Some arterioles course along the cortico-medullary border and supply blood primarily to the cortex via arcades o f anastomosing capillaries which join postcapillary venules . The capillary arcades are tightly covered on the outside with a layer of epithelioid reticular cells. Another branch enters the medulla. It gives rise to fenestrated capillaries. The postcapillary venules allow entrance of thymocytes coming from the cortex to enter the circulation and be transported to secondary lymphoid organs.
Veins leave the thymus along with the efferent lymphatics via the septa.
Veins drain to the thymic vein . The thymus becomes functionally dormant with age and involution begins at puberty . Most of the parenchyma is replaced by adipose tissue and the medulla becomes fibrous. See Figure 4. Under low power of your microscope study the organization of the thymus.