any structural anomaly of the complex non-epithelial mammary gland tissue composed of fibrous connective tissue extracellular matrix (ECM) proteins, and a wide variety of cell types, including inter- and intralobular fibroblasts, adipocytes, endothelial cells, and innate immune cells (both macrophages and mast cells); the mammary gland stroma is the support network for the epithelium, providing both nutrients and blood supply, and immune defenses, as well as physical structure to the gland; each of the different stromal cell types secrete instructive signals that are crucial for various aspects of the development and function of the epithelium