The thymus was covered by a dense irregular connective tissue capsule in all the age groups of both Hansli chicken and Vencobb broiler chicken. The connective tissue septa or trabeculae araised from the thymic capsule and extended into each thymic lobe (Fig. 1).

The trabeculae divided the lobes into lobules. The present observations were in accordance with the reports given by Singh et al. (2010) and Lochi et al. (2014) in chicken, Gulmez and Aslan (1999) in native geese, Sultana et al. (2011) in ducklings of Bangladesh and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. The lobulation pattern of the thymus was comparatively more distinct with much wider trabeculae in the thymus of 4^th week age Vencobb broiler chicken than Hansli chicken of the same age group. In some cases, the trabeculae even extended deep into the medulla coursing the whole thickness of the thymus in 4^th week age Vencobb broiler chicken. Most of the trabeculae resembled like crypts in 4^th week age Vencobb broiler chicken. The amount of blood vessels was also comparatively more in number in both cortex and medulla of the thymus of 4^th week age Vencobb broiler chicken than Hansli chicken of the same age group. The cortex was stained much darker in 12^th week age Hansli chicken and it was comprised of numerous small lobules as compared to the cortex of 4^th week age Hansli chicken. Most of the trabeculae extended deep into the medulla in 12^th week age Hansli chicken. Some of the trebeculae travelled obliquely and anastomosed with the adjacent trabeculae forming cross connections in 12^th week age Hansli chicken. Further, the amount of blood vessels was also comparatively more in number in both cortex and medulla of the thymus of 12^th week age Hansli chicken than 4^th week age Hansli chicken.

The collagen (Fig. 2) reticular (Fig. 3) and elastic fibers were reported in the thymic capsule, septa or trabeculae and tunics of capsular and trabecular blood vessels of both Hansli chicken and Vencobb broiler chicken in all the age groups. The splenic parenchyma had reticular fibers. The amount of collagen and reticular fibers in capsule and septa increased with age in both the Hansli chicken and Vencobb broiler chicken.

However, the amount of elastic fibers in the thymic capsule and septa decreased in 24^th week age. The amount of reticular fibers increased with age at 24^th week of age that subsequently replaced the lymphocytes and reticuloepithelial cells which marked the process of regression of the organ in both the birds at this age. The present observations were in accordance with the reports given by Mahanta (2018) in local hill fowl of Uttarakhand and RIR.

Each thymic lobe was distinctly divided into peripheral darkly stained cortex and central lightly stained medulla in both Hansli chicken and Vencobb broiler chicken in all the age groups. The cortex of thymic lobe comprised of densely packed lymphocytes of different size in the network of reticuloepithelial cells in both Hansli chicken and Vencobb broiler chicken in all the age groups. Hence, it was stained dark. The medulla of thymic lobe comprised of loosely packed lymphocytes of different size in the network of reticuloepithelial cells in both Hansli chicken and Vencobb broiler chicken in all the age groups. Hence, it was stained light. The thymic lobes had numerous small lobules with darkly stained cortex in 12^th week age Vencobb broiler chicken. Angiogenesis was quite evident in this age in Vencobb broiler chicken. The cortex was stained comparatively lighter in 24^th week age Hansli chicken as compared with the cortex of 12^th week age Hansli chicken. Further, the medulla was populated with connective tissue elements in 24^th week age, which indicated the process of regression of the organ in Hansli chicken at this age.

There was presence of collagen, reticular and elastic fibers in the medulla of the thymus in both Hansli chicken and Vencobb broiler chicken. The amount of the connective tissue fibers increased with age in both the birds and replaced the lymphocytes during involution. The present observations were in accordance with the reports given by Singh et al. (2010) in chicken, Sultana et al. (2011) in ducklings of Bangladesh, Song et al. (2012) in Ostrich chicks, Lochi et al. (2014) in Aseel chicken and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. The reticuloepithelial cells appeared as large vesicular cells in both cortex and medulla of each thymic lobe in both Hansli chicken and Vencobb broiler chicken. There was presence of small, medium and large sized lymphocytes in both cortex and medulla of each thymic lobe in both Hansli chicken and Vencobb broiler chicken. The reticuloepithelial cells along with lymphocytes were arranged concentrically to form thymic corpuscle or Hassall’s corpuscle in the cortex, cortico-medullary junction and medulla of each thymic lobe in both Hansli chicken and Vencobb broiler chicken (Fig. 4). There was also presence of erythrocytes, heterophils, plasma cells and fibroblasts in the Hassall’s corpuscle. The fibroblasts were spindle shaped cells having central round nucleus and often found in the capsule and trabeculae. The neuroendocrine cells were oval shaped with centrally located light stained nucleus surrounded by darkly stained cytoplasm located in the medulla of the thymus.

