Use of serial semithin frozen sections to evaluate the co-localization of estrogen receptors and progesterone receptors in cells of breast cancer tissues.

The extent of co-expression of estrogen receptors (ER) and progesterone receptors (PgR) in breast cancer cells was examined immunocytochemically. Eight surgical cases of infiltrating ductal carcinoma designated as ER-positive and PgR-positive (ER+/PgR+) by enzyme immunoassay (EIA) were used. They were fixed with 4% formaldehyde and cut into serial frozen semithin sections. Using sections stained with either anti-ER or anti-PgR antibody, we ascertained the co-localization of ER and PgR in a single cell and estimated the ratio of the number of cells co-expressing ER and PgR. Twenty-six to 95% of the cells were immunopositive for both ER and PgR, 2-25% of them, varying in cases, were positive for ER but not for PgR, and <3% of the cells were positive for PgR but not for ER. The remaining 5-60% cells were positive for neither ER nor PgR. A significant percentage of breast cancer cells in tissues designated as ER+/PgR+ by EIA showed the phenotype of ER-positive but PgR-negative. The co-expression ratio of ER and PgR in biochemically detected ER+/PgR+ breast cancer may reflect a particular clinical parameter, such as the heterogeneous responsiveness of ER+/PgR+ breast cancers to hormonal treatment. Immunostaining of serial semithin frozen sections for two or more different antigens is a useful method to assess the correlation of localization of antigens.