Thymus

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:

• Cortex, which is extremely basophilic in H&E. It is packed with T lymphocytes and numerous macrophages, the latter involved in phagocytosis of T cells that do not fulfill education requirements. Can be demonstrated with PAS (called PAS cells). The cortex contains Type I, II, and III epithelioreticular cells:
• Cytoplasmic processes of Type I cells separate the cortical parenchyma from the CT of the capsule, trabeculae, and blood vessels adventitia. These processes exhibit occluding junctions and barrier between the parenchyma and CT.
• Type II cells form meshwork to support developing T lymphocytes. They have large euchromatic nuclei and cytoplasmic processes exhibiting desmosomes.
• Type III cells are located between the cortex and the medulla, creating a barrier. The cell processes reveal occluding junctions between them.
• Medulla stains lighter due to large lymphocytes containing pale staining nuclei and more cytoplasm. They contain type IV, V, and VI epithelioreticular cells:
• Type IV cells are located between the cortex and the medulla, which possesses occluding junctions.
• Type V cells, whose cytoplasmic processes form meshwork between groups of lymphocytes. The processes are joined by desmosomes.
• Type VI cells form Hassall’s corpuscles, which are packed together and concentrically arranged. The center may appear keratinized (due to oropharyngeal epithelial derivative), and they may contain calcium deposits.

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.