Milk and Rat Mammary Tumors 63 milk and estrone sulfate groups. At autopsy, the cumulative number of tumors in the milk, estrone sulfate, and water groups was 53, 33, and 17, respectively. Mean number of tumors per rat in the milk, estrone sulfate, and water groups was 3.3, 2.1, and 1.0, respectively. Mean tumor number per rat in milk group was significantly higher than that in the water group. Histology of mammary tumors Several histological types were recognized in mam- mary tumors induced by DMBA. According to histological findings, tumors were classified into 3 histological types: intraductal papilloma, fibroadenoma, and adenocarcinoma. Intraductal papilloma is the most common type of tumor in this experiment. Figure 1A shows a typical intraductal papilloma. The papillae containing both epithelial cells and myoepithelial cells are composed of a core of fibrovascu- lar tissue lined by a single layer of cuboidal or low cylindri- cal epithelial cells. The epithelial cells are homogeneous in size and shape; they have leptochromatic nuclei, and the nucleolus is either absent or inconspicuous; some cellular atypia and mitoses may be present but scarce. Fibroade- noma is a benign tumor composed of mammary epithelium and numerous connective tissues. Ductal and lobular struc- tures are surrounded with layers of fibrous tissue. As with normal rat mammary gland, the epithelium underlies the myoepithelium and basement membrane (Fig. 1B). Adeno- carcinoma is characterized by the loss of the tubular-alveolar pattern. Solid sheets of neoplastic epithelial cells are inter- rupted by secondary lumina of round, oval or irregular shape and variable size, representing a cribriform pattern. No single metastasis was noted, while vascular local invasion can be seen sporadically in many tumors. Individual neo- plastic cells are moderately to markedly pleomorphic, the degree of pleomorphism varies from tumor to tumor and even in different areas of the same tumor (Fig. 1C). As in human mammary carcinoma, the different types were not clearly defined and were found in different parts of the same tumor. The results of histological classification are summa- rized in Table 2. Most of the tumors were diagnosed as intra- ductal papilloma or fibroadenoma. There was no significant difference in the percentages of histological types among the three groups. However, the milk group and estrone sulfate group developed adenocarcinoma but the water group did not. Immunohistochemistry of SMA, ERα, and PCNA Immunoperoxidase reaction with anti-alpha SMA anti- body revealed elongated myoepithelial-like cells at the periphery of epithelial cell nests and glandular structures in all tumors. Appearance and number of myoepithelial cells showing positive for SMA varied among different histo- logical types of rat breast tumors. Most of the epithelial cell nests in intraductal papilloma are lined by myoepi- thelial cells (Fig. 2A). Adenocarcinoma were also positive for SMA immunostaining and myoepithelial cells were stretched and scattered at the base of the epithelial cell nests. Additionally, the number of myoepithelial cells in adeno- carcinoma was apparently lower than that in intraductal Fig. 1. HE staining of rat mammary tumor. A) Intraductal papilloma. Numerous papillary projections sustained by thin connective tissue cores can be seen. HE×25. B) Fibroadenoma. Fibrous stroma is compressing the tubular alveolar components. HE×25. C) Adeno- carcinoma. Numerous secondary lumina can be seen. HE×25.