T-cell development stages in relation to thymic architecture.

Differentiation of TN thymocytes occurs within the subcapsular zone of the thymus. This region contains a network of epithelial reticular cells. At the DP stage, thymocytes migrate to the cortex, where they encounter cortical epithelial cells with long processes, fibroblasts, and macrophages. These cells are important for MHC class restriction and negative selection. Thymocytes then migrate to the medulla, where CD4 or CD8 lineage commitment occurs. This region contains medullary epithelial cells with shorter processes, dendritic cells, and macrophages. Mature T cells exit the thymus from the medullary region and enter the peripheral circulation.