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. Thereby they create and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex. Macrophages occur in both cortex and medulla. They are difficult to distinguish from the reticular cells in out preparations. The vast majority of them will be developing T-lymphocytes.