Inhibins and activins regulate mammary epithelial cell differentiation through mesenchymal-epithelial interactions

Inhibins and activins are members of the transforming growth factor beta (TGFbeta) family. Female mice in which both alleles encoding the inhibin betaB subunit have been deleted are unable to nurse their pups. We have now identified a cause of lactation failure in these mice. Ductal elongation and alveolar morphogenesis are retarded. During puberty and pregnancy, ductal outgrowth and alveolar development are limited and morphologically abnormal endbuds persist in the glands of postpartum females. The alveolar lumina fail to expand at parturition due to the absence of secreted milk. Transplantation experiments have been performed to determine whether the absence of systemic- or mammary-derived betaB subunits are the cause for the incomplete and aberrant development. While transplanted intact glands from wild-type mice grew normally in betaB-deficient hosts, betaB-deficient glands remained underdeveloped in wild-type hosts. However, betaB-deficient epithelium developed normally when transplanted into the fat pad of wild-type hosts. This demonstrates that ductal elongation and epithelial cell differentiation during puberty and pregnancy require activin/inhibin signalling from the stroma. The results further show that distinct, though related, activins and inhibins perform unique functions and are not able to compensate for the absence of activin B and AB and inhibin B in the process of mammogenesis. The betaB-deficient mice provide the first genetic evidence for stromal signalling in the adult mammary gland in vivo.