TRIPLOIDY AND TETRAPLOIDY MOSAICISM
The presence of ploidy mosaicism, while relatively common in preimplantation embryos, is very rare at prenatal diagnosis. This suggests that mosaicism for tetraploidy and haploidy is either generally lethal to the embryo or harmless due to selective growth of the normal cells. Complete haploidy is most certainly lethal prior to implantation while completely tetraploid abortuses are seen in early miscarriages. However, tetraploid cells are not uncommonly found in prenatal diagnosis and are generally considered ‘harmless’ and not reported. Because tetraploid cells are so common in CVS samples—particularly in long term cultures, Noomen et al. (2001) has developed criteria for reporting of these cells in order to reduce the number of prenatal follow-up investigations. There are very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism and one has to consider that this would not likely be reported unless abnormalities were present (i.e. the abnormalities could be coincidence in some cases).
Triploid mosaics are also extremely rare. Daniel et al 2003 reported on 4 such cases, 3 detected at prenatal diagnosis and the other was of an intellectually handicapped, dysmorphic boy. In two of the cases, prenatal diagnosis was preformed because multiple abnormalities were dected on ultasound. In the third case detected at prenatal diagnosis by CVS, the triploid line seemed to be sequestered into the extra-fetal tissues (confined placental mosaicism). This fetus developed normally and a normal infant was born with no evidence of triploidy in newborn blood or cord blood at three months of age. Three different mechanisms of origin for these apparent mosaics were detected: (1) chimaerism with karyotypes from two separate zygotes developing into a single individual, (2) delayed digyny, by incorporation of a pronucleus from a second polar body into one embryonic blastomere, and (3) delayed dispermy, similarly, by incorporation of a second sperm pronucleus into one embryonic blastomere.
Parent support discussion group for triploid/diploid mosacism
Daniel A, Wu Z, Darmanian A, Collins F, Jackson J. Three different origins for apparent triploid/diploid mosaics. Prenat Diagn. 2003 Jul;23(7):529-34. Abstract
Noomen P, van der Berg C, de Ruyter JL, Van Opstal D, Los FJ. Prevalence of tetraploid metaphases in semidirect and cultured chorionic villi. Fetal Diagn Ther. 2001 May-Jun;16(3):129-32. Abstract
Vatish M, Sebire NJ, Allgood C, McKeown C, Rees HC, Keay SD Triploid/diploid
mosaicism (69XXY/46XX) presenting as severe early onset preeclampsia
with a live birth: placental and cytogenetic features. Eur J Obstet Gynecol Reprod Biol. 2004 Feb 10;112(2):233-5.