Mammary gland development is marked by numerous cellular changes that begin in the embryo, continue postnatally, are modified at puberty, and come to full fruition during the adult cycles of pregnancy, lactation, and involution. These changes are initiated by hormones produced at distant sites and by the microenvironment within the gland itself. The latter, which includes soluble factors and the insoluble extracellular matrix (ECM), will be the focus of this chapter.
