Mammary gland during its development goes through several stages , which are regulated by hormones , the interaction with epithelial cells and the tissue factors from the extracellular matrix ( extracellular matrix) . Molecular mechanisms of most of these processes are little explored. One group of genes for the epithelium - mesenchyme interaction in embryonic Hox genes belonging to the family Muscle -Segment Homeobox ( Msx ) . Expression of Msx interaction occurs in the epithelium - mesenchyme and is essential for the development of the limbs, the teeth and bearings .