Precursors of T cells originate in the bone marrow from which they migrate via bloodstream into thymic cortex , where they encounter stromal cell s including cTECs , which form the microenvironment crucial for proliferation and development of T cells by expression of DLL4 ( delta-like notch ligand 4 ) , cytokine s IL-7 , TGFβ or stem cell factor and chemokine s CCL25 , CXCL12 or CCRL1 etc .