Early fusion of two human embryos?

SUMMARY An infant chimaera with an XX and an XY cell line is described. She had a macroscopically and histologically normal ovary on the right and an ovotestis on the left. Biopsies from the gonads revealed 1–15% of XY cells as judged by fluorescent Y body findings of uncultured cells as well as by karyotypes of mitoses from long term cultures. Ovarian and testicular structures had similar frequencies of the two cell lines. There was no evidence of in vitro selection of cells during tissue culture. In addition to the sex chromosomal evidence, two paternal contributions could be deduced from the segregation pattern in the proposita and her parents of the following systems: a fluorescent marker on chromosome 22, HL‐A antigens, Xgaand MNSs. Evidence for two maternal contributions came from a fluorescent marker on chromosome 3, HL‐A and ADA. The distribution of the two cell lines in different tissues was unequal, the frequency of the XY cells being 1–15% in gonadal tissues and approximately 90% in lymphocytes and erythrocytes. The inheritance of the maternally derived fluorescent marker on chromosome 3 indicates that the two maternal nuclei cannot have been the ovum and the second polar body provided no crossing‐over had occurred between the centromere and band 3q11. Participation of one or both of the products of the first meiotic division (i.e. secondary oocyte nuclei) was discussed but thought to be improbable. It is therefore likely, but not formally proven, that the chimaera had originated from the early fusion of two separate embryos.