Histological analysis of the primary tumor tissue used to derive MAM-1. The tumor was obtained from the #8 mammary gland of a BALB-NeuT transgenic mouse. A. Hematoxylin and Eosin staining revealed a prominent fibrovascular response associated with the tumor (4×). The tumor forms a discrete nodule within a thin rim of collagen (pseudocapsule). The tumor is made of epithelial cells, arranged in ill-defined nests (arrow) that are separated by fine fibrous septa. Discrete acini, delimiting central lumens, and filled with lightly eosinophilic secretions, confer to these epithelial nests a distinct overall, sieve-like (cribriform) configuration. Necrotic material and focal hemorrhage accumulates within the tumor, in a punctate fashion (comedo-type necrosis). B. Tumor cells show homogenous cytological features and only mild pleomorphism. They are polygonal, with a relatively low N/C ratio, eosinophilic cytoplasm, round central nuclei with finely dispersed chromatin and infrequent macronucleoli and undergo frequent mitoses (40×). C. Immunohistochemical analysis of HER2/neu shows strong staining in the majority of the tumor cells, in a cytoplasmic and membranous distribution. D. Immunohistochemical analysis for α-SMA detected strong cytoplasmic staining associated exclusively with the stromal cells (arrow) surrounding the tumor cell nests. The antigens were detected by DAB (brown) and nuclei counterstained with hematoxylin (blue). Images were photographed under 40× objective.