5-Fluorouracil-Induced Apoptosis in the Rat Thymus

Cho, Keun Ja; Kim, Soo Il; Yang, Eun Jin; Zheng, Guo Dong; Kim, Won Sik

Abstract

Thymocyte death has been recognized as one of the best characterized experimental models of apoptosis that can be induced by a variety of stimuli such as glucocorticoids, ionizing radiation, anticancer drugs and toxins. The aim of this study was to investigate 5-fluorouracil (5-FU)-induced apoptosis in the rat thymus morphologically. Sprague Dawley rats were injected intramuscularly with 5-FU 100mg/kg body weight and 200mg/kg body weight, and were then sacrificed on day 1, 3, 6, 9 and 12 after treatment. H-E stain and the TUNEL method using the ApopTag kit were used. The results were as follows: 1. On day 1 and 3 after 5-FU treatment, thin cortex and prominent reduction of thymocytes in the cortex were detected. The degree of change was more extensive in the 5-FU 200 mg/kg-treated group than in the 5-FU 100 mg/kg-treated group. Pleomorphic apoptotic cells detected in the cortex were increased in the 5-FU 200mg/kgtreated group and on day 3 after treament. 2. In the 5-FU 100mg/kg-treated group, germinal centers appeared in the cortex on day 6 after treatment starting recovery. Formation of germinal centers continued to day 9 after treatment. In the germinal centers, there were large numbers of typical round apoptotic cells. The thymus exhibited normal architecture on day 12 after treatment. In the 5-FU 200 mg/kg-treated group, however, there were some that did not recover from involution of the thymus on day 6 after treatment and showed fibrosis with decrease of thymocytes and increase of connective tissue to day 9 after treatment. Others recovered and occurred similar changes to the 5-FU 100mg/kgtreated group. 3. 5-FU-induced pleomorphic apoptosis appeared mainly in the cortex and increased gradually depending on dose up to day 1, 3 after treatment. Apoptosis in the germinal centers first occurred from day 6 after treatment and revealed typical large round apoptotic cells. According to the above results, it is thought that 5-FU has an inhibitory effect on DNA synthesis, cell death and involution on surrounding tissue by direct action on thymocytes. The effect continues upto day 3~4 after treatment. Meanwhile, along with the recovery of the thymus from day 6 after treatment, new germinal centers appeared and active lymphocytes production occurred in the germinal centers, which was considered as compensatory reaction. To eliminate enormous B cells newly produced in the germinal center, active apoptosis would occur.