S100A4+ stromal cells are required for functional nipple development and successful nursing

Abstract

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 signals from mesenchymal cells. S100A4 (S100 calcium binding protein A4, also known as fibroblast-specific protein 1) is expressed by mesenchymal cells and has been associated with hair follicle regeneration. S100A4-expressing cells have been implicated in the development of eccrine glands, and studies using S100a4-Cre to manipulate gene function have suggested that S100A4-expressing cells may contribute to 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 analysis 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 staining identified S100A4+ cells as fibroblasts and immune cells in the nipple, while in the mammary gland S100A4 was expressed only in immune cells. 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.