Fig. 8.

Model illustrating the plasticity of stromal progenitors in the mammary gland. The mammary stroma is home to PDGFRα⁺ PREF-1⁺ mesenchymal progenitor cells that are found in close promixity to the parenchymal epithelium. These cells can differentiate into adipocytes (Adiponectin⁺) in the mammary fat pad, or switch to an epithelial cell fate (EpCAM⁺). To undergo this stromal-to-epithelial transition, mammary epithelial cells are stimulated by adult sex hormones such as estrogen and progesterone (E+P) to secrete PDGFCC, a PDGFRα ligand, which induces chemotactic migration of PDGFRα⁺ mesenchymal progenitors into the epithelial niche. In the epithelium, these stroma-derived cells generate epithelial progeny, primarily giving rise to the hormone receptor (HR)- negative luminal lineage in the adult during epithelial expansion