Morphologic characterization of spontaneous nervous system tumors in mice and rats

Spontaneous rodent nervous system tumors, in comparison to those of man, ar e less well differentiated. Among the central nervous system (CNS) rumors, the "embryonic" forms (medulloblastoma, pineoblastoma) occur both in rodent s and humans, whereas the human "adult" forms (gliomas, ependymomas, mening iomas) have fewer counterparts in rodents. In general, the incidence of spo ntaneous CNS tumors is higher in rats (<1%) than in mice (>0.001%). A chara cteristic rat CNS tumor is the granular cell tumor. Usually it is associate d with the meninges, and most meningeal tumors in rats seem to be totally o r at least partly composed of granular cells, which have eosinophilic granu lar cytoplasm, are periodic acid-Schiff reaction (PAS)-positive, and contai n lysosomes. Such tumors are frequently found on the cerebellar surface or at the brain basis. Rat astrocytomas are diffuse, frequently multifocal, an d they invade perivascular spaces and meninges. The neoplastic cells with r ound to oval nuclei and indistinct cytoplasm grow around preexisting neuron s, producing satellitosis. In large tumors, there are necrotic areas surrou nded by palisading cells. Extensive damage of brain tissue is associated wi th the presence of scavenger cells that react positively with histiocytic/m acrophage markers. The neoplastic astrocytes do not stain positively for gl ial fibrillary acidic protein; they probably represent an immature phenotyp e. In contrast to neoplastic oligodendroglia, they bind the lectin RCA-1. A strocytomas are frequently located in the brain stern, especially the basal ganglia. Rat oligodendroglial tumors are well circumscribed and frequently grow in the walls of brain ventricles. Their cells have water-clear cytopl asm and round, dark-staining nuclei. Atypical vascular endothelial prolifer ation occurs, especially at the tumor periphery. Occasionally in the oligod endrogliomas, primitive glial elements with large nuclei occur in the form of cell groups that form rows and circles. Primitive neuroectodermal rumors of rats, such as pineal tumors or medulloblastomas, appear to have feature s similar to those found in man. In mice, the meningeal tumors are mostly d evoid of granular cells and the astrocytomas are similar to those occurring in rats. whereas spontaneous oligodendrogliomas are observed extremely rar ely. Tumorlike lesions, such as lipomatous hamartomas or epidermoid cysts, are occasionally encountered in the mouse CNS. It is suggested that we clas sify rodent CNS lesions as "low grade" and "high grade" rather than as "ben ign" and "malignant." The size of CNS turners is generally related to their malignancy. Tumors of the peripheral nervous system are schwannomas and ne urofibromas or neurofibrosarcomas consisting of Schwann cells, fibroblasts, and perineural cells. Well-differentiated schwannomas are characterized by S-100 positivity and the presence of basement membrane. They show tither A ntoni A pattern with fusiform palisading cells or Antoni B pattern, which i s sparsely cellular and has a clear matrix. The rat develops specific forms of schwannomas in the areas of the submandibular salivary gland, the exter nal ear, the orbit, and the endocardium. Spontaneous ganglioneuromas occur in the rat adrenal medulla or thyroid gland. Compared to experimentally ind uced neoplasms, the spontaneous tumors of the rodent nervous system are poo r and impractical models of human disease, although they may serve as gener al indicators of the carcinogenic potential of tested chemicals.