Practical pathology of aging mice

Pettan-Brewer, Christina; Treuting, Piper M.

Subcutaneous masses in the inguinal or pelvic region often were enlarged cystic or infected preputial or clitoral glands (Fig. 4E,F) . The subcutaneous lymph nodes, especially the submandibular, may be enlarged due to physiologic lymphoid reactive hyperplasia secondary to systemic or localized inflammation. Systemic lymphadenopathy involving subcutaneous and internal lymph nodes is most frequently due to malignant lymphoma. Other subcutaneous structures such as the mammary fat pads or salivary glands may also become inflamed or neoplastic and present as palpable masses. Often, the masses can be diagnosed as inflammatory or neoplastic at necropsy with the combination of the gross presentation and cytology. However, histopathology is definitive as neoplasia may also be infected or inflamed. For example, the retro-orbital mass in Fig. 4C was suspected clinically, and at necropsy, to be a chronic retrobulbar abscess. Final histopathological diagnosis was a Harderian gland adenoma (Fig. 3D) . For malignancies within the lung, determining whether neoplastic masses are primary lung tumors versus metastatic disease may be challenging without the aid of histology (Fig. 4G,H) . Inbred strains vary in their typical spectrum of disease and neoplasia is no exception (9, 35, 36) . In the B6 background, hematopoietic neoplasias (malignant lymphoma and histiocytic sarcoma) are extremely common (Table 1) (21, 36, 37) . Necropsy findings for these hematopoietic neoplasias often overlap, for example, enlarged mesenteric lymph nodes (Fig. 4I) can be affected by either type. While there are differences in organ distribution and cellular morphology (Fig. 5IÁL ) that allow distinction between the malignant lymphomas and histiocytic sarcoma, definitive diagnosis may require immunohistochemical or molecular analysis (38Á40).