Decidual cell differentiation is evolutionarily derived from fibroblast activation

What the molecular mechanisms underlying the evolutionary origin of novel cell types are is a major unresolved question in biology. The uterine decidual cell is a novel cell type of placental mammals which serves as the interface between maternal and fetal tissues during pregnancy. In this paper, we investigate two models for the nature of the differentiation of decidual cells: first, that it represents a mesenchymal-epithelial transition (MET), and second, that it evolved from wound-induced fibroblast activation (WIFA). Immunocytochemistry and RNA-seq analysis of decidualizing human endometrial fibroblasts cast doubt on the MET hypothesis and instead demonstrate a similarity between decidualization and fibroblast activation, including a central role for TGFB1. Through single-cell RNA-seq, we found a transient myofibroblast-like cell population in the in vitro differentiation trajectory of human decidual cells and found that these cells represent a pre-decidual state approaching the inferred transcriptomic transition to decidual cells. We propose an evolutionary developmental model wherein the decidual cell is a novel cell type not equivalent to the myofibroblast, but the process of decidual differentiation itself evolved as an endometrial-specific modification to fibroblast activation in response to the wound caused by embryo implantation.