Regulation of mammary morphogenesis: Evidence for extracellular matrix-mediated inhibition of ductal budding by transforming growth factor-β1

Abstract

Branching morphogenesis in the mammary gland involves focal regions of cell proliferation, the terminal and lateral ductal buds, that exist simultaneously with extensive regions of differentiated ducts in which budding and growth are actively suppressed. Exogenous transforming growth factor-β1 (TGF-β1) has previously been shown to locally inhibit the formation and growth of mammary ductal buds. Here we report that endogenous TGF-β1, produced by epithelial and stromal mammary cells, forms complexes with extracellular matrix (ECM) molecules surrounding those ductal structures in which budding is inhibited. The largest amounts of immunostainable TGF-β1 are found in mature periductal ECM, and the least in newly synthesized ECM. In all areas of active ductal growth, where DNA-synthetic buds were forming new ductal branches, we found a highly focal loss of TGF-β1 from the periductal ECM at the bud-forming region of the duct. When growth of the new buds terminated, the structures again became associated with TGF-β-rich ECM. These findings indicate that ECM must reach a certain state of maturity before it becomes associated with TGF-β1 and that TGF-β1 can be depleted selectively from the periductal ECM at focal growth points. A different type of growth point, the alveolar (secretory) buds, was also investigated. These buds are known not to be inhibited by exogenous TGF-β1, and we found them not to be associated with changes in ECM-bound TGF-β1. Our results support the concept that the periductal ECM acts as a reservoir for TGF-β1 that functions to maintain an open pattern of mammary branching by inhibiting ductal, but not alveolar, bud formation.