Architecture and morphology of the mammary gland. (a) A cartoon representation of the structure of the epithelial tissue of the human mammary gland indicating a large duct branching into a lobule. (b) A representation of a cross section cut through the bilayered epithelia: many bilaryered acini that are part of the lobule would be apparent yet their direct connection to the lobule 'disappears' in the 2D cross section. (c) A magnified cross section of the terminal ductal lobular unit (TDLU) referred to as an acinus. Acinar polarity is demonstrated where apical proteins face the lumen formed by luminal epithelial cells and the basement membrane (BM) is in contact with myoepithelial cells (d) S1 cultured cells form a single layered acinus-like structure in 3D culture with apico-basal polarity despite the lack of the myoepithelial layer.