author(s)
TA Kannan, Geetha Ramesh, S Ushakumari, S Vairamuthu and M Sivakumar

abstract
Transmission electron microscopic studies on thymus, spleen and caecal tonsil of layer chicken was done from day-old to forty weeks of age. The thymic gland showed a thin connective tissue capsule. In thymic parenchyma, lymphocytes, retiucloepithelial cells, myoid cells and macrophages were the predominant component. Erythrocytes, granulocytes, mast cells and plasma cells were also observed. The Hassall’s corpuscles and myoid cells were also noticed. Marked involutary changes were noticed in forty weeks. The spleen was encapsulated by a connective tissue capsule with ill developed trabeculae. The white pulp was composed of lymphoblasts, lymphocytes, follicular dendritic and reticulum cells. Red pulp was composed of pulp cords consisted of erythrocytes and other cellular population. In caecal tonsil, the capsule of the germinal centre consisted of many layers of flattened reticular cells separated with an intercellular substance. The germinal centre consisted of lymphoblasts, lymphocytes, reticular cells, plasma cells, mast cells and macrophages. In forty week-old birds, the lymphocytic population was observed to be comparatively reduced and more of fibroblasts and collagen fibres were noticed.