Thymus is the site where stem lymphocytes (CFU-Ls) differentiate into T lymphocytes (or thymocytes). It contains epithelial cells (or oropharyngeal origin) called epithelioreticular cells. There are 6 types (I-VI): three in the cortex and three in the medulla. They remain attached by desmosomes or occluding junctions, and contribute to various compartment barriers in the thymus (Types I, III, and IV). They form a cytoplasmic reticulum within the parenchyma of the thymus, serving as a framework for T cell development, and correspond to reticular cells and their fibers in other lymph organs. Reticular fibers are not present in the thymus. The parts of the thymus include:

Blood-Thymic Barrier:

Components of the barrier include capillary endothelium (a major structural component of the barrier with extensive tight junctions), endothelial basal lamina, thin perivascular connective tissue sheath with residing macrophages, basal lamina of epithelioreticular cells, and epithelioreticular cell sheath (made of type I epithelioreticular cells). There are no afferent lymphatics, as the thymus does not react to lymph-borne antigens.