Contractile fibroblasts are recruited to the growing mammary epithelium to support branching morphogenesis

Fibroblasts are stromal cells found in connective tissue that are critical for organ development, homeostasis, and disease. Single-cell transcriptomic analyses have revealed a high level of inter- and intra-organ heterogeneity of fibroblasts. However, the functional implications and lineage relations of different fibroblast subtypes remain unexplored, especially in the mammary gland. Here we provide a comprehensive characterization of pubertal mammary fibroblasts, achieved using single-cell RNA sequencing, spatial mapping, and in vivo lineage tracing. Notably, we discovered a transient niche-forming population of specialized contractile fibroblasts that exclusively localize around the tips of the growing mammary epithelium and are recruited from the surrounding fat pad. Using functional organoid-fibroblast co-cultures we reveal that different fibroblast populations can acquire contractile features when in direct contact with the epithelium, promoting morphogenesis. In summary, our exhaustive characterization of these specialized cells provides new insights into mammary fibroblast heterogeneity and implicates their functional relevance for branching morphogenesis and lineage hierarchy during mouse mammary gland development.