Development of T cells in thymus:
In the embryonic life the thymus generates from the third pharyngeal pouch. Preliminary epithelial rudiment is seeded with blood borne stem cells. The ectoderm of the third branchial cleft forms the epithelium of thymic cortex. The endoderm of the third pharyngeal pouch distinguishes into epithelium of thymic medulla.
Stem cells migrate into thymus due to the chemotactic signals emitted from rudiment, β2 micro-globulin and components of MHC Class I molecule. The prethymic bone marrow derived stem cells which enter the thymic rudiment is a multipotent cell. The nurse cells existed in the cortex support the proliferation of stem cells arriving from bone marrow. Such stem cells develop into large, actively proliferating, self renewing lymphoblasts that generate the thymocyte population. Cortical thymocytes are less mature than medullary thymocytes. The cortical cells migrate to and grown-up in the medulla. The most mature T cells leave the thymus through post capillary venules situated at the cortico- medullary junction.
The T cells modify their phenotype throughout maturation. Differentiation markers of functional importance is either obtained or lost during the progression from stem cells to T cells.
During T cell maturation, sequential modifications take place in the membrane antigens. In the primary stage, cells state CD4- and CD8- phenotype. In this phase cells are capable of giving mount to other lineages. In the second phase cells state CD4+ and CD8+ phenotype. In the third phase cells exhibit major phenotypic changes. They become either CD4 T cells or CD8 T cells. Immature cell markers are not present as they become mature T cells. During this maturation in thymus, T cell receptor variety and positive and negative selection take place.
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