Identify lymphocytes and thymic corpuscles. Take a close look at the medulla and try to find some cells which contain large and light nuclei. They will be either macrophages or reticular cells. Sketch part of the thymus at low magnification. Identify medulla and cortex.
Stem cells invade the cortical regions of the thymus, where they divide to form lymphocytes.
They migrate via the medulla into the blood stream to populate the T-lymphocyte areas of other lymphoid tissues and organs. Since the function of the thymus is to produce lymphocytes for the other lymphoid tissues it is a primary lymphoid organ . After puberty much of the parenchyma of the thymus, in particular cortical lymphoid tissue, is replaced by adipose tissue. The process, which is called involution , initially proceeds rapidly but slows down in adulthood. Although most pronounced in the thymus, involution is a common feature of all lymphoid tissues .