Abstract
The endometrium is a dynamic tissue that exhibits remarkable resilience to repeated episodes of differentiation, breakdown, regeneration, and remodeling. Endometrial physiology relies on a complex interplay between the stromal and epithelial compartments with the former containing a mixture of fibroblasts, vascular, and immune cells. There is evidence for rare populations of putative mesenchymal progenitor cells located in the perivascular niche of human endometrium, but the existence of an equivalent cell population in mouse is unclear. We used the Pdgfrb‐BAC‐eGFP transgenic reporter mouse in combination with bulk and single‐cell RNA sequencing to redefine the endometrial mesenchyme. In contrast to previous reports we show that CD146 is expressed in both PDGFRβ + perivascular cells and CD31 + endothelial cells. Bulk RNAseq revealed cells in the perivascular niche which express the high levels of Pdgfrb as well as genes previously identified in pericytes and/or vascular smooth muscle cells (Acta2, Myh11, Olfr78, Cspg4, Rgs4, Rgs5, Kcnj8, and Abcc9). scRNA‐seq identified five subpopulations of cells including closely related pericytes/vascular smooth muscle cells and three subpopulations of fibroblasts. All three fibroblast populations were PDGFRα+/CD34 + but were distinct in their expression of Ngfr/Spon2/Angptl7 (F1), Cxcl14/Smoc2/Rgs2 (F2), and Clec3b/Col14a1/Mmp3 (F3), with potential functions in the regulation of immune responses, response to wounding, and organization of extracellular matrix, respectively. Immunohistochemistry was used to investigate the spatial distribution of these populations revealing F1/NGFR + cells in most abundance beside epithelial cells. We provide the first definitive analysis of mesenchymal cells in the adult mouse endometrium identifying five subpopulations providing a platform for comparisons between mesenchymal cells in endometrium and other adult tissues which are prone to fibrosis.
Highlights
The endometrium of women and mice share a similar architecture with epithelial cells lining the glands and lumen supported on a complex stroma containing fibroblasts and a well-developed vasculature
In the current study we were able to target our analysis to a purified population of 5000 green fluorescent protein (GFP)+/platelet-derived growth factor receptor beta (PDGFRβ) + cells detecting 18,000 unique genes and this allowed us to identify five populations of cells including a vascular smooth muscle cell (VSMC) cluster that expresses Acta[2] and Mcam with the latter in a population we identified as pericytes which did not appear to be separated in this human endometrial study
We have demonstrated that PDGFRβ and cluster of differentiation 146/melanoma cell adhesion molecule/MUC18 (CD146) are reported to be ubiquitously expressed by pericytes in a wide range of tissues[36] in endometrium they are not exclusive to this cell type
Summary
The endometrium of women and mice share a similar architecture with epithelial cells lining the glands and lumen supported on a complex stroma containing fibroblasts and a well-developed vasculature. A menstrual-like event can be simulated in other mouse species following artificial induction of decidualization.[7,8] In both natural and induced menstrual cycles the surface of the endometrium resembles a bloody wound during tissue breakdown but repair is both rapid and scar-free with evidence of epithelial cell proliferation, mobilization of stromal cells, and evidence of mesenchymal to epithelial cell transformation.[3,8] It has been postulated that the rapid restoration of endometrial tissue integrity is in part due to mobilization and differentiation of tissue-resident endometrial progenitor cells.[9] Many studies have searched for candidate endometrial progenitor cells using in vitro assays based on clonogenicity and self-renewal as well as whole tissue analysis including recovery of side population cells (SP) and those labeled with BrdU in label retention assays.[10,11] In 2004, Chan et al, reported the existence of small populations of epithelial and stromal cells in human endometrium that were able to form colonies from single-cell suspensions in vitro (0.22% and 1.25%, respectively.[12] These cells exhibited key features of stem-like cells including self-renewal, a high proliferative potential, and the capacity for multilineage differentiation.[13] A population of perivascular cells from human endometrium with clonogenic and multi-lineage differentiation potential in vitro was initially isolated based on the co-expression of PDGFRβ and CD146 12,14 but in further studies sushi domain-containing protein 2 (SUSD2), recognized by the W5C5 monoclonal antibody has emerged as a definitive candidate marker for putative endometrial mesenchymal progenitors.[9]. Did these studies identify and characterize a population of perivascular cells in the mouse endometrium but novel findings revealed evidence of unexpected complexity in the fibroblast populations that make up the bulk of the endometrial stromal mesenchyme
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