Breast cancer is heavily influenced by the reproductive history of the individual. Pregnancy has a protective effect which is attributed to differences in the degree of differentiation of the breast. The purpose of this work was to determine whether the quantity and the type of parenchymal structures present in the human breast were related to the age and parity history of a woman. Fifty-one human breast samples were obtained from bilateral or unilateral reduction mammoplasties performed in 40 parous women ranging in age from 18 to 57 years, and 11 nulliparous women ranging in age from 14 to 54 years. An average of 100 grams of tissue/specimen were processed for whole mount. A total of 650 slides were examined and 31,222 structures were classified and counted under the light microscope. The following mammary structures were identified: terminal structures (TS), and lobules (LOB) type 1, 2, and 3. Results were plotted for the total patient population and separately for nulliparous and parous women against age. The total patient population contained similar proportions of LOb1, 2, and 3 between ages 14-18, with a reduction of percentage of Lob1 and increase in Lob3 between ages 23 to the middle forties, when Lob3 decreased and Lob1 increased to 70%. Lob2 and TS did not exhibit significant changes throughout the period of life analyzed. When analyzed separately it was found that the breasts of nulliparous women were predominantly composed of Lob1, fewer Lob2, with Lob3 almost completely absent, whereas parous women had a high frequency of Lob3, which were the predominant structures until the fourth decade of life.(ABSTRACT TRUNCATED AT 250 WORDS)

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