Epidemiological and clinical observations indicate that breast cancer incidence is greater in nulliparous women, whereas early parity confers protection. Since the initiation of breast cancer is related to the degree of development of the organ, this study was designed with the purpose of determining what basic differences exist between the parous and the nulliparous women's breast, and whether these differences correlated with the presence or absence of malignancies. For this purpose, the architecture of the mammary gland of parous and nulliparous women with breast cancer was compared with that of women free of mammary pathology. The women ranged in an age from 20 to 63 years. Normal whole breasts obtained at autopsy or cancer-bearing breasts surgically removed by modified radical mastectomy were studied in whole mount preparations in which the number and relative proportion of normal structures, i.e., lobules type 1, 2, and 3 (Lob 1, 2, and 3), were determined. In the breast of nulliparous women, the predominant structure present was the Lob 1; the presence of cancer did not modify the basic architectural pattern of the breast, which contained a higher proportion of Lob 1, as well. In parous women free of cancer, the breast contained a greater percentage of Lob 3 and a moderately increased number of Lob 2, with a concomitant reduction in Lob 1. Parous women with breast cancer, on the other hand, exhibited a different architecture in the mammary gland, which had a greater percentage of Lob 1 and a lower of Lob 3 than the noncancerous group, approaching the percentages found in nulliparous women.(ABSTRACT TRUNCATED AT 250 WORDS)

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