Progesterone is not essential to the differentiative potential of mammary epithelium in the male mouse

In pursuit of a model system in which to determine whether or not exposure to progesterone is necessary for mammary epithelial cells to develop their differentiative potential, we explored hormone-dependent growth of the mammary epithelial rudiment in adult male mice. Initiation of the formation of ductal cells can be effected by administration of estradiol in the absence of endogenous progesterone and glucocorticoid using adrenalectomized-castrated animals. The resulting epithelium contains three times more lactose synthetase activity per epithelial cell than that in midpregnant mice. The blood spermidine level in these doubly operated animals was similar to the concentration of spermidine required to substitute effectively for glucocorticoid during mammary differentiation in vitro. It is suggested that spermidine can partially supplant glucocorticoid in vivo in milk protein synthesis. We also concluded that, unlike other secondary sex tissues, mammary cells do not require exposure to progesterone during their ontogeny in order to realize their differentiative potential. The positive role of this steroid in mammary development is apparently limited to its effect on the formation of alveolar structures.