Clear-cell carcinoma in the trachea of a rat.

Necropsy examination of a control Sprague-Dawley male rat from a carcinogenicity study disclosed the presence of a neoplastic mass in the trachea. Microscopic examination disclosed that the tumor consisted of closely packed cells arranged in lobules and cords separated by thin connective tissue stroma. The cells had distinct cell borders and abundant pale cytoplasm. Electron microscopic examination further disclosed that the cytoplasm contained numerous argyrophilic neurosecretory granules measuring 298 nm in diameter. These granules were positive for Grimelius silver stain. The histopathologic and electron microscopic features of this tumor are consistent with a diagnosis of clear-cell carcinoma.