The development of the breast evolves through progressive formation of lobular structures. Lobules type 1 (Lob 1) are the most undifferentiated ones; they evolve to lobules type 2 (Lob 2), and those to type 3 (Lob 3), in response to mainly ovarian hormones. Although estradiol and progesterone act by binding to their specific receptors, their content in these structures and how they correlate with proliferative activity are not known. Normal breast tissues obtained from 40 reduction mammoplasties performed for cosmetic reasons in women free of mammary pathology were processed for light microscopy. Paraffin sections were immunoreacted with antibodies against estrogen receptor (ER), progesterone receptor (PgR), and proliferating cell nuclear antigen (PCNA). The number of cells positively labeled with each one of the three antibodies was quantitated in Lob 1, 2 and 3, and expressed as a percentage of the total number of cells. ER were positive in Lob 1 (21%) and Lob 2 (10%), but negative in Lob 3. PgR were positive in Lob 1 (66%) and 2 (68%), decreasing in Lob 3 (31%). The highest percentage of PCNA positive cells were found in Lob 1 (40%), decreasing in Lob 2 (33%) and Lob 3 (11%). These results indicate that the normal breast epithelium contains ER and PgR, but the former is at higher levels in Lob 1, which also expresses greater proliferative activity as demonstrated with PCNA.

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