Dual origin of mesenchymal tissues participating in mouse mammary gland embryogenesis

By the 14th day of gestation, two different mesenchymes can be identified which affect mouse mammary gland embryogenesis: the fibroblastic mammary mesenchyme (MM) closely surrounding the epithelial rudiment, and a condensed mesenchymal tissue (FP) appearing separately, posterior to the mammary rudiment, the precursor tissue of the fat pad. Late on the 16th day, the mammary epithelium (ME), surrounded by MM, starts to elongate, puts out branches, and penetrates the FP. A fatty substance appears in the FP at this stage. Interaction between ME and FP is necessary for typical mammary morphogenesis. When 17-day ME is combined with 14- or 17-day FP, the resulting mammary gland has the normal mammary pattern, but when 17-day ME is combined with 12- to 17-day MM, a ductal hyperplasia is formed by frequent branching, without the “stretching out” of these ducts. All the glands formed by combining ME with either FP or MM will lactate, if the mice carrying the grafts are allowed to mate and give birth. Adult ME also shows a different response to MM and FP.