thymus /thī″məs/ pl. thymuses, thymi [Gk, thymos, thyme, flowers] , a single unpaired lymphoid organ that is located in the mediastinum, extending superiorly into the neck to the lower edge of the thyroid gland and inferiorly as far as the fourth costal cartilage. The thymus is the primary central gland of the lymphatic system. The endocrine activity of the thymus is believed to depend on the hormone thymosin, which is composed of biologically active peptides critical to the maturation and the development of the immune system. The T cells of the cell-mediated immune response develop in this gland before migrating to the lymph nodes and spleen. The gland consists of two lateral lobes closely bound by connective tissue, which also encloses the entire organ in a capsule. Superficial to the gland is the sternum. Lying deep to the thymus are the great vessels and the cranial part of the pericardium. The two lobes of the gland differ in size, and in many individuals the right lobe overlaps the left lobe. The thymus is about 5 cm long, 4 cm wide, and 6 mm thick. The lobes are composed of numerous lobules, which are separated by delicate connective tissue. Each lobule is composed of a dense cellular cortex and an inner, less dense medulla. The thymus develops in the embryo from the third branchial pouch and increases in size until attaining a weight of 12 to 14 g before birth. The size of the organ relative to the rest of the body is largest when the individual is about 2 years of age. The thymus usually attains its greatest absolute size at puberty, when it weighs about 35 g. After puberty the organ undergoes involution. With aging the gland may change from pinkish-gray to yellow and in the elderly individual may appear as small islands of thymic tissue covered with fat and surrounded by the yellowish capsule. The normal involution of the thymus may be superseded by rapid accidental involution caused by starvation or by acute disease. Compare spleen.
