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Cell division in normal and neoplastic mammary gland tissue in the rat

Mitotic rates of epithelial cells in Sprague‐Dawley rat mammary glands were investigated in the virgin rat, and during pregnancy, lactation and involution. In addition, rates of cell division were determined in mammary gland neoplasms induced by dimethylbenz(a)anthracene (DMBA). The mammary gland epithelium of the virgin rat exhibited a slight but continuous mitotic activity through all phases of the estrous cycle, indicating a low rate of cell renewal. During pregnancy the rate increased markedly, attaining a peak on the twelfth day. The rates of mitosis declined during lactation and involution. The majority of DMBA induced mammary gland neoplasms exhibited mitotic rates lower than the normal mammary gland during pregnancy.