The normal microenvironment directs mammary gland development.

Normal development of the mammary gland is a multidimensional process that is controlled in part by its mammary microenvironment. The mammary microenvironment is a defined location that encompasses mammary somatic stem cells, neighboring signaling cells, the basement membrane and extracellular matrix, mammary fibroblasts as well as the intercellular signals produced and received by these cells. These dynamic signals take numerous forms including growth factors, steroids, cell-cell or cell-basement membrane physical interactions. Cellular growth and differentiation of the mammary gland throughout the developmental stages are regulated by changes in these signals and interactions. The purpose of this review is to summarize current information and research regarding the role of the mammary microenvironment during normal glandular development.