Thymus: cortex

The cortex is composed primarily of closely packed, immature T lymphocytes (thymocytes), supported by a framework of epithelial reticular cells.  A few macrophages are also present.  A thin connective tissue capsule surrounds the thymus; blood vessels enter the thymus through the capsule and its septa.  400x

The cortex is composed primarily of closely packed, immature T lymphocytes (thymocytes), supported by a framework of epithelial reticular cells.  A few macrophages are also present.  A thin connective tissue capsule surrounds the thymus; blood vessels enter the thymus through the capsule and its septa.  400x

Lymphoblastic stem cells migrate into the thymus from the bone marrow where they mature in the cortex to form T lymphocytes. These mature cells exit the cortex and migrate to the medulla, where they enter the blood stream to be transported to T-dependent regions of the lymphoid system (diffuse lymphoid tissue of MALT, paracortex of lymph node and PALS of spleen).

The stroma of the thymus is formed only by epithelial reticular cells; no fibers are present in this stroma.  Thymic reticular cells are derived from embryonic epithelium of the pharynx rather than embryonic connective tissue, as are most reticular cells.  Consequently, stromal cells of the thymus are called epithelial reticular cells rather than simply reticular cells.

This area is shown at higher magnification in the next image.