Lymphoid Tissue

Thymus of a newborn monkey

The gland has a covering capsule whih sends in septa dividing it into many incomplete lobules. Under low power, locate a lobule and note that the capsule covering it carries blood vessels with closely packed blood cells in their lumen. Examine a lobule. The periphery contains densely packed lymphocytes forming the cortex. In the inner zone is the medulla in which the cells are not as tightly packed. As a consequence, the cortex appears darker in H & E preparations. Under X40, study the cellular and structural components of the lobule. In the cortex, identify the densely stained components of the lymphocytes, scattered epithelial reticular cells(lightly stained) and macrophages. Do you see any blood vessels in the cortex? The medulla possesses numerous circular bodies called Hassall's or thymic corpuscles. These are large concentric layers of eosinophilic matter derived from epithelial reticular cells. Between the Hassall's corpuscles, identify the numerous epithelial reticular cells, lymphocytes and blood vessels(capillaries, venules).