The thymus gland, despite containing glandular tissue and producing several hormones, is much more closely associated with the immune system than with the endocrine system. The thymus serves a vital role in the training and development of T-lymphocytes or T cells, an extremely important type of white blood cell. T cells defend the body from potentially deadly pathogens such as bacteria, viruses, and fungi. It has two distinct but identical lobes that are each surrounded by a tough, fibrous capsule. Within each lobe is a superficial region of tissue called the cortex and a histologically distinct deep region called the medulla. Epithelial tissues and lymphatic tissues containing dendritic cells and macrophages make up the majority of both regions of the thymus.
The function of the thymus is to receive immature T cells that are produced in the red bone marrow and train them into functional, mature T cells that attack only foreign cells. T cells first reside within the cortex of the thymus where they come in contact with epithelial cells presenting various antigens. The immature T cells that respond to the antigens corresponding to foreign cells are selected to survive, mature, and migrate to the medulla while the rest die via apoptosis and are cleaned up by macrophages. This process is known as positive selection.
Upon reaching the medulla, the surviving T cells continue to mature and are presented with the body’s own antigens. T cells that bind to the body’s own antigens test positively for autoimmunity, whereby they attack the body’s own cells instead of only foreign cells. Autoimmune T cells are eliminated by apoptosis in a process known as negative selection, resulting in only around 2% of the immature T cells reaching maturity.
Several hormones produced by the thymus promote the maturation of the T cells prior to their release into the bloodstream. The now mature T cells circulate through the body where they recognize and kill pathogens, activate B cells to produce antibodies, and store the memory of past infections. Unlike most organs that grow until the age of maturity, the thymus enlarges throughout childhood but slowly shrinks from the onset of puberty and throughout adulthood. As the thymus shrinks, its tissues are replaced by adipose tissue. The shrinking is due to the reduced role of the thyroid in adulthood – the immune system produces most of its T cells during childhood and requires very few new T cells after puberty.
Structure of Thymus Gland:
The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces. At birth it is about 5 cm in length, 4 cm in breadth, and about 6 mm in thickness. The organ enlarges during childhood, and atrophies at puberty. Unlike the liver, kidney and heart, for instance, the thymus is at its largest in children. The thymus reaches maximum weight (20 to 37 grams) by the time of puberty. The thymus of older people is scarcely distinguishable from surrounding fatty tissue. As one ages the thymus slowly shrinks, eventually degenerating into tiny islands of fatty tissue. By the age of 75 years, the thymus weighs only 6 grams. In children the thymus is grayish-pink in color and in adults it is yellow.
- • Cortex: The cortical portion is mainly composed of lymphocytes, supported by a network of finely-branched epithelial reticular cells, which is continuous with a similar network in the medullary portion. This network forms an adventitia to the blood vessels. The cortex is the location of the earliest events in thymocyte development, where T cell receptor gene rearrangement and positive selection takes place.
- • Medulla: In the medullary portion, the network of reticular cells is coarser than in the cortex, the lymphoid cells are relatively fewer in number, and there are concentric, nest-like bodies called corpuscles. These concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelial cells. They are the remains of the epithelial tubes, which grow out from the third pharyngeal pouches of the embryo to form the thymus. Each follicle is surrounded by a vascular plexus, from which vessels pass into the interior, and radiate from the periphery toward the center, forming a second zone just within the margin of the medullary portion. In the center of the medullary portion there are very few vessels, and they are of minute size.
- • Blood supply: The arteries supplying the thymus are derived from the internal thoracic artery, and from the superior thyroid artery and thyroids. The veins end in the left brachiocephalic vein (innominate vein), and in the thyroid veins.