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 car milk cans had been soldered from three pieces, but they were later replaced by rolled and molded single-piece cans, which avoided uncleaned cracks in the solder joints. Milk tank cars were first made of glass-lined steel, and later of stainless steel. These tanks were often enclosed for insulation within a car body resembling a boxcar. These tank cars were usually filled with milk pre-cooled to at a central collection point just prior to pickup by a milk train with a delivery schedule avoiding need for additional cooling during transit. Milk was shipped from Wisconsin to Florida as a test; and