Cortical thymic epithelial cells  Cortical thymic epithelial cells (cTECs) form unique parenchyma cell population of the thymus which critically contribute to the development of T cells. Thymus tissue is compartmentalized into cortex and medulla and each of these two compartments comprises its specific thymic epithelial cell subset. cTECs reside in the outer part- cortex, which mostly serves as a developmental site for T cells. Precursors of T cells originate in the bone marrow from which they migrate via bloodstream into thymic cortex, where they encounter stromal cells including cTECs, which form the microenvironment crucial for proliferation and development of T cells by expression of DLL4 (delta-like notch ligand 4), cytokines IL-7, TGFβ or stem cell factor and chemokines CCL25, CXCL12 or CCRL1 etc. Essential part of T cell development forms process called VDJ recombination, mediated by RAG recombinases, that stochastically changes DNA sequences of T cell receptors (TCR) and endows them with diverse recognition specificity. Thanks to this process, T cells can recognize vast repertoire of pathogens, but also self-peptides or even their TCRs don´t respond to any surrounding signals. Major role of thymic epithelial cells is to test, whether TCRs are "functional" and on the other hand "harmless" to our body. While cTECs control the functionality of TCRs during the process called positive selection, Medullary thymic epithelial cells (mTECs) that home in the inner part of the thymus- medulla, present on their MHC molecules self-peptides, generated mostly by protein Autoimmune regulator, to eliminate T cells with self-reactive TCRs via processes of central tolerance e.g. negative selection and protect the body against development of autoimmunity. "Major function of cTECs is to positively select those T cells that are capable to recognize and interact with MHC molecules on their surface" "." Once T cell precursors enter the thymic cortex, they start their transformation from double negative stages (T cell without surface expression of CD4 and CD8 co-receptors) to a double positive stage (T cell with surface expression of both co-receptors) that expresses fully recombined TCR.