Schematic diagram of the blood-thymus barrier. The blood-thymus barrier consists of three major elements: (1) capillary endothelium and its basal lamina, (2) perivascular connective tissue space occupied by macrophages, and (3) type I epithelioreticular cells with their basal lamina. The perivascular connective tissue is enclosed between the basal lamina of the epithelioreticular cells and the endothelial cell basal lamina. These layers provide the necessary protection to the developing immature T cells and separate them from mature immunocompetent lymphocytes circulating in the bloodstream.
• Type I epithelioreticular cells with their occluding junctions. These cells provide further protection to the developing T cells. The epithelioreticular cells surround the capillary wall in the cortex and with their basal lamina represent another major structural component of the blood-thymus barrier.
During fetal life, the thymus is populated by multipo-tential lymphoid stem cells that originate from the bone marrow and are destined to develop into immunocompetent T cells. Stem cell maturation and differentiation into immunocompetent T cells is called thymic cell
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