الفهرس 
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Abstract
Studies of the immune organs and/or immune mechanisms contribute together in the clearance of the phylogeny and ontogeny of the different animal classes. Previous studies on both invertebrates and vertebrates greatly participated in these two fields
Fish represent the largest and most diverse group of vertebrates and their studies should add more information to both phylogeny and ontogeny of the immune system of vertebrates. Several studies were performed on various kinds of fish such as Salmo gairdneri, Rutilus rutilus, Cyprinus carpio and Tilapia mossambica.
In the present study the seasonal changes of the lymphoid system of Oreochromis niloticus, aimed to add to the previously developed ontogenic information in fish.
To fulfil this study, histological analysis were performed on thymus, spleen, head-kidney and peripheral blood using haematoxylin and eosin staining techniques.
Evaluation of macrostructure of these organs was performed by the naked eye.Microscopic evaluation of different types of cells was done using a triocular microscope attached to fully automated camera.
0. nlloticus were studied in groups , each group comprised 10 ± 2 fish per month. The study was extended for 12 month to cover the 4 seasons of the year. Each organ was studied by analysing not less than 5 slides. Counts of the different
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cells were calculated and difference was evaluated by student
t-test.
Each lymphoid organ, in addition to peripheral blood, was
studied separately to evaluate the absoulute count of the cellular elements. The study showed the following summarized result in accordance to each organ.
LYMPHOID ORGANS
Thymus :
Lymphocytes were observed as islands or groups through the thymus. Neither cortex nor medulla could be seen as it is common in the thymus of vertebrates.In thymus, lymphocytes showed maximum count of 654.6 t 40 during summer. They also exhibited elevated count during autumn (415 ± 30). This observation was the same while studying myoid and melano-macrophage cells. The elevated counts of these two types of cells during autumn were nearly as high as in summer.
On the other hand Hassal’s-like corpuscles were frequently observed during winter. The corpuscles were composed of multilayerd squamous epithelium. Also amorphous material that seem to be a secretion, was observed encapsulated in cystic cells. These cystes were referd to as cystic structure.
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Spleen :
Lymphocytes showed maximum density during summer(absolute count were not possible because of scattering presentation). On the other hand melano-macrophages were countable and expressed their maximum during both summer and autumn (140 ± 0.9 and 144.8 ± 7 respectively).
By following the erythropoiesis during different seasons, erythrocytes were remarcably decreased during summer. During all other seasons flactuation in presence of erythrocytes was observed, within each season.
Head-kidney t
By studying the lymphocyte and melano-macrophage distributions and counts in the head-kidney, it was observed that summer was the season in which maximum lymphocytes count was reached (398±16). Melano-macrophages recorded their maximum during Autumn (88.6± 7).
BLOOD :
Peripheral blood cells were analysed according to their five categories, namely lymphocytes, monocytes, neutrophils, eosinophils and basophils. Count of both lymphocytes and neutrophils expressed significant changes correlated to seasonal variation. Lymphocytes were maximum during summer in accordance with lymphocytes in thymus, spleen and
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head-kindney.
On the other hand monocytes, eosinophils and basophilas showed irregular percentage during the four seasons with no frank maximum values.
In conclusion lymphocytes were the type of cells which constantly expressed increased counts during summer. In other words, it was thus obvious that counts of these cells were affected by increased temperature both in lymphoid organs and peripheral blood.
Melano-macrophages also expressed this pattern of count in relation to increased temperature but only in lymphoid organs. All other types of cells of the immune system did not show such pattern of responce to temperature.