Depletion of S100A4+ stromal cells results in abnormal nipple development and nursing failure

The nipple and mammary gland are essential for the survival of mammalian offspring, providing postnatal nourishment. Their development, like that of epidermal appendages, depends on instructive mesenchymal signals. S100A4 (fibroblast-specific protein 1) is expressed by mesenchymal cells and has been implicated in the development of eccrine glands, hair follicles, and mammary branching morphogenesis. However, the identity and functional contribution of S100A4-positive (S100A4+) cells to nipple and mammary development remain unclear. Here, we used a cell-depletion mouse model, S100a4-Cre;DTA, to investigate their role during lactation. S100a4-Cre;DTA dams exhibited a severe nursing defect leading to complete litter loss within the first day postpartum. Immunofluorescence and oxytocin stimulation assays revealed no abnormalities in mammary morphology, milk production, or alveolar contractility, but defective nipple development was observed. Bulk RNA sequencing of nipple tissue indicated inflammatory signatures. Lineage tracing and immunofluorescence identified S100A4+ cells as fibroblasts and immune cells in the nipple, while only immune cells expressed S100A4 in the mammary gland. Our study uncovers a previously unrecognized role of S100A4+ cells in nipple development, highlighting their importance for successful lactation and offering new insights relevant to breastfeeding medicine.