During the present study, grossly it was observed that the thymus was present as a series of separated lobes within the adipose tissue. Histomorhologically, in all age group of birds, each thymic lobe was enclosed by thin connective tissue capsule of uneven thickness (Fig 1). The present observations confirm the reports made by
(Haseeb et al., 2014) in Aseel chicken,
(Khalil et al., 2003) in chicken and
(Senapati et al., 2015) in quail, chicken and duck. They reported that the thin connective tissue capsule of thymic lobes gives rise to septa to divide lobes in to lobules. Similar reports were also made by
(Treesh et al., 2014) in chicken,
Gulamez and Aslan (1999) in Geese,
(Song et al., 2012) in ostrich chick and
(Davison et al., 2008) in avian. The findings of the present study regarding the components of thymic capsule and septa and lobulation is in accordance with those reported by
(Ali et al., 2016) in turkey,
El- Zoghby and Attia (2007) in ostrch and
(Tamilselvan et al., 2017) in guinea fowl. These workers reported that capsule and vascularised speta was mainly composed of collagen and reticular fibres.
The each thymic lobule during the present study was found to be composed of outer basophilic stained cortex and inner acidophilic stained medulla in all age group of birds (Fig 2). This observation is in line with earlier reports made by
Hashimoto and Sugimura (1976) in pekin duck,
Gulmez and Aslan (1999) in Geese,
(Ciriaco et al., 2003) in avian,
El- Zoghby and Attia (2007) in ostrich,
(Davison et al., 2008) in avian,
(Treesh et al., 2014) in chicken,
(Haseeb et al., 2014) in Aseel chicken,
(Ali et al., 2016) in turkey,
(Tamilselvan et al., 2017) in guinea fowl and
(Senapati et al., 2015) in quail, chicken and duck.
The cortex of each lobule was predominantly composed of densely populated small lymphocytes with medium lymphocytes, lymphoblasts and few numbers of epithelial reticular cells. The epithelial reticular cells were comparatively larger with spherical to oval euchromatic nucleus and widely spread in the cortex. The evident of mitotic activity was found among the cortical cellular population in all age group of birds under present study (Fig 3). This observation is in agreement with the findings reported by
(Haseeb et al., 2014) and
(Treesh et al., 2014) in chicken. They reported that cortex was enriched with lymphocytes.
Gilmore and Bridges (1974) in fowl and
Hashimoto and Sugimura (1976) in pekin duck reported the numerous densely packed small lymphocytes in cortex within meshwork of epithelial reticular cells. In addition,
(Tamliselvan et al., 2017) in guinea fowl presence of medium lymphocytes and
El- Zoghby and Attia (2007) in ostrich mentioned the presence of lymphoblasts in the cortex.
During the present study, few myoid cells, macrophages and plasma cells were observed in the thymic cortex of all age group birds (Fig 3). The macrophages and plasma cells were very few at age group of one week, however their population was found to be increase with the advancement of age. Present observations are also in accordance with the
(Tamliselvan et al., 2017) in guinea fowl and
Gulmez and Aslan (1999) in Geese. In agreement with the present findings,
(Tamliselvan et al., 2017) reported increase in number of plasma cells with the advancement of age.
During the present work, it was observed that, thin connective tissue strands extended from interlobular connective tissue septa in to the each lobular parenchyma up to the corticomedullary junction. These connective tissue strands carried the blood vessels up to the corticomedullary junction. From group II age of birds, the connective tissue strands found to be more prominent with the advancement of age and divided the cortical tissue in compartments giving lobular appearance to thymic cortex (Fig 1). In agreement with this finding,
(Treesh et al., 2014) in chicken reported the presence of blood capillaries at the corticomedullary junction. The observation of the present study confirms the report made by
(Davison et al., 2008) in Avian. They reported that the invasion of the connective tissue septa resulted in to the thymic cortical lobulation.
The medulla was composed of cellular population of lymphocytes, lymphoblast, epithelial reticular cells, myoid cells, macrophages, plasma cells and structures like cyst and Hassall’s corpuscles (Fig 4). This is in line with the reports made by
(Tamilselvan et al., 2017) in guinea fowl,
(Haseeb et al., 2014) in chicken and
Hashimoto and Sugimura (1976) in pekin duck. These workers reported the similar components of the thymic medulla.
