A study has been made of the selective influence of the ovarian steroids, estradiol and progesterone, on three distinct parts of cultured, explanted human adult resting mammary tissue: lobules, interlobular and large ducts. The addition of estradiol elicited the highest stimulation of3H-thymidine incorporation in large ducts. When progesterone was added DNA synthesis occurred mainly in interlobular ducts. The isolated action of both steroids on lobules was quantitatively comparable, but the time sequence of DNA synthesis was different. The possible significance of these findings is discussed.
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