Lymphoid tissues represent the sites of proliferation and differentiation of lymphocytes. The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes. The thymus is situated in the upper parts of the thorax, behind the sternum and the upper four costal cartilages, in the anterior and superior mediastina. The size of the thymus changes in the course of life. The thymus consists of a right and left lobe which are joined by connective tissue.
Blood vessels enter and leave the thymus via the connective tissue septae.
Each lobulus is divided into a darker peripheral zone, the cortex , and a lighter, central zone, the medulla . Medullary tissue is continuous from lobule to lobule throughout each lobe. Reticular cells and macrophages are present in addition to the lymphocytes, which are the dominant cell type within the lobules. Reticular cells are quite abundant. Their cytoplasm is eosinophilic, and their large, ovoid and light nuclei may contain one or two nucleoli.