中文摘要：

本研究旨在为进一步探讨胸腺生理或免疫功能提供形态学资料。选取3个年龄段（1日龄、5月龄及成年）健康牦牛的胸腺样本,通过组织学、免疫组织化学和统计学的方法对牦牛胸腺组织结构增龄性变化进行研究。结果显示,随着增龄,牦牛胸腺皮、髓质分界不清,细胞排列稀疏,小叶间结缔组织和脂肪组织增多占据了胸腺原有的功能区,被膜增厚;各年龄段S100阳性胸腺树突状细胞（TDCs）位于皮髓交界、髓质区域,单位面积内此种细胞的数量随增龄下降,各年龄段组间差异极显著（P〈0.01）;单位面积内Caspase-3阳性胸腺凋亡细胞平均数目随增龄上升,各年龄段牦牛胸腺皮质单位面积内此种细胞数目组间差异极显著（P〈0.01）,而5月龄与成年牦牛髓质单位面积内此种细胞数目差异不显著（P〉0.05）,但二者均与1日龄时差异极显著（P〈0.01）。结果提示,随着增龄,牦牛胸腺结构呈退化趋势;胸腺功能减退,这可能与抗原递呈细胞（TDCs）的数量下降和皮质胸腺细胞凋亡现象显著相关,其机制有待于进一步研究。

英文摘要：

This experiment was conducted to study thymuses physiological and immunological functions.The age-associated changes in histology of the healthy yak＇s thymus at three different age groups（one-day-old,five-month-old and adult） were observed by histology,immunochemistry and statistics methods.The results showed that as the age increased,identifiable corticomedullary delineation became unclear;thymocytes in both the cortical and the medulla became sparse.These changes were accompanied by a marked increase in the content of interlobular connective and adipose tissue and thickening of the thymic capsule.S100 positive thymic dendritic cells（TDCs） were localized in the corticomedullary junction and medullary zones in each group.The average number of S100 positive thymic dendritic cells decreased per area while the age increased.And differences between each age group were significant（P0.01）.Moreover,the average number of Caspase-3 positive thymocytes per area increased while the age increased.Particularly,the average number of this cell in the cortex increased and there were significant differences between each age group（P0.01）.No difference（P0.05） were observed between 5-month-old and adult group,but these two groups were significantly higher（P0.01） than the 1-day-old group,respectively.The results indicated that the yak＇s thymus showed obvious degeneration trend in structure and function which is possibly associated with the decrease of TDCs and increase of cortical apoptotic thymocytes.The underlying mechanism need to be investigated further.