The splenic parenchyma was surrounded by a thin fibrous dense irregular connective tissue capsule in both the Hansli chicken and Vencobb broiler chicken in all the age groups. Few amount of smooth muscle fibers were also reported in the capsule of the organ. The splenic capsule mainly comprised of collagen fibers (Fig. 5) with very few amount of reticular and elastic fibers in both Hansli chicken and Vencobb broiler chicken. The splenic capsule and sub-capsular area had very few amount of elastic fibers in both Hansli chicken and Vencobb broiler chicken in all the age groups. Further, these connective tissue fibers increased in the spleen with age in both the birds and they imparted elasticity to the organ. Numerous trabeculae or septa araised from the splenic capsule and invaded the substance of splenic parenchyma. The present observations were in agreement with the findings of Venkatesan and Vijayaragavan (1999) in Japanese quail, Akter et al. (2006) in broiler chicken and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. These septa formed passage for the trabecular arteries and trabecular veins. Lymphocytes of various sizes, reticular cells, plasma cells, dendritic cells, fibroblasts, macrophages, erythrocytes and other leukocytes were reported in the splenic parenchyma in both Hansli chicken and Vencobb broiler chicken in all age groups. The present observations were in agreement with the findings of Venkatesan and Vijayaragavan (1999) in Japanese quail, Akter et al. (2006) in broiler chicken, Sultana et al. (2011) in duckling of Bangladesh, Kannan and Ramesh (2013) in Nandanam chicken and Mahanta (2018) in local hill fowl of Uttarakhand and RIR.

The splenic parenchyma was comprised of round to oval lightly stained regularly organized white pulp and darkly stained irregularly arranged red pulp in all the age groups in both Hansli chicken and Vencobb broiler chicken (Fig. 6). The splenic white pulp consisted of central artery and arterioles surrounded by densely packed mass of lymphocytes of various sizes forming the peri arterial lymphatic sheath (PALS) in all the age groups of birds. The white pulp was present in diffused form among the red pulp in 4^th and 12^th week age Vencobb broiler chicken, whereas the splenic white pulp was reported in discrete patches among the red pulp in Hansli chicken at same age groups.

The collagen, reticular and few amounts of elastic fibers were noted in the tunics of the central artery and arterioles of the splenic white pulp. There was presence of very few lymphatic nodules or splenic nodules in the white pulp of deeper part of spleen in 4^th week age Hansli chicken, whereas these were present in the white pulp areas of subcapsular region and deeper part of spleen in 4^th week age Vencobb broiler chicken.

Further, their number increased in 12^th week of age in both the birds. The splenic nodules were reported in the white pulp areas of subcapsular region and deeper part of the spleen in 12^th week age of both the birds. Very few splenic nodules were reported in the white pulp of deeper part of spleen in 24^th week age of Hansli chicken. The splenic nodules had lymphocytes of various sizes and categorized into small, medium and large lymphocytes. Some of the lymphatic nodules contained lightly stained central area surrounded by darkly stained area. This lightly stained area was referred as germinal centre. There was presence of a dark basophilic zone known as marginal zone in between the splenic red pulp and white pulp. This zone had lymphocytes of various sizes, reticular cells and macrophages. Further, venous sinuses known as marginal sinus were reported at the periphery of marginal zone. The present observations were in agreement with the findings of Mahanta (2018) in local hill fowl of Uttarakhand and RIR.

The splenic red pulp consisted of splenic cords and venous sinuses or venules. The splenic cords had reticular fibers, reticular cells, macrophages, erythrocytes, lymphocytes, plasma cells and other leukocytes. The arterioles those entered the splenic red pulp from the white pulp were divided into capillaries and these were surrounded by the lymphocytes of various sizes, reticular cells and macrophages known as sheathed or ellipsoid capillaries. This lymphoid sheath was referred as peri-ellipsoidal lymphatic sheath (PELS). The capillaries had narrow lumen and their endothelium was cuboidal type with centrally located nucleus surrounded by lightly stained pinkish cytoplasm (Fig. 7). Fine networks of reticular fibers and elastic fibers were also recorded in the PELS areas of spleen in both the birds.

The wall of the bursa of Fabricius had innermost tunica mucosa, middle tunica muscularis and outermost tunica serosa in both Hansli chicken and Vencobb broiler chicken in all the age groups (Fig. 8).

The present finding was in accordance with the reports of Gulmez and Asian (1999) in native geese, Indu et al. (2005) in White Pekin ducks, Singh et al. (2010) in chicken, Hassan et al. (2011) in quail and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. But it was contradicting to the finding of Song et al. (2012) who observed that the bursa was comprised of a mucosa, a submucosa, a muscular layer and an adventitia in ostrich chicks.

The Tunica mucosa of the bursa was arranged into so many longitudinal folds known as plicae. They were categorized into smaller and larger plical folds. Each bursal plica comprised of surface epithelium and lamina propria. The lamina propria was filled with numerous lymphoid follicles. There was presence of connective tissue core inside each bursal plicae that originated from the base of plicae and ran up to their apices. The connective tissue trabeculae or septa originated from this connective tissue core and ran into the substance of each bursal plicae dividing them into different compartments. The central connective tissue core of each bursal plica was comprised of numerous collagen fibers (Fig. 9), reticular fibers (Fig. 10) and very few elastic fibers as reported by Indu et al. (2005) in White Pekin ducks, Jayachitra et al. (2009) in turkeys, Singh et al. (2010) in chicken and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. There was presence of large blood vessels in the connective tissue core of the bursal plica.

