Mammary gland gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells' branching. When the lactiferous duct tree is almost ready, "leaves" alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates
Milk substitute Milk substitute A milk substitute (or, more precisely, an analogue of mammalian milk) (also called milk alternative or milk analogue) can refer to a liquid considered (particularly in the USA and other western countries) as a replacement for the milk from a mammal (mainly cows and humans). Mammalian milk analogues are white or whitish liquids that resemble either dairy or human breast milk. Milk substitutes for non-infant consumption take two very different forms: plant milks, which are liquids made from plants and may be home-made or commercially produced, and coffee creamers, synthetic products invented in the USA in the 1900s