The thymus is a pyramid shaped primary lymphoid organ that is located beneath the breastbone, at the same level as the heart. It is the initial site for the development of T cell immunological function and the first of the lymphoid organs to be formed. The organ is named so because its shape resembles that of a thyme leaf.
The thymus grows rapidly during fetal life and the first year after birth. This is in response to postnatal antigen stimulation and the demand for a large number of mature T cells. The organ continues to grow later, but at a slow pace. At the onset of puberty, the organ begins the slow process of shrinking. It continues to shrink as the years go by till the end of the individual's life.
Structure of thymus
The thymus is divided into two lobes, which are further subdivided into lobules. The two lobes lie on either side of the midline of the body. The organ is covered by a dense connective-tissue capsule. This sends fibers into the body of the organ for support. Each lobule has an outer cortex and an inner medulla. The lobules are separated by septa (connective tissue).
The organ is composed of two types of cells: lymphocytes and reticular cells. In this it is similar to the other organs of the lymphatic system. The reticular cells form a loose meshwork. The space between the reticular cells is filled by the lymphocytes. The cortex has the highest concentration of lymphocytes. Some of the daughter cells that are produced in the cortex migrate to the medulla, where they enter the bloodstream through the medullary veins. These cells add to the lymphocytes seen in the peripheral blood and the lymphoid organs.
During the shrinking phase, the cortex becomes thin. Lymphocytes are gradually replaced by fat tissue from the partitions between the lobules. The organ continues to shrink till the individual dies. Experts believe that bits of thymus tissue that remain are sufficient to maintain its function.
Function of the thymus
The thymus can be removed from an adult human being without impacting the person's health. However, the removal of the organ in the newborn results in the depletion of the T cells in the blood, depletion of the lymphoid tissue and failure of the immune system. It is a major site of lymphopoiesis in the embryo and the newborn. Lymphocytes migrate from the thymus to seed the spleen, lymph nodes and other lymphoid organs.