Mammary gland reprogramming: metalloproteinases couple form with function

The adult mammary structure provides for the rapid growth, development, and immuno-logical protection of the live-born young of mammals through its production of milk. The dynamic remodeling of the branched epithelial structure of the mammary gland in response to physiological stimuli that allow its programmed branching morphogenesis at puberty, cyclical turnover during the reproductive cycle, differentiation into a secretory organ at parturition, postlactational involution, and ultimately, regression with age is critical for these processes. Extracellular metalloproteinases are essential for the remodeling pro-grams that operate in the tissue microenvironment at the interface of the epithelium and the stroma, coupling formwith function.Deregulated proteolytic activity drives the transition of a physiological mammary microenvironment into a tumor microenvironment, facilitating malignant transformation. Milk production is an evolutionary survi-val strategy that allows the rapid growth and development of live-born young, as well as being a defining trait of mammals. Milk pro-duction at parturition and not at other time