The breast consists of three main components, the skin, the subcutaneous adipose tissue and the functional glandular tissue that comprises both parenchyma and stroma. The collecting ducts open at the nipple. The breast is divided into 15-25 lobes, each based on a branching duct system that leads from the collecting ducts to the terminal duct-lobular units. The terminal duct-lobular units are the functional sites of milk production. Each collecting duct drains a lobe made up of 20-40 lobules.
In the nipple the stratified squamous epithelium from the surface extends into the collecting ducts for a variable short distance. Here, there is an abrupt change into glandular epithelium that is present throughout the duct and lobular system. The glandular epithelium is composed of two distinct types of cells, the secretory or luminal cells and the myoepithelial cells. In the collecting ducts the lining cells are usually columnar whereas in the acini they are usually cuboidal. Two types of luminal secretory cells have been identified. Basal cells, which have relatively clear cytoplasm and an oval nucleus lacking a visible nucleolus, and the superficial luminal cells with darker, basophilic cytoplasm. The myoepithelial cells usually form a discontinuous layer between the luminal secretory cells and the basement membrane. The myoepithelial cells appear small, flattened and with dark nuclei. In addition to the glandular cells, the lobe is composed chiefly of adipose tissue and fibrous stroma - referred to as the inter and perilobular connective tissue.