reproduction is a very selective system in which the conceptions with abnormal karyotypes are eliminated by several mechanisms, particularly by spontaneous abortions, the restriction of chromosomally abnormal clones to the placenta, and the ‘rescue’ of trisomic conceptuses. All of these mechanisms contribute to optimization of the final outcome of the pregnancy and acknowledge that their existence makes placental genetic testing an important source of pathogenetic information in all abnormal pregnancies (Semprini and Simoni, 2000). Detailed review of the present understanding of the etiology and pathogenesis of confined placental mosaicism (Kalousek and Vekemans, 1996) was presented by Dr D. Kalousek. In her review, she has shown that, although in most human pregnancies the chromosomal complement detected in the fetus is also present in the placenta, in 1–2 per cent of viable pregnancies the cytogenetic abnormality, most often trisomy, is found to be confined to the placenta, a situation known as confined placental mosaicism (CPM). The three types of CPM are categorized according to the specific placental cell lineage(s) exhibiting the abnormal cell line (Figure 1 and Table 1). Placental mosaicism can be confined to trophoblast (type I), chorionic stroma (type II), or both cell lineages (type III). CPM is the result of viable postzygotic mitotic mutation(s) occurring in either the progenitor cells of specific placental cell lineages or the true embryoblasts. It can arise either in diploid conception (mitotic CPM-abnormal mitosis produces trisomy) or in a viable chromosomally abnormal conceptus (meiotic