New Aspects of Stroma-Parenchyma Relations in Mammary Gland Differentiation

This chapter reviews the morphological and biochemical evidence for stromal–parenchymal relationships in mammary gland development, mainly in mice, and discusses the possible cellular and molecular mechanism of these interactions. The mammary gland is a reproductive organ unique to the Class Mammalia. The mammary gland is composed of epithelial parenchyme and two different types of mesenchymal stroma—that is, dense mammary mesenchyme and fatty stroma. The dense mammary mesenchyme is present immediately surrounding the epithelium at the 14- to 15-day embryonic stage, in endbuds at puberty, and in cancers. This mesenchyme determines mammary epithelium and fixes the ability of the epithelium to interact with the fatty stroma. Androgen receptors appear in dense mesenchyme at the embryonic stage as a result of the inductive interaction with mammary epithelium. The fatty stroma is essential for typical mammary gland morphogenesis. Living mammary stroma with three-dimensional structure is essential for normal morphogenesis. The two types of mammary stroma synthesize different extracellular matrix proteins.