The capacity of mammary gland tissue slices to incorporate [3H]thymidine into DNA was studied at intervals after intraductal injection of prolactin. Prolactin markedly increased the rate of incorporation of thymidine into DNA (as extracted by HCIO4 or by sodium dodecyl sulphate). The increase in incorporation rate occurred within 6 h of prolactin injection. Autoradiography of mammary tissue, after administration of [8H]thymidine in vivo, showed that the increased rate of incorporation of thymidine was caused by an increase in the number of alveolar cells entering S-phase. These results are discussed in relation to current theories concerning functional differentiation of mammary tissue.

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