Abstract
Prolactin administration to mature virgin mice leads to prolilferation of epithelium of the mammary gland. In organ culture, prolactin is not mitogenic, but insulin and insulin-free serum are. These observations are reconciled by the finding that insensitive epithelium is sensitized to insulin and/or serum by prolactin treatment in vivo. Thus, it appears that prolactin acts indirectly as a mitogen by rendering the cells susceptible to the aforementioned mitogens.
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