The epithelial reticular cells were more in number and larger with spherical to ovoid vacuolated nucleus as compared to epithelial reticular cells of cortex. The increase in number of hypertrophied epithelial reticular cells was found to be increase with the advancement of age (Fig 5). In agreement with the present findings,
El- Zoghby and Attia (2007) in ostrich,
Ali (2016) in turkey,
(Tamilselvan et al., 2017) in guinea fowl,
Gilmore and Bridges (1974) in fowl and
(Davison et al., 2008) in avian reported more number of epithelial reticular cells in the thymic medulla.
The myoid cells were found to be more in the medulla as compared to the cortex. The medullary myoid cells showed the same characteristic structures as observed in the cortex (Fig 5). Similar observations were also made by
Hashimoto and Sugimura (1976) in pekin duck and
Gilmore and Bridges (1974) in fowl, who they reported more number of myoid cells in medulla than the cortex.
The number of macrophages and few plasma cells were found in the medulla. It was observed that the number of macrophages and plasma cells in thymic medulla increased with the advancement of age (Fig 5). This observation is in concurrence with the findings of
(Treesh et al., 2014) in chicken.
The increase in number of macrophages with the advancement of age may be attributed to the necessity of more number of macrophages to remove the degenerative cellular debris.
During the present work, the cyst of various shapes and sizes were observed in cortex and medulla of all age group of birds. The cysts were characterized by the presence of flattened epithelial wall surrounded by degenerative cells. It was observed that, group of cyst coalesced to form the larger cysts (Fig 6 and 7). These observations are in agreement with the findings reported by
(Kannan et al., 2015) in fowl.
Hashimoto and Sugimura (1976) in Pekin duck reported the similar morphology of cyst.
(Tamilselvan et al., 2017) in Guinea fowl reported more number of cysts of varied shape and size.
In all age group of birds, the Hassall’s corpuscles in various stages of development and form were observed. In all age group of birds, unilamilar and multilamilar Hassall’s corpuscles were observed. In unilamilar Hassall’s corpuscles, a single layer of flattened epithelial reticular cell around the central acidophilic mass or degenerating epithelial reticular cell. The multilamilar Hassall’s corpuscles were characterized by two or more concentric layers of flattened keratinized epithelial reticular cells or other cells around the central degenerating acidophilic mass (Fig 8 and 9).
The coalition of two or more developing or unilamilar Hassall’s corpuscles to form the large multilamilar Hassall’s corpuscles was observed in all age group of birds (Fig 8 and 9).
In the present study, the Hassall’s corpuscles associated with one or more cysts were observed in all age group of birds. These cysts subsequently increased in size or merged with each other to form larger cysts around the Hassall’s corpuscles (Fig 10).
These observations of the present findings are in line with reports made by
(Kannan et al., 2015) in fowl,
Gulmez and Aslan (1999) in Geese,
(Tamilselvan et al., 2017) in Guinea fowl,
Hashimoto and Sugimura (1976) in Pekin duck and
(Senapati et al., 2015) in quail, chicken and duck. These workers reported the similar morphology of Hassall’s corpuscles. In agreement with the present observation,
Hashimoto and Sugimura (1976) in Pekin duck,
(Treesh et al., 2014) in chicken,
(Haseeb et al., 2014) in Aseel chicken and
(Kannan et al., 2015) in fowl reported the increase in number of Hassall’s corpuscles with the advancement of age.
The increase in number of Hassall’s corpuscles and macrophages with the advancement of age recorded during the present study may be attributed to the increased function for destruction of self-antigen responsive lymphocytes by the process of apoptosis to generate tolerance and necessity of more number of macrophages to digest the debris of large number of apoptotic lymphocytes.
The infiltration of cortical cells in to the medulla was observed from group III age of birds (Fig 11). This cortical infiltration along with increase in number of Hassall’s corpuscles with the advancement of age may be indicative of start of regressive changes in thymus.
Similarly, the increase in medullary vascularisation and presence of lymphocytes in the lumen of vein as well as infiltration of cortical cells in to the medulla with the advancement of age during the present study indicated the differentiation of more lymphocytes in the cortex and their migration to the peripheral circulation or to the secondary lymphoid organs.
