Aim: To observe the changes in thymocyte and its microenvironment in aged mice after bilateral testicular resection. Methods: In male old mice, at the 25th day after testicular resection, the peripheral blood and thymus were collected. Blood and thymus suspension smears were prepared for quantitative histochemistry and immunohistochemistry study under light and electron microscopes. Results: In testes resected mice the size and the weight of thymus were markedly increased. The demarcation between cortex and medulla was clear. The cortex was thickened and the cell density was increased. The ratio of cortex/medulla stereometry was increased. The total cell count, thymocyte count, the percentage of acid -naphthyl acetate esterase (ANAE) positive thymocytes, nonlymphocytes and the rosette formation of macrophages and thymocytes were all increased. The thymocytes surrounded closely to the light thymic epithelial cells, dendritic cells or macrophages. The lymphocytes, particularly the ANAE positive lymphocytes of peripheral blood were increased. Conclusion: After bilateral testicular resection, the thymus of aged male mice showed morphological regeneration and the thymocytes and its microenvironment appeared to be definitely improved. It is suggested that testicular resection may improve immune function

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