The thymus is a primary lymphoid organ, which is part of the immune system. It is located in the mediastinum, neighboring the aortic arch and vena cava, right above the heart, behind the upper part of the sternum. The size of the thymus varies considerably throughout life. At birth, it only weighs 15 grams; then it continues to grow during childhood and puberty until it weighs 40 grams. From then on, it undergoes gradual regression and fatty degeneration, and by age 40, it weighs 10 grams.
Not only is the thymus a lymphoid organ but it is also an endocrine gland. As a lymphoid organ, it produces lymphocyte T cells. As an endocrine gland, it secretes thymic hormones, which support proliferation and differentiation of T cell progenitors (prothymocytes). It is composed of two lobes and is characterized by thymic epithelial reticular cells, which are mostly arranged in a loose network. A dense irregular connective tissue capsule wraps around the entire gland as trabecular branches of the capsule surround each lobules that form the two lobes.
Histologically, the thymus is composed of a cortex, which is on the outer portion of the organ, and a medulla, which lies in the inner region. The cortex is rich in lymphocytes, while the medulla is rich in epithelial reticular cells. These epithelial reticular cells form a cellular network similar to the networks formed by reticular cells in lymph nodes, bone marrow, and the spleen. The medulla also contains concentric whorls (coils) of degenerating epithelial cells, which are called Hassal's corpuscles or thymic corpuscles.
Function
The thymus is a key organ in the immune system because it is here that lymphocytes that arise in the bone marrow proliferate and differentiate into T cells. Thus, cells destined to become T cells are bone marrow stem cells. Stem cells from the bone marrow mature into T cells and undergo clonal expansion in the thymus, and then they seed distant sites in lymph nodes, the spleen, and elsewhere in the body. Lymphocyte differentiation is mediated by lymphokine called thymosin.
The thymus medulla also contains marrow-derived thymic dendritic cells, which are involved in T cell selection and maturation.
Below, light micrograph of the thymus, exhibiting the medulla (M) and the cortical lobules (CL).
Picture of the lymphoid organ, showing its location in the human body and its shape.
