Regulation of mammary gland development by tissue interaction.

Development of the mammary glands is initiated in the embryo but the major part of their development occurs in the adult. While development in puberty and pregnancy is dependent on hormones, prenatal and early postnatal development appear to progress autonomously. Mutual and reciprocal epithelial-mesenchymal interactions are critical for both phases of development. Specific steps such as the formation of the bud, the first appearance of hormone receptors, formation of the primary sprout and ductal elongation have been shown to be governed by epithelial-mesenchymal signaling. In recent years, some of the signaling molecules that are required in these processes have been identified through gene inactivation. We discuss the potential role of these factors in mediating growth and differentiation. In addition we provide evidence that mammary epithelial cells from late embryonic stages are already capable of synthesizing milk proteins when subjected to appropriate hormonal stimulation.