Naturally occurring schwannoma in a Fischer 344 rat.

Fig. 3. Fibrin thrombus with organization in right basal vein. HE. Bar = 250 wm. great cerebral vein, the difference in cerebral venous anatomy may account for the thrombosis of the right basal vein and the lack ofthrombosis in the great cerebral vein ofthe present canine c a ~ e. ~. ~ The fresh fibrin thrombus in the midportion of the left basal vein was considered to be responsible for the lesion in the left hypothalamus. The thrombus within the right basal vein was histologically a subacute lesion undergoing fibrous organization, whereas clinical signs, neuropil lesions, and thrombi in the small blood vessels and capillaries were compatible with a more acute process.6Jo Such multi-aged throm-bi also have been reported in cases of human and nonhuman primates with deep cerebral vein thrombosi~.~,~ The large 320 Brief Communications venous thrombus in the right basal vein in the present case may have aggravated the hypoxic injury elicited by the acute thrombosis of small blood vessels and contributed to clinical signs and lesions. Tumors of cranial, spinal, and peripheral nerves have been reported in rats treated with chemical carcinogen^,^.'^ but naturally occurring peripheral nerve tumors of rats rarely have been reported.L,5,6 Here we report the gross, histologic, ultrastructural, and immunocytochemical characteristics of a naturally occurring peripheral nerve neoplasm in a Fischer 344 (F344) rat. A 14-month-old male F344 rat from a commercial supplier fed a commercial pelleted diet ad libitum died 2 months after arrival in the colony with no prior clinical signs. The rat was necropsied, and an ovoid, 3-x 2-x 1-cm, soft, fluctuant nodular mass was located in the caudal mediastinum between the diaphragmatic lung lobes and the diaphragm. The mass was mottled diffusely deep purple to red-black and surrounded the aorta. The cut surface of the mass was deep red-black. The lungs were diffusely red and edematous. The urinary bladder contained red urine, and the mucosal and serosal surfaces of the bladder were hyperemic. There were several punctate yellow foci on the capsular surface of the right kidney , which on section extended as fine pale linear streaks to the corticomedullary junction. The liver was extremely pale and had a marked lobular pattern. Tissue samples were fixed in 10% neutral buffered for-malin, embedded in paraffin, sectioned at 5 wm, and stained with hematoxylin and eosin (HE). Sections of the tumor were stained with Masson's trichrome, periodic acid-Schiff, Alcian