Quantitative comparison of stem cell marker-positive cells in fetal and term human amnion

Scattered in the amniotic epithelium of the human term placenta are pluripotent stem cell marker-positive cells. Unlike other parts of the placenta, amniotic epithelial (AE) cells are derived from pluripotent epiblasts. It is hypothesized that most epiblast-derived fetal AE cells are positive for stem cell markers at the beginning of pregnancy and that the stem cell marker-positive cells scattered through the term amnion are remaining, epiblast-like stem cells. To test this hypothesis, human fetal amnia from early-stage pregnancies were evaluated for expression of the stem cell-specific cell surface markers TRA 1-60 and TRA 1-81 and of the pluripotent stem cell marker genes Oct4, Nanog, and Sox2. Whole-mount immunohistochemical analysis demonstrated that a greater proportion of AE cells in the 17-19 week human fetal amnion are positive for both TRA 1-60 and TRA 1-81 than in the term amnion. Quantitative real-time RT-PCR analysis confirmed that the fetal AE cells exhibit greater stem cell marker gene expression than those in term placentae. Expression of both Nanog and Sox2 mRNAs were significantly higher in the fetal amnion, while Oct4 mRNA expression was not significantly changed. These differences in abundance of stem cell marker-positive cells and stem cell marker gene expression together indicate that some stem cell marker-positive cells are conserved over the course of pregnancy. The results suggest that stem cell marker-positive AE cells in the term amnion are retained from epiblast-derived fetal AE cells.