Chemical Reprogramming of Fibroblasts to Functional Mammary Epithelial Cells via a Single BMP Inhibitor

Direct lineage reprogramming offers a highly promising non-genetic pathway for generating functional somatic cells. In this study, a single small molecule inhibitor of the BMP pathway, LDN193189, was able to directly reprogram goat ear fibroblasts into Chemically induced Mammary Epithelial Cells (CiMECs) in vitro within 8 days. The generated CiMECs displayed typical epithelial morphology, expressed key mammary epithelial markers (including KRT8/14/18/19, CD49f, and EpCAM), and exhibited potent secretory functions: they could form lipid droplets and produce milk proteins such as lactoferrin (LTF) and αS2-casein (αS2-CSN). Single-cell RNA sequencing revealed a stepwise reprogramming trajectory: starting from mesenchymal-epithelial transition (MET), through ectoderm-like and epidermal-like intermediate states to differentiate into basal and lumenal mammary epithelial cells, which involves multiple signaling pathways related to mammary gland development, such as TGF-β, MAPK, and PI3K-Akt. Functionally, CiMECs could self-assemble in vitro to form three-dimensional mammary-like organs. In addition, in vivo transplantation experiments in nude mice showed that these CiMECs survived and contributed to ductal- and basal-like tissue structures, confirming their in vivo regenerative potential. In conclusion, this study establishes a simple somatic cell-based platform with a well-defined chemical composition to efficiently generate functional mammary epithelial cells, providing an important model for mammary developmental studies and opening up promising strategies for regenerative medicine and bioreactor engineering applications.