The regenerative capacity of the endometrium has been attributed to stem/progenitor cells, but the origin of these cells is unclear. Reports of a bone marrow–derived contribution to endometrial stroma and epithelium [1-4], suggest a role for bone marrow stem/progenitor cells in endometrial regeneration.
Telomerase reverse transcriptase is a component of the telomerase complex and a stem cell marker. We recently used a GFP reporter for mouse telomerase reverse transcriptase promoter activity (mTert-GFP) to identify putative epithelial and endothelial stem/progenitors, and immune cells with telomerase activity in the mouse endometrium . To assess whether mTert-GFP+ endometrial progenitors were derived from bone marrow, mTert-GFP bone marrow was transplanted into irradiated wild type female recipients. Microscopy of recipient endometrium 4 months after transplantation (n=8 mice, >6000 epithelial & >25,000 stromal cells examined) revealed mTert-GFP+ immune cells expressing pan-leukocyte marker CD45+, but provided no evidence of bone marrow-derived CD45- mTert-GFP+ cells in the epithelial or stromal compartments.
To address the possibility that the mTert-GFP reporter failed to mark bone marrow-derived stem/progenitors, we recreated previous studies [2&4] by transplanting wild type female recipients with bone marrow containing ubiquitously expressed chicken-beta actin-GFP. 4 months after transplantation (n=7 mice, >15,000 epithelial & >55,000 stromal cells examined) chicken-beta actin-GFP+ cells in the stromal compartment were CD45+ immune cells and no evidence of bone marrow-derived epithelial cells was observed.
We conclude it is unlikely that bone marrow stem cells give rise to endometrial epithelial or stromal cells. The misidentification of bone marrow-derived immune cells is the most probable explanation for previous reports of a bone marrow origin for these endometrial cell types.