Hormone-dependent differentiation of immature mouse mammary gland in vitro.

Explants from the mammary glands of 3-week-old mice can be induced to synthesize casein in vitro in the absence of lobuloalveolar development. Maximum biochemical differentiation requires the presence of insulin, hydrocortisone, and prolactin in the culture medium. In contrast to explants from adult mice, the mammary epithelium of immature animals undergoes DNA synthesis and mitosis in vitro in the absence of exogenous insulin; however, such proliferation does not lead to the formation of differentiated daughter cells. Insulin acts in at least two ways during the proliferative phases of the cell cycle of differentiating mammary epithelial cells.