The origins of placental mesenchymal stromal cells: Full spectrum flow cytometry reveals mesenchymal heterogeneity in first trimester placentae, and phenotypic convergence in culture

Single-cell technologies (RNA-sequencing, flow cytometry) are critical tools to reveal how cell heterogeneity impacts developmental pathways. The placenta is a fetal exchange organ, containing a heterogeneous mix of mesenchymal cells (fibroblasts, myofibroblasts, perivascular, and progenitor cells). Placental mesenchymal stromal cells (pMSC) are also routinely isolated, for therapeutic and research purposes. However, our understanding of the diverse phenotypes of placental mesenchymal lineages, and their relationships remain unclear. We designed a 23-colour flow cytometry panel to assess mesenchymal heterogeneity in first-trimester human placentae.. Four distinct mesenchymal subsets were identified; CD73+CD90+ mesenchymal cells, CD146+CD271+ perivascular cells, podoplanin+CD36+ stromal cells, and CD26+CD90+ myofibroblasts. CD73+CD90+ and podoplanin+CD36+ cells expressed markers consistent with cultured pMSCs, and were explored further. Despite their distinct ex-vivo phenotype, in culture CD73+CD90+ cells and podoplanin+CD36+ cells underwent phenotypic convergence, losing CD271 or CD36 expression respectively, and homogenously exhibiting a basic MSC phenotype (CD73+CD90+CD31-CD144-CD45-). However, some markers (CD26, CD146) were not impacted, or differentially impacted by culture in different populations. Comparisons of cultured phenotypes to pMSCs further suggested cultured pMSCs originate from podoplanin+CD36+ cells. This highlights the importance of detailed cell phenotyping to optimise therapeutic capacity, and ensure use of relevant cells in functional assays.