الفهرس 
Chapter I : IntroductionOnly 14 pages are availabe for public view
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Abstract
The present work was directed to throw some spots of light on the histological picture and ultrastructure of the immune organs of Tilapia nilotica during different seasons to show the influence of temperature and photoperiod on immune response.
This study was carried out on thymus, head kidney and spleen of hundred fish of Tilapia nilotica which collected from many commercial farms in El-Kanater El-Khaieria.
1- THYMUS
The thymus was situated on the superior edge of the gill cover close to the opercular cavity, the thymus was surrounded by connective tissue capsule which consisted of collagen fibers and reticular fibers sending trabeculae into the parenchyma which organized into cortex and medulla. The thymocytes (T- lymphocytes) form the main type of cells present in thymus embedded within network of epithelial reticular cells.
Also the thymus was characterized by presence of hassall’s body-like structures, myoid cells, mucin secreting cells (type of epithelial cells) and melanomacrophage cells.
The thymus showed great involution in winter in form of decrease number of lymphocytes and proliferation of connective tissue and adipose tissue. It was formed from three zones, inner and outer zones consisted of thymocytes and epithelial reticular cells while middle zone consisted of adipocytes and collagen fibers.
Thymus showed regeneration of tissue during spring, the parenchyma was divided into cortico-medullary like zones with significant increase in number of red blood cells especially under the capsule. Sharp increase in lymphocytes number was observed during summer as the lymphocytic foci became denser and increased in size also melanomacrophages increased in number and appeared in the form of clusters instead of their presence in solitary form in other seasons.
In autumn the parenchyma was divided into cortico-medullary like zones with increase in connective tissue proliferation especially in the medulla. With decrease in lymphocytes number during this season as also the lymphocytic foci become less dense and decreased in size.
The ultrastructural study showed presence of lymphocytes, macrophages, epithelial reticular cells and red blood cells.
2- HEAD KIDNEY
This study revealed that the head kidney was located extraperitoneally, ventrally with respect to vertebral column posterior to thymus and anterior to trunk kidney.
The head kidney was covered externally by connective tissue capsule formed from collagen fibers and reticular fibers. The parenchyma was formed from collections of lymphocytes and melanomacrophage centers surrounding blood vessels and these collections were separated by lymphocytes and red blood cells in solitary form. Also, some mast cells were observed mainly beside MMC.
Melanomacrophages consisted of fragments derived from phagocytosed cells, mainly erythrocytes, and pigments such as haemosiderin, lipofuscin, and melanin and were large, pleomorphic and were surrounded by an accumulation of esinophilic granulocytes. It may be present in the form of clusters or scattered randomly in the parenchyma.
Head kidney endocrine tissue was found along the central branches of the veins; its cells were columnar, with irregular or elongated nuclei
In respect of effect of seasonal changes, the following results obtained increased number of mast cell in winter and spring, high reactivity to scmorl’s technique in spring and high reactivity to PAS stain during summer.
Erythrocytes, granulocytes, lymphocytes, mononuclear cells (macrophages) and plasma cell make up the major cell populations of the hemopoietic tissue as demonstrated by the ultrastructural study.
3-SPLEEN
The present investigation revealed that the spleen of Tilapia nilotica, was dark red often pyramidal structure located behind the stomach.
The organ was covered by thin connective tissue capsule but usually no distinct trabeculae were found. The parenchyma was formed from indistinct white pulp and red pulp scattered through the tissue section.
Red pulp was composed of erythrocytes, reticular cells supporting blood sinusoids, macrophages and few numbers of lymphocytes, while the white pulp was formed from arterioles or ellipsoids, lymphocytes, melanomacrophages in solitary form or in clusters with some mast cells and some erythrocytes.
In respect of effect of seasonal changes, the following results obtained a decrease in density of lymphocyte observed during winter with the presence of mast cells and noticeable high increase in melanin granules inside melanomacrophage centers. But in spring, a decrease in the density of lymphocytes, also there was obvious increase in number of red blood cells and low amount of melanin granules inside melanomacrophage center and high reactivity to PAS stain.
In summer, it was found that there was an increase in the density of lymphocytes, with more increase in red blood cells number and melanomacrophage centers reacted negatively to schmorl’s technique with decrease in melanin granules inside MMC. But in autumn it was found that there was a noticeable sharp increase in the number of red blood cells, decrease in density of lymphocytes also observed during this period of year with low amount of melanin granules inside melanomacrophage center which reacted positively to schmorl’s technique.