Persistence of responsiveness of adult mouse mammary gland to induction by embryonic mesenchyme

Persistence of the capacity for embryogenic morphogenesis in adult mammary epithelium was demonstrated by allowing it to interact with grafted embryonic mesenchyme in vivo. When 14-day embryonic mammary or salivary mesenchyme was transplanted in the mammary gland of syngeneic young adult virgin mice, organogenetic development of the mammary epithelial cells occurred responding to closely attached mesenchyme. An early change, within 2–4 days, that was observed equally in both types of the mesenchymes was proliferation of mammary epithelial cells in multiple layers resembling rudimental architecture. Subsequently, ductal branching occurred from the rudimental architecture by mesenchyme-dependent branching pattern, of mammary gland type with mammary mesenchyme and of salivary gland-like type with salivary mesenchyme. This developmental response did not require hormones secreted from ovaries since it was observed similarly in ovariectomized mice. The mammary epithelium at the lactating stage did not show such a potential to the transplanted salivary mesenchyme.