Olfactory undifferentiated carcinoma with endocranial extension in a Fischer-344 rat

A spontaneous tumor in the nasal cavity, extending through the cribriform plate to the cranial cavity, was detected in a 103-wk-old female Fischer-344/DuCrj rat. The tumor appeared to arise from the olfactory epithelium and was composed of round to polygonal, undifferentiated cells arranged in solid sheet-like and lobular patterns with scant fibrovascular stroma. Ultrastructurally, the tumor cells often formed desmosomes between contiguous cells and had no neurosecretory granules. Immunohistochemically, the tumor cells did not react with any antibodies specific for intermediate filaments, neuronal, muscular, and endothelial elements. Based on these results, the tumor was diagnosed as undifferentiated olfactory epithelial carcinoma.