Extensive expression of stage-specific embryonic antigen-1 (SSEA-1) has been documented in some animal species, but not in human embryos. In this study, SSEA-1 was detected during human embryogenesis by whole-mount immunohistochemistry. Alkaline phosphatase (Ap) activity was detected to identify human primordial germ cells. SSEA-1 was expressed steadily and restrictedly in some cells/tissues, especially in the nephric duct and nephric tubule (including the pronephric duct and tubule, mesonephric duct and tubule, metanephric tissues) besides embryonic ectodermal cells and yolk sac from 3 to 7 weeks. High level of Ap activity was observed in vessels, part of the mesonephric duct, especially in embryonic primordial germ cells localized in the yolk sac, primitive gut, dorsal mesenteries and genital ridges. No colocalization of AP and SSEA-1 cells was observed. SSEA-1 was expressed in human embryos in a different pattern at early stages compared to that in mouse embryos. It was expressed in the nephric duct, nephric tubule, yolk sac and on the surface of embryonic ectodermal cells of the epidermis, but not in human primordial germ cells.

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