Abstract
Stem/progenitor cells are the naughty cells of the endometrium! The term “naughty” has a number of connotations, one being immaturity which I will apply to the rare stem/progenitor cell populations hiding in the endometrium, where they have eluded scientists for so long. Despite their rarity, these immature cells have the capability of growing up and differentiating into the functional cells of the endometrium, producing their progenies in the process. The self-willed human endometrial epithelial progenitor cells (eEPC) and mesenchymal stem cells (eMSC) first revealed themselves through their clonogenic activity, shunning their mates and setting up clones of cells on their own. Their risqué production of identical copies of themselves ensures their continuity, much to the chagrin of their mature counterparts. They are sneaky and can produce large numbers of mature progeny, but rarely proliferate themselves preferring to take life easy and do little. They also spit out viability dyes (Hoechst) at a greater rate than mature endometrial cells to become Side Population (SP) cells.
A number of approaches have been used to tame these naughty endometrial stem/progenitors. In order to determine the identity and location of these elusive cells, specific markers had to be found. The immature endometrial epithelial progenitor cells play tricks with the specific markers they express. For example, clonogenic eEPC are N-cadherin+, an epithelial mesenchymal transition marker, found by unbiassed gene profiling, revealed their hiding place in the bases of glands deep in the endometrial basalis. Similarly, SSEA-1+ basalis epithelial progenitors pirated their marker from mature neutrophils and differentiating human pluripotent stem cells. In mice the stem/progenitor cells like to play chase, with lineage tracing of individual genetically marked cells revealing their location in the intersection zone of the glands and luminal epithelium, and also in the gland bases (Axin2+ and Lgr5+).
The identity of eMSCs has also been determined by discovery of specific markers, but even here the eMSC play games in human endometrium where sometimes they are pericytes (CD140b and CD146 double positive cells), sometimes perivascular cells (SUSD2+) and sometimes CD34+ cells in the adventitia of blood vessels. They are also adventitial perivascular cells in ovine endometrium, but this time they are CD271+. Mature endometrial stromal cell progeny are also naughty, often pretending to be eMSC, particularly when shed into menstrual fluid, confusing many of their status. Adding further to their misbehaviour, they express the same official MSC surface markers. To get even immature endometrial MSC strike back, claiming immunomodulatory properties in attempt to upstage their mature stromal progeny, also endowed with these properties.
Finally, other endometrial cells such as macrophages may also be naughty as their mischievousness in evading detection can trick us to consider them as stem cells from the bone marrow, masquerading as endometrial epithelial or stromal cells.
Naughty implies behaving badly and I will show data suggesting that stem/progenitor cells may escape the endometrium to cause a nasty disease, endometriosis. They may also become wayward and unruly, invading the myometrium to form adenomyosis. Some naughty epithelial progenitors defiantly pick up mutations to become cancer stem cells and initiate endometrial cancer. They may also malfunction because they do not obey estrogen signalling instructions, failing to proliferate and causing thin unresponsive endometrium. In their naughtiness, they may run away or get totally lost, thereby resulting in Asherman’s syndrome. Therefore, for numerous reasons, stem/progenitor cells are the naughty cells of the endometrium.
© The Author(s) 2020. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For permissions, please e-mail: [email protected].
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