الفهرس 
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Abstract
The present work was conducted on 80 fertilized chicken eggs for pre-hatching development and 64 samples from thymus gland, bursa of fabricius and spleen of healthy growing chickens for post-hatching development collected from the hatchery unit of Al-azzab farm in Fayom governorate. Samples were collected prepared to be studied by both light and transmission electron microscopy. Our investigation aimed to study the developmental stages and fate of the thymus gland, bursa of Fabricius and spleen
- The obtained results were:-
Thymus gland
Pre-hatching:-
The thymic primordia appeared in the neck region of 4 days-old chick embryos on both sides just beside the jugular vein. At the 8th day of incubation, the developing thymus gland showed the beginning of partial lobulation. The gland in this stage showed the first appearance of lymphopieotic cells intermingled with epithelial cells. The chick embryo at the 12th day of incubation showed well developed capsule and short septa resulting separated thymic cortex and common medulla. The cortex contained numerous thymoblast with few reticular cells while the medulla housed few thymoblast and numerous reticular cells. At the 16th day of incubation the thymic lobule showed definite demarcation between cortex and medulla. The latter contained large homogenous acidophilic irregular masses. The fully developed form of the thymus gland obtained at the day nineteen. The parenchyma of the thymus gland during this period appeared to be supported by a network of reticular fibers and few elastic fibers.
Post-hatching:-
In one day-old chick, the thymus stroma represented by well developed capsule, septa and interstitial stroma represented by fine reticular fibers and the long cytoplasmic processes of the epithelial reticular cells. The thymic cortex in one day-old chick consisted of a huge number of thymocytes and few reticular cells. A post capillary venule frequently noticed at the corticomedullary junction. The thymic medulla showed the classical form of thymic corpuscle. At one month of age, other immunologic cells housing the thymic cortex clearly noticed as macrophage and eosinophiles. The chicken thymus gland at two months of age showed various degrees of degeneration in both thymocytes and reticular cells. The degeneration progressively increased by the advancement of age. The thymus gland at five month old chick showed marked loss of the cortical architecture and numerous extra cellular vacuolation as well as accumulation of abundant acidophilic medullary myoid cells. The thymus gland of one year-old chick showed complete loss of the cortical tissue and complete degeneration of the all thymic cells either epithelial reticular cells or immunologic cells.
Bursa of fabricius
Pre-hatching:-
At the 4th day of incubation the bursal primordia could be recognized as an invagenation of the dorsal cloacal wall. Later on increasing in the depth of invagination, coalesceing of intra epithelial vacuoles occur to form the further bursal lumen then the developing bursa separated from the cloacal tissue and had a smooth lumen. The bursal epithelium showed several plicae at the day ten which increased in size and height at the day twelve in addition to huge number of invading hemopeiotic cells. The chicken bursa of 13th day-old embryos revealed the appearance of invagenation in the epithelium covering the bursal folds. At the day fourteen the bursa contained many lymphoid follicles. Each follicle enclosed by one to two layers of epithelial cells and contained undifferentiated epithelial cells beside hemopioetic cells. Large lymphoid follicle appeared at the day fifteen covered by 2-3 layers of flattened follicular epithelium. The bursa of 17 days-old chicken embryos showed many bursal follicles of different sizes. The epithelial lining bursa clearly differentiated into follicular epithelium lacking basal lamina and inter follicular epithelium. Each follicle contained series of lymphoblasts as well as reticular cells. At the nineteenth day of incubation, the bursal follicular epithelium became high cuboidal while inter follicular epithelium became pseudostratified columnar. The epithelial layer enclosing the bursal follicle became flat cells. The connective tissue separating the bursal follicles predominantly consisted of reticular fibers.
Post-hatching:-
The wall of chicken bursa of one day-old chick consisted of mucosa which thrown into bursal plicae. Each one contained two rows of bursal follicles separated by axial filament. The muscular coat of the bursa was formed mainly from two smooth muscle layers. The bursal follicles covered by high cuboidal follicular epithelium. The cortex of the bursal follicles were separated from the medulla by corticomedullary epithelium. Both cortex and medulla contained reticular cells as well as groups of lymphocytes. At one month-old, the chicken bursal mucosa was studded with primary plicae which carried secondary one. Reticular fibers occurred in the connective tissue capsule enclosing the bursal follicles as well as in the corticomedullary membrane. The main cellular contents of the bursal follicles were series of lymphocytes, reticular cells, plasma cells, eosinophiles and macrophages. Involutive changes including increasing in the amount of connective tissue between the lymphoid follicles, losing the connection between the bursal follicle and the follicular epithelium noticed at the bursa of 3 months-old chick. At the fifth month-old chick involutive changes clearly noticed in form of invagenation in the bursal epithelium lined by mucous cells, separated to form mucous cyst. Eighth months-old chick showed massive proliferation of the connective tissue between the lymphoid follicles, appearance of acidophilic materials in the follicle cortex and degenerative changes in both reticular cells and lymphocytes.
Spleen
Pre-hatching:-
The chick embryo at the 4th day of incubation showed the primordia of spleen as an oval structure bulging into the coelomic cavity. At the day six of incubation the splenic primordia increased in all dimensions, cellular contents and blood sinusoids. The ceolomic epithelium changed into flattened epithelial cells and the mesenchymal cells under ceolomic epithelium differentiated into fibroblasts at the ninth day of incubation. At the day twelve the fibroblasts became arranged parallel to the ceolomic epithelium in the form of several layers inter mingled with reticular fibers. The splenic parenchyma contained several splenic blood sinusoids, red blood cells, reticular cells and lymphoblasts. A well developed slenic capsule covered by a single layer of mesothelial cells noticed at the 15th day of incubation. During this stage the splenic parenchyma of chicken spleen possessed first demarcation between the red pulp and white pulp. At the day seventeen of incubation the splenic parenchyma showed red pulp contained numerous blood sinusoids, reticular cells and lymphoblasts. The white pulp represented by peri arterial lymphatic sheath. At the 20th day of incubation the splenic stroma contained a network of reticular fibers in the splenic capsule, around the blood vessels and supporting the splenic parenchyma.
Post-hatching:-
At one day-old chick the splenic stroma was represented by dense fibrous capsule and septa. A well developed splenic parenchyma in one day-old chick showed red pulp including blood sinusoids and splenic cords as well as white pulp including peri arterial and peri ellipsoidal lymphatic sheath. The chicken spleen at one month-old possessed prominant demarcation between the red pulp and white pulp. The peri ellipsoidal lymphatic sheath contained many reticular cells, lymphocytes and macrophage. The splenic corpuscle appeared to be enclosed by fine connective tissue contained many reticular cells, lymphocytes and plasma cells in the chicken spleen at the two months-old. Chicken spleen up to eight months-old revealed well developed spleen with thick capsule and voluminous red pulp as well as well distinct white pulp.