The surface of the bursal plica was wavy in outline and consisted of raised and depressed pitted area. The pitted areas of tunica mucosa were mostly lined by pseudostratified ciliated columnar cells with numerous goblet cells. Further, the surface epithelium of bursa was categorized into Follicle-Associated Epithelium (FAE) and Inter Follicular Epithelium (IFE). The interfollicular epithelium (IFE) shared a small portion of the surface epithelium of the bursa in both Hansli chicken and Vencobb broiler chicken. The present report was contradicting to the findings of Jayachitra et al. (2009) in turkeys who observed that the interfollicular epithelium shared the bulk of the surface epithelium of the bursa of Fabricius. These differences might be due to the age, breed and strain variations.

The lamina propria had loose connective tissue and lymphoid follicles in both Hansli chicken and Vencobb broiler chicken in all the age groups. The bursal follicles were irregular round, triangular, oval or elliptical in shape in both the birds. These lymphoid follicles were separated from each other by interfollicular connective tissue in both the birds. The bursal cysts were prominent and numerous in the lymphoid follicle of 4^th week age Vencobb broiler chicken. The follicular pattern were absent in the plicae and the connective tissue elements of lamina propria occupied most of the portions of the bursal plicae at 24^th week age in both the birds (Fig. 11). It indicated the process of regression of the organ at 24^th week of age in both Hansli chicken and Vencobb broiler chicken.

The collagen, reticular and a few elastic fibers were reported in the interfollicular area, sub-epithelial connective tissue, centre of bursal plicae and blood vessels present in the interfollicular region of the organ in all the age groups in both the birds. The amount of the collagen fibers increased with the age in both the birds which imparted protection to the organ and played role in the replacement of cellular elements of the spleen. But the amount of elastic fiber gradually decreased with age in both the birds. Further, each bursal follicle consisted of peripheral cortex and central medulla and they were separated by a cortico-medullary junction. The cortex of the follicle consisted of densely packed lymphocytes of various sizes, whereas the medulla had loosely arranged lymphocytes of various sizes. Both the regions had networks of reticular cells in both the birds in all the age groups. There was also presence of dendritic cells, erythrocytes and macrophages inside the lymphoid follicles in both Hansli chicken and Vencobb broiler chicken in all the age groups. The peripherally located lymphoid follicles were in direct contact with the surface epithelium of the bursa, whereas the rest of the bursal follicles were surrounded by the interfollicular connective tissue. The amount of reticular fibers increased in the bursa at 24^th week of age in both the Hansli chicken and Vencobb broiler chicken that might be attributable to the decreased number of lymphocytes and gradual replacement of the cellular elements which led to the involution of the bursa with age in both the birds.

There was presence of small, medium and large sized lymphocytes in both the cortex and medulla of the lymphoid follicle in both Hansli chicken and Vencobb broiler chicken. The reticuloepithelial cells were located in the cortex, medulla and connective tissue elements of the bursal follicles. They consisted of large vesicular nucleus with lightly stained cytoplasm (Fig. 12).

The macrophages had indented nucleus with vesiculated cytoplasm. The dendritic cells were elongated cells having long extended cytoplasmic processes. They had elongated vesicular nucleus. The cytoplasm of these cells was granular in appearance and light pink in colour. The plasma cells were reported in the cortex, medulla and interfollicular areas of the bursa having eccentric nucleus in both the birds. It was contradicting to the finding of Kumar et al. (2014) who observed few plasma cells in the cortex of the lymphoid follicle only in Khaki Campbell duck. The presence of these cells in the bursa might be attributed to their role in providing humoral immunity to both the birds. The fibroblasts were noted in the connective tissue elements of the organ. These cells were spindle shaped with centrally located round nucleus.

The tunica muscularis of the bursa of Fabricius had inner circular and outer longitudinal smooth muscle layer in Hansli chicken and Vencobb broiler chicken in all the age groups (Fig. 13). It was contradicting to the finding of Indu et al. (2005) who reported that the tunica muscularis of bursa had outer circular and inner longitudinal smooth muscle layers in White Pekin ducks. There was presence of blood vessels between the smooth muscle layers of tunica muscularis and at the base of each bursal plica.

The tunica serosa was the outermost layer of the bursa in Hansli chicken and Vencobb broiler chicken in all the age groups. It had dense irregular type of connective tissue with fibroblasts, blood vessels and connective tissue fibers mostly collagen fibers as reported by Sari and Kurtdede (2007) in turkeys, Ingole et al. (2010) in CARI Shyama and Vanaraja breeds of poultry, Singh et al. (2010) in chicken and Mahanta (2018) in local hill fowl of Uttarakhand and RIR. The collagen, reticular and elastic fibers (Fig. 14) were noted in the tunica muscularis, tunica serosa and walls of blood vessels located in both the tunica muscularis and serosa in Hansli chicken and Vencobb broiler chicken in all the age groups. The amount of collagen fibers increased with age in both the birds which imparted protection to the organ. But the elastic fibers gradually decreased with age that might be attributable to the decreased elasticity of the bursa with age in both Hansli chicken and Vencobb broiler chicken.