The thymus gland is a primary lymphoid organ located in the neck or upper thorax of all vertebrates. It is a site of lymphocyte development. T-cell precursors (prothymocytes) migrate from the bone marrow to the thymus gland, where they undergo a stringent selection process. The microenvironment of the thymus is composed of nonlymphoid cells (epithelial cells, macrophages, dendritic cells, and fibroblasts) and extracellular matrix components. This environment stimulates the differentiation (maturation) of prothymocytes. The maturation includes the acquisition of T-cell function, leading to the mature and self-tolerant T cells that are found in the peripheral lymphoid organs. Classical hormones and neural innervation also affect lymphoid development and function.