NCIt definition : Development of bone marrow-derived stem cells in the thymus through stages of positive
selection under the influence of epithelial cells in the thymic cortex and negative
selection under the influence of bone marrow-derived dendritic cells at the corticomedullary
junction. Thymic T cell precursors (thymocytes) progress through phases expressing
both CD4 and CD8 coreceptors to a single positive stage expressing only CD4 or CD8
in addition to the T cell receptor. They are then allowed to leave the thymus and
enter the periphery via the lymphatics or venules of the circulatory system as mature
T cells. (NCI/OSP);