Malignant stromal cell tumor of the spleen in a WBN/Kob rat

Primary splenic stromal tumors have rarely been reported in rodents. We report the case of a 90-week-old male WBN/Kob rat with a nodular demarcated mass in the spleen, which was kept as a non-treated animal in a long-term animal study. Histopathology revealed round to short spindle-shaped tumor cells arranged in a solid growth pattern. Invasive growth, anisokaryosis, and high mitotic activity (46 per 10 high-power fields [2.37 mm²]) were observed to be multifocal, but most tumor cells showed mild nuclear pleomorphism. The pattern of silver impregnation corresponded to that of the marginal zone of the red pulp. Immunohistochemistry revealed that the tumor cells were double positive for fascin and desmin and focally positive for Iba-1 and OX-6 expression. These characteristics were similar to those observed in fibroblastic reticular cells and dendritic cells in the marginal zone of the red pulp. These findings suggest that the malignant stromal cell tumor of the spleen in this case had characteristics of both fibroblastic reticular cells and dendritic cells.     

Malignant Lymphoma with Severe Infiltrative Growth into Skeletal Muscles in WBN/Kob Rats

Although spontaneously occurring neoplasms have been reported repeatedly in F344, SD and Wistar rats, which are commonly used strains for routine toxicologic and carcinogenicity studies, there are only a few reports of malignant lymphoma or lymphatic leukemia except for large granular lymphocytic leukemia (LGL) in F344 rats. Malignant lymphoma (lymphosarcoma) is thought to be uncommon in F344 rats. The authors encountered malignant lymphomas of the non-LGL leukemia type with characteristic pathologic features in WBN/Kob rats. The mean age at onset of the disease in all 13 affected rats (8 males and 5 females) was about 60 weeks. Common and characteristic clinical signs were abnormal gait with hind limb paralysis. Macroscopically, the enlargement of the lymph nodes, spleen and liver was slight to moderate. Scattered multiple white-to-gray nodules encompassed the aorta and assumed a bead-like appearance near the thoracic and lumbar vertebrae. Histopathologically, neoplastic proliferative changes were predominant in the bone marrow tissue of the entire body, and many tumor cells infiltrated the spleen and several lymph nodes. The most striking histological features were constant and severe infiltration of tumor cells in the adipose tissue and skeletal muscle adjacent the thoracic and lumber vertebrae. Immunohistochemically, all tumor cells were positive for B-cell markers (PAX-5, CD79a and CD45) and negative for CD3. From the results of immunohistochemistry and morphological examination, these tumors were diagnosed as malignant B-cell lymphomas.     

Three-dimensional structural analysis of mitochondria composing each subtype of fast-twitch muscle fibers in chicken

In a previous study, the three-dimensional structures of mitochondria in type I and type IIb muscle fibers of chicken were analyzed. The study reported differences in the shape of the mitochondria and the distribution of lipid droplets. In this study, we three-dimensionally analyzed mitochondria and lipid droplets of type II muscle fiber subtypes IIa, IIb, and IIc of chicken lateral iliotibial muscle in the same field of view using correlative light electron microscopy (CLEM) and array tomography methods. The reconstructed images showed that the mitochondria of type IIa muscle fiber were thick and aligned along the myofibrils, and many lipid droplets were embedded in the mitochondria. The mitochondria of type IIb muscle fibers were intermittent, aligned along the myofibrils, and showed contact between adjacent horizontal mitochondria. No lipid droplets were observed in type IIb muscle fiber. In type IIc muscle fiber, we observed irregularly shaped mitochondria with small diameters aligned along the myofibrils. Lipid droplets not only were embedded in the mitochondria but also existed independently in some cases. The combination of array tomography and CLEM methods enabled three-dimensional electron microscopic observation of mitochondria in different subtypes of type II muscle fibers. The subtypes of type II muscle fibers differed in mitochondrial occupancy and morphology and in lipid droplet distribution, and characteristics that had been demonstrated biochemically were also demonstrated ultrastructurally.