Epithelial hyperplasia of usual type expresses both S100-alpha and S100-beta in a heterogeneous pattern but ductal carcinoma in situ can express only S100-alpha in a monotonous pattern

Immunohistochemical identification of myoepithelial cells using alpha-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 alpha-subunit (S100-alpha) and beta-subunit (S100-beta) 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-alpha and/or S100-beta were intermingled with non-expressing cells, resulting in a mosaic-like pattern. On the contrary, DCIS (n = 32) uniformly lacked immunoreactive S100-beta; S100-alpha 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-alpha and S100-beta in a heterogeneous pattern, DCIS can express only S100-alpha in a monotonous pattern, possibly signifying unidirectional differentiation toward secretory glandular epithelium.