AGE RELATED HISTOLOGICAL CHANGES IN THE THYMUS OF TANUVAS ASEEL BIRDS

Abstract

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.