Background: Thymus is the primary lymphoid organ in avian. It is a site of differentiation and maturation of T lymphocytes. Quail thymus showed age related involution with decrease in its volume and cellular contents. It is a lymphoepithelial organ and thus plays a major role in the anti-infection defense mechanism.Methods: For the present study 48 Japanese quail birds reared and thymus was collected from 12 birds each at end of first week, second week, third week and fourth week. The tissue was processed for routine paraffin embedding and processed for different staining procedures for histomorphological studies.Result: In all age group of birds, each thymic lobe was enclosed by thin connective tissue capsule. Thymic lobule was found to be composed of outer cortex and inner medulla. Cortex was composed of lymphocytes, lymphoblasts, myoid cells, macrophages, plasma cells and epithelial reticular cells and that of medulla was composed of lymphocytes, lymphoblast, epithelial reticular cells, myoid cells, macrophages, plasma cells, cyst and Hassall's corpuscles. The cyst of various shapes and sizes were observed in cortex and medulla. In all age group of birds, unilamilar and multilamilar Hassall's corpuscles were observed. Number of Hassall's corpuscles and macrophages were increased with the advancement of age.
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