AGE RELATED HISTOLOGICAL CHANGES IN THE THYMUS OF TANUVAS ASEEL BIRDS

The present study was undertaken to investigate the age-related histological changes in the thymus of TANUVAS Aseel chickens. Healthy birds were selected from five different age groups, namely day-old, 4, 8, 18 and 32 weeks, with six birds representing each group. Thymic samples were collected from birds maintained at the Poultry Research Station, Madhavaram Milk Colony, Chennai. The tissues were thoroughly rinsed in normal saline and fixed in 10 per cent neutral buffered formalin, Bouin’s fluid and Zenker’s fluid for histological evaluation. Following fixation, the tissues were processed by routine dehydration, clearing and paraffin embedding. Sections of 4–5 µm thickness were prepared and stained with Haematoxylin and Eosin for general histological examination and Masson’s Trichrome for demonstration of connective tissue fibres. The thymus of day-old chicks was characterized by a thin connective tissue capsule and delicate septa, with well-defined cortical and medullary regions. The cortex contained a greater number of lymphoblasts, whereas the medulla showed a higher occurrence of reticuloepithelial cells and Hassall’s corpuscles. At four weeks, the capsule and septa showed slight thickening, while the general cortical and medullary proportions remained comparable to those of day-old chicks. By eight weeks, connective tissue thickening and more distinct lobular compartmentalization were evident, accompanied by a slight increase in the medullary component. Hassall’s corpuscles of Types I–III were identified in the medulla, with occasional Types I and II in the cortex. At 18 weeks, progressive changes became apparent, including increased thickness of the capsule and septa, lymphoid depletion, reduced lymphocyte size, degeneration of reticuloepithelial cells and vacuolation. Numerous Hassall’s corpuscles of Types I–V were observed in the medulla. By 32 weeks, marked thymic involution was evident, characterized by collagen-rich capsular thickening, cortical lymphocyte depletion, connective tissue invasion and cyst formation. The findings demonstrate progressive structural maturation followed by age-associated involution of the thymus in TANUVAS Aseel chickens.