Abstract
Mouse mammary gland, which shows a progressive decline in rate of ductal growth during serial propagation in vivo, was passaged in both normal virgin hosts and in hosts bearing pituitary isografts. In both groups ductal proliferation declined, and there is no evidence that the mammogenic stimulation provided by additional pituitaries prevented loss of ductal growth potential. In the pituitary-stimulated group, it was found that alveolar proliferation, which was conspicuous in early transplant generations, also declined with age. Finally, mammary gland was propagated in normal virgins without pituitaries until ductal growth was slowed, and a single exposure to alveolar-mammogenic stimulation was provided by breeding. Alveolar proliferation was then normal and extensive, but ductal elongation was not stimulated.
These results suggest that aging of mammary cells, when measured by reduced growth potential, occurs because of declining ability to respond by proliferation to any set of endocrine stimuli that is constant and which continues over a long period of time. The ability of gland which is aged under one particular set of humoral conditions to proliferate in response to another, different set, suggests that the observed aging effect represents progressive alterations to hormone-responsive regulatory mechanisms, and does not indicate a permanent inability of mammary cells to divide.