Thymus: Is a central lymphoid organ situated in the mediastinum, the thymus has dual embryonic origin mesodermal and endodermal origin. The thymus originated from branchial epithelium. During development, the epithelium grows caudally and proliferates to form tubular projections. Multipotential lymphoid stem cell originated from bone marrow and destined to form immunocompetent T lymphocytes which invade the epithelium and occupy the spaces between the epithelial cells. The thymus has connective tissue capsule which penetrates the parenchyma and divide it into incomplete lobules, so that there is continuity between the cortex and medulla of adjoining lobules. Each lobule has a peripheral dark zone known as cortex and a central light zone called the medulla. Figure1:Photomicrograph of a section of thymus showing the lobules. Two lobules show the dark cortical and the light medullary zones. At the upper left are blood vessels and the connective tissue capsule. The cortex is composed of an extensive population of T cell precursors (also called thymocytes) occupy spaces within an extensive meshwork of epithelioreticular cells. Macrophages are also dispersed among the cortical cells. Because the cortex is richer in small lymphocytes than the medulla, it stains more darkly. The developing T cells arise from CFU-Ls, which originate in bone marrow. As development proceeds in the thymus, the cells derived from CFU-Ls pass through a series of developmental stages that are reflected by their expression of different CD molecules. As their name implies, epithelioreticular cells have features of both epithelial and reticular cells. They provide a framework for the developing T cells; thus, they correspond to the reticular cells and their associated reticular fibers in other lymphatic tissues and organs. Epithelioreticular cells exhibit certain features characteristic of epithelium such as intercellular junctions. Six types of epithelioreticular cells are recognized on the basis of function: Three types in the cortex and three types in the medulla (figure 4) 1- Type I epithelioreticular cells
Thymus: Is a central lymphoid organ situated in the mediastinum, the thymus has dual embryonic origin mesodermal and endodermal origin. The thymus originated from branchial epithelium. During development, the epithelium grows caudally and proliferates to form tubular projections. Multipotential lymphoid stem cell originated from bone marrow and destined to form immunocompetent T lymphocytes which invade the epithelium and occupy the spaces between the epithelial cells. The thymus has connective tissue capsule which penetrates the parenchyma and divide it into incomplete lobules, so that there is continuity between the cortex and medulla of adjoining lobules. Each lobule has a peripheral dark zone known as cortex and a central light zone called the medulla. Figure1:Photomicrograph of a section of thymus showing the lobules. Two lobules show the dark cortical and the light medullary zones. At the upper left are blood vessels and the connective tissue capsule. The cortex is composed of an extensive population of T cell precursors (also called thymocytes) occupy spaces within an extensive meshwork of epithelioreticular cells. Macrophages are also dispersed among the cortical cells. Because the cortex is richer in small lymphocytes than the medulla, it stains more darkly. The developing T cells arise from CFU-Ls, which originate in bone marrow. As development proceeds in the thymus, the cells derived from CFU-Ls pass through a series of developmental stages that are reflected by their expression of different CD molecules. As their name implies, epithelioreticular cells have features of both epithelial and reticular cells. They provide a framework for the developing T cells; thus, they correspond to the reticular cells and their associated reticular fibers in other lymphatic tissues and organs. Epithelioreticular cells exhibit certain features characteristic of epithelium such as intercellular junctions. Six types of epithelioreticular cells are recognized on the basis of function: Three types in the cortex and three types in the medulla (figure 4) 1- Type I epithelioreticular cells