X-chromosome inactivation in the human cytotrophoblast

Preferential paternal X-chromosome inactivation occurs in the cell lineages that differentiate first within the female rodent blastula (trophectoderm and extraembryonic endoderm). The present studies were designed to test the nature of X-chromosome inactivation (XCI) in the earliest differentiating cell lineage of the human placenta, the cytotrophoblast. Using glucose-6-phosphate-dehydrogenase (G6PD) polymorphisms as a marker system, term placentae were obtained from 13 female heterozygotes where parental allelic contributions could be determined. Chorionic villi were enzymatically digested and centrifuged in a Percoll density gradient to isolate a pure population of cytotrophoblasts, which was ascertained by cell culture, differentiation to syncytiotrophoblasts, and histochemical staining for alpha-human chorionic gonadotrophin (alpha-HCG). On electrophoresis, all 13 samples exhibited exclusive or near exclusive expression of the maternally derived X-linked enzyme variant, regardless of whether it was G6PD A or G6PD B. No intermediate bands were seen, indicating a single active G6PD locus per cell. The stromal cells of the villi, which derive from the mesoderm and differentiate later than the cytotrophoblast, exhibit random XCI. These findings establish preferential paternal XCI in the cytotrophoblast, the cell type first to differentiate within the human blastula.