Primary mammary epithelium in 3D Matrigel culture.
The mammary gland, which produces milk, is composed of a dense network of ducts that gradually forms after birth through a complex process of budding, invasion, and branching. At birth, the system is a rudimentary duct tree, but in response to hormones, cells in the ducts proliferate and migrate, allowing the ducts to elongate and invade through the mesenchyme layer of mammary tissue into the mammary fat pad, where they then begin to branch. Puberty brings further structural changes to the system of ducts, whereas pregnancy induces the formation of alveolar structures within the ducts for milk production.
Credit: Andrew J. Ewald, Johns Hopkins University.