The inner cell layer of the mammary epithelium bilayer that lines the luminal surface of mammary gland ducts and alveoli; luminal cells have only limited contact with the underlying basement membrane and surrounding connective tissue; they are secretory epithelial cells characterized by the expression of the receptors for systemic hormones, i.e. estrogens, progesterone and prolactin. [ http://www.informatics.jax.org/accession/anna ]