| Abstract: | Immunohistochemical identification of myoepithelial cells using α-smooth muscle actin provides little information about the nature of solid or quasi-solid portions of epithelial hyperplasia and ductal carcinoma in situ (DCIS) because actin-rich myoepithelial cells are usually demonstrated only in the stromal–epithelial junction of both lesions. We studied the differential distribution of α-subunit (S100-α) and β-subunit (S100-β) of S100 protein in actin-negative areas of usual epithelial hyperplasia and DCIS by employing the streptavidin method with monospecific rabbit antibodies against each subunit. All usual epithelial hyperplasias (n=17) were composed of heterogeneous epithelial cell types; cells expressing S100-α and/or S100-β were intermingled with non-expressing cells, resulting in a mosaic-like pattern. On the contrary, DCIS (n=32) uniformly lacked immunoreactive S100-β; S100-α was diffusely expressed in 24 (68.8%) DCIS (three solid/comedo, 13 cribriform, four endocrine, one micropapillary, three papillary variants) and negative in the remaining eight (31.2%) DCIS (one cribriform, two micropapillary, four papillary and one apocrine variants). In conclusion, in contrast to usual epithelial hyperplasia that expresses both S100-α and S100-β in a heterogeneous pattern, DCIS can express only S100-α in a monotonous pattern, possibly signifying unidirectional differentiation toward secretory glandular epithelium. |
