Thymus Gland, name applied to a structure located just beneath the upper portion of the sternum in almost all vertebrates. The thymus gland consists chiefly of lymphatic tissue and contains a few small areas of epithelial tissue known as Hassall's corpuscles. The human thymus gland increases in weight in the first two years of life, and from then until puberty it grows slowly to a weight of about 43 g (about 1.5 oz). After puberty, it shrinks gradually and the lymphatic tissue of the thymus gland is replaced by fat. In the adult human the organ is chiefly composed of fatty tissue. Scientists generally agree that the thymus gland plays an important role in the development of immune responsiveness in early life (see Immune System). It is a site of formation of lymphocytes and a site of antibody production. Whether or not it has any other endocrine functions is uncertain. Clearly the adult animal is not affected by its removal. Abnormal enlargement of the thymus or development of tumors of the gland may occur in myasthenia gravis, but the reason is not known.