Meena MrigeshMahanta, Dwipjyoti2019-02-092019-02-092018-05http://krishikosh.egranth.ac.in/handle/1/5810094277The present study was conducted on the thymus, spleen and bursa of Fabricius of 24 birds of the local hill and 24 birds of RIR divided into four age groups viz. day old, 1 month, 3 months and 6 months old birds with six birds in each age group. The thymus consisted of two long chains of lobes located on either side of neck, which were embedded in the subdermal connective tissue in both Local hill fowl and Rhode Island Red (RIR). The number of lobes in each half of thymus varied between the two birds. The average weight and size of thymus increased up to 3 months of age, thereafter started declining and its rate of regression was more rapid in local hill fowl than RIR. Histologically, the thymus was covered by a capsule and consisted of outer darker cortex and inner lighter medulla. Both the cortex and medulla were composed of lymphocytes, reticular cells, macrophages, plasma cells, mast cells, dendritic cells, fibroblasts and erythrocytes. The Hassall’s corpuscles were formed by the concentric arrangement of reticuloepithelial cells and were found in cortex, cortico-medullary junction and medulla of the thymus. Their number increased, but their size decreased during involution of thymus. The collagen, elastic and reticular fibres were the connective tissue fibres present in the different regions of thymus. The capsule, trabeculae and Hassall’s corpuscles showed PAS positive and Alcian blue activities. The spleen was almost triangular in shape with rounded edges, dark-reddish in colour and located on the right side of the junction between the proventriculus and gizzard in both the birds. Histologically, the spleen was covered a capsule and the parenchyma consisted of white pulp and red pulp regions. The white pulp was having lymphocytes, macrophages, dendritic cells and several types of blood vessels. The packed mass of lymphocytes gathered around the central artery and capillaries formed the PALS and PELS. The red pulp comprised of venous sinuses and splenic cords. The pulp cords consisted of erythrocytes, reticular cells, various sized lymphocytes, plasma cells and heterophils. The collagen, elastic and reticular fibres were the connective tissue fibres present in the different regions of spleen. The capsule, blood vessels and some cells of white pulp of spleen showed moderate PAS positive activity. Under ultrastructural studies fenistrated basement membrane of splenic sinus was cearly visible. The bursa of Fabricius originated from the proctodeal wall of the cloaca and was placed dorsal to the rectum. It was oval in shape and creamy white in colour in both local hill fowl and RIR. The bursa of Fabricius became largest at 3 months of age and then it completely disappeared at the age of 6 months in Local hill fowl, but in RIR, it became regressed, but still persisted at this age. Histologically, the wall of the bursa of Fabricius comprised of innermost tunica mucosa, middle tunica muscularis and outermost tunica serosa. The mucosa was arranged into longitudinal folds forming larger and smaller plicae. The follicle-associated epithelium of the bursal plicae was made up of pseudostratified ciliated columnar and few patches of simple cuboidal epithelium. Each plicae of bursa of Fabricius was filled with number of irregular shaped lymphoid follicles and the individual follicles were separated by layer of interfollicular connective tissue. The cortex and medulla of lymphoid follicles had lymphocytes, reticular cells, macrophages, plasma cells, mast cells, dendritic cells and fibroblasts. The collagen, elastic and reticular fibres were present in the different regions of bursa of Fabricius. The apical border of surface epithelium showed PAS positive and Alcian blue activities in both the birds. Ultrastructural studies revealed that the thymus, spleen and bursa of Fabricius of both local hill fowl and RIR were composed of lymphocytes, reticular cells, macrophages, plasma cells, mast cells, dendritic cells, fibroblasts and erythrocytes. The present study provided a detailed comparative age wise development of thymus, spleen and bursa of Fabricius in the local hill fowl and RIR.ennullThesis