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.