| Primary lymphoid tissues include the fetal yolk sac, the embryonic liver; the fetal and adult bone marrow and the thymus
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| Lymphocytes originate in primary lymphoid tissue and undergo early development and differentiation there. Their location alters during gestation: in the early embryo the main site of lymphocyte production is the fetal yolk sac but later in embryonic life this shifts to the liver. Later there is yet another shift to the bone marrow and the thymus, the sites at which it will remain throughout the rest of the individual's life. Within the bone marrow, pluripotential stem cells become committed to the lymphoid lineage. Those that remain within the bone marrow for the rest of their maturation become B cells. In T-cell development, the primitive lymphocytes have to travel to the thymus. It is still uncertain at which point in their development lymphocytes become committed to a T-cell lineage.
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| The thymus has epithelial and mesenchymal (supporting) components in addition to the lymphoid tissue. The thymus is a two-lobed lobulated structure found in the anterior chest or mediastinum. At the microscopic level, there is an outer cortex and an inner medullary area within each lobule. In the thymus, T-cell development progresses further as the immature T cells migrate from the cortex to the medulla. During this time, the early T cells interact with nonlymphoid elements, including thymic epithelia and dendritic cells. These cells are thought to be responsible for the important processes of positive and negative selection that take place as part of the 'thymic education of T cells'. During this process the T cells are assessed for their ability to discriminate between what is and is not 'self'. During early development and differentiation, the T and B cells demonstrate changes in expression of surface molecules that would determine their future role and functional capabilities. The development of both early T and B cells in the primary lymphoid tissues is independent of extrinsic antigen stimulation.
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