Figure 1. A cartoon showing archetypical age-related states of breast tissue.

With increased age there is an increase in fat and a decrease in connective tissue in the stromal that surrounds the pseudostratified mammary epithelia. The mammary epithelium is a branching bilayered tissue, but the diagram shown is meant to convey the age-related changes that can arise at cellular resolution. With age, multipotent progenitors accumulate, luminal cells lose lineage fidelity and take on characteristics of the more basal myoepithelial cells. The proportions of tumor suppressive and contractile myoepithelial cells themselves also are diminished with age. Reduction of myoepithelial cells might help explain putative discontinuities in the basement membrane that are thought to arise with age because those cells produce many key basement membrane components.