Long-term label retaining cells localize to distinct regions within the female reproductive epithelium

The uterus is an extremely plastic organ that undergoes cyclical remodeling including endometrial regeneration during the menstrual cycle. Endometrial remodeling and regeneration also occur during pregnancy and following parturition, particularly in hemochorial implanting species. The mechanisms of endometrial regeneration are not well understood. Endometrial stem/progenitor cells are proposed to contribute to endometrial regeneration in both humans and mice. BrdU label retention has been used to identify potential stem/progenitor cells in mouse endometrium. However, methods are not available to isolate BrdU label-retaining cells (LRC) for functional analyses. Therefore, we employed a transgenic mouse model to identify H2B-GFP LRCs throughout the female reproductive tract with particular interest on the endometrium. We hypothesized that the female reproductive tract contains a population of long-term LRCs that persist even following pregnancy and endometrial regeneration. Endometrial cells were labeled (pulsed) either transplacentally/translactationally or peripubertally. When mice were pulsed transplacentally/translactationally, the label was not retained in the uterus. However, LRCs were concentrated to the distal oviduct and endocervical transition zone (TZ) following natural (i.e., pregnancy/parturition induced) and mechanically induced endometrial regeneration. LRCs in the distal oviduct and endocervical TZ expressed stem cell markers and did not express ERα or PGR, implying the undifferentiated phenotype of these cells. Oviduct and endocervical TZ LRCs did not proliferate during endometrial re-epithelialization, suggesting that they do not contribute to the endometrium in a stem/progenitor cell capacity. In contrast, when mice were pulsed peripubertally long-term LRCs were identified in the endometrial glandular compartment in mice as far out as 9 months post-pulse. These findings suggest that epithelial tissue of the female reproductive tract contains 3 distinct populations of epithelial cells that exhibit stem/progenitor cell qualities. Distinct stem/progenitor-like cells localize to the oviduct, endometrium, and cervix.