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.