A Basal-Cell Epithelioma of the Rat

Abstract

The tumor to be described arose spontaneously in a young male rat, about five months old, weighing 60 grams. When first observed, it appeared as a flattened oval mass, about 2.5 by 1.5 cm. in diameter, occupying the skin of the left thoracic region, and resembled an epithelioma such as occurs frequently on the human face. At the center the tumor was ulcerated, with an uneven granular surface. The margins of skin about this were nodular, raised, thickened, and slightly undermined, and of a reddened shiny appearance which shaded rather abruptly into the normal tissues. The consistency of the mass was distinctly firm, and it was freely movable on the deeper parts. The tumor was removed by operation, and the major part of it was inoculated into 204 rats and, in two places, into its bearer. Serial paraffin sections which were made of the remaining tissue showed the following histological characteristics: The tumor was in direct connection with the skin (fig. 1) and extended into the subcutaneous tissue but not into the muscle; it was composed of rounded and irregular alveolar masses of cells having the same staining and morphological characteristics as the basal cells of the epidermis and skin appendages (fig. 2). There were all gradations from typical basal cells to those of a spindle shape. The cells were of small size, with a deeply staining nucleus and relatively small amounts of protoplasm giving the alveoli a very dark appearance. In places these cells were in strands and whorls resembling a spindle-cell sarcoma (fig. 3). Here and there, in the center of cellular areas, were glandular (fig. 4) or cystic structures filled with cell debris and lined by squamous cells in layers; there was, however, no production of keratin. The overlying skin was not abnormal except at one point where the strands of tumor cells and the basal layer of the epidermis were continuous. Staining with the Bielschowski method showed no intercellular collagen fibrils. The interalveolar connective tissue showed marked proliferation and in places resembled sarcoma. As may be seen in the photomicrographs there was an unusual amount of lymphocytic infiltration about the tumor alveoli.