Reproduction Abstracts

Rare epithelial and stromal stem/progenitor cells (SPC) have been identified in the mouse endometrium. However the role of these SPC in endometrial regeneration is unclear due to the lack of a traceable marker. We used transgenic mice with reporter constructs for the stem cell marker telomerase reverse transcriptase (mTert) to identify and characterize mouse endometrial SPC. Endometrial mTert expression was examined by microscopy and flow cytometry in mice expressing GFP under the control of the mTert promoter (mTert-GFP). The fate of the endometrial mTert lineage was determined in mTert-CreER::R26R mice. A rare population of intrinsic (CD45 −ve) endometrial stromal cells expressed mTert-GFP during development and in adult mTert reporter mice. Rare focal regions of epithelial mTert-GFP were observed in both the glandular and luminal epithelium in adult mice. These mTert-GFP cells were distinct from stromal and epithelial label-retaining cells previously identified in mouse endometrium. Stromal mTert-GFP cells did not express ERα but epithelial mTert-GFP cells were ERα positive. Epithelial mTert-GFP cells were not slow cycling and displayed a level of proliferation comparable to mTert −ve epithelium. Lineage tracing showed that mTert lineage cells were largely confined to the stroma of prepubertal endometrium, but contributed extensively to glandular and luminal epithelium in the endometrium of adult cycling mice. In adult mice, removal of endogenous ovarian hormones by ovariectomy blocked expansion of the epithelial mTert lineage. Our data suggest that epithelial structures of the adult cycling endometrium originate from an mTert positive stromal SPC that undergoes mesenchymal-to-epithelial transition during cyclic endometrial regeneration.