ABSTRACT
This chapter describes the major lesions morphologically and many are documented with photographs. One reason for this is to provide a general reference for investigators who only sporadically evaluate pathologic findings in aging rats. Lesions of the mouth were limited to gross findings such as overgrown incisor teeth and grossly observed masses. Microscopically the lesions consisted of several patterns such as tubular structures, spaces or small cystic structures lined by papillary mesothelium, and areas of spindle cells. Nonneoplastic age-associated lesions of the thyroid glands, with the exception of parafollicular cell hyperplasias, were not common in the aging Brown Norway (BN)/Bi, Wistar-derived (WAG)/Rij, or (WAG × BN)F1 rats. Medullary thyroid carcinomas were described in WAG/Rij rats by G. A. Boorman et al. They described the light microscopic and ultrastructural features of the tumors and confirmed the parafollicular cell as the cell of origin.
