Schematic representation of ECM receptor expression and adhesive function throughout thymocyte differentiation. A high expression of the ECM adhesion receptors, in special FN receptors, and an increased adhesiveness to TEC cells hallmarks the early steps of thymocyte differentiation. Recently generated DP thymocytes keep an upregulated expression of L-selectin, 4, and 5 integrin chains, while DP thymocytes emerging later during thymus reconstitution exhibit a downregulated profile of these ECM receptors. During SP cell maturation the 4 chain is downregulated before the 5 chain, and L-selectin is upregulated in a fraction of fully mature thymocytes, while a more marked downregulation of the 6 chain is observed in CD4⁺ cells than in CD8⁺ cells. The dashed line splits thymocyte subsets according to their high or low TEC adhesive capabilities. The arrow tagged with a question mark shows a CD3ⁱⁿCD45RC⁻ subset that is enriched after irradiation and most likely represents IL-2Rβ⁺and/or NK1.1⁺ thymocytes of still undefined origin in C57Bl/6 mouse thymus, according to the literature.