Genetic regulation of slowly progressing mild muscle atrophy in fast-twitch muscles of BUF/Mna rats

Cervical thymuses exist, but no cervical thymomas develop in thoracic thymoma-prone BUF rats

To confirm the existence of the cervical thymus and the development of cervical thymoma in thymoma-prone BUF/Mna (BUF) rats, we examined cervical organs and adjacent tissues, and thoracic thymic tissues of the three inbred strains, BUF, ACI/NMna (ACI), and WKY/NCrj (WKY), and 11 congenic strains, in which genetic regions of rat nude (Rnu), thymus enlargement-1 and thymus enlargement-2 (Ten1 and Ten2), thymoma susceptibility of rat-1 (Tsr1), atrophy of fast-twitch muscles-1 (Aftm1) and proteinuria of rat-1 (Pur1) were transferred into BUF, ACI or WKY rats. These organs and tissues were fixed en block in 10% formalin and cut coronally into four to six slices of 3-mm thickness, depending on the age of the rat, and embedded together in one block for each rat. Sections were cut and stained with haematoxylin and eosin and examined microscopically. Cervical thymuses were detected in 12-21% of rats from these inbred and congenic strains. No cervical thymuses were found in BUF-Rnu/Rnu rats, which were athymic. All of 42 BUF, 2 of 55 BUF-Rnu/+ and 28 of 33 ACI-Tsr1/Tsr1 rats survived more than 52 weeks, and developed thoracic thymoma, but no cervical thymomas did. It is therefore clear that cervical thymuses behave differently from thoracic thymuses in spontaneous thymomagenesis in BUF rats.

Spontaneous thymoma rat as a model for myasthenic weakness caused by anti-ryanodine receptor antibodies

The mechanism of muscle weakness in myasthenia gravis and its possible relation to antibodies that are directed against the ryanodine receptor (RyR) were studied by the use of the spontaneous thymoma rat (Buffalo/Mna strain). The present study focused on the motor dysfunction as complicated by impaired subcellular machineries and noted particularly in patients with thymus abnormalities. Rats began to develop skeletal muscle weakness soon after birth and worsened progressively. Rats aged 3 months showed a benign thymoma characterized by proliferative lymphocytes; epithelial cells were stained with anti-RyR peptide antibody. The rat serum contained anti-RyR antibodies, but no anti-acetylcholine receptor antibodies. The electrophysiological study in muscle showed a reduction of contractile force without abnormality in synaptic transmission and membrane properties, suggesting a defect in excitation-contraction coupling. Hypothetically, thymic epithelial cells and skeletal muscles share a common RyR antigen, so that anti-RyR antibodies that target the thymic tissue may react with a homologous target in the muscle.