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thymus gland(thī`məs), mass of glandular tissue located in the neck or chest of most vertebrate animals. In humans, the thymus is a soft, flattened, pinkish-gray organ located in the upper chest under the breastbone. It is relatively large in the newborn infant (about the size of the baby's fist), and continues to grow throughout childhood up to the age of puberty when it weighs about 1.2 oz (35 grams). Then it gradually decreases in size until it blends in with the surrounding tissue. The functions of the thymus were not well understood until the early 1960s, when its role in the development of the body's system of immunity was discovered. Beginning during fetal development, the thymus processes many of the body's lymphocytes, which migrate throughout the body via the bloodstream, seeding lymph nodes and other lymphatic tissue. The main cells undergoing this processing are the T cells, a heterogeneous groups of cells essential in protecting the body against invasions by foreign organisms (see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
..... Click the link for more information. ). If the thymus fails to develop or is removed early in fetal life, the immune system cannot develop completely. Normally, by the time the infant is a few months old, the immune system has sufficiently formed so as to function throughout life. However, further growth and development of lymphoid tissue still depends on intervention by the thymic cells. After the initial seeding process, the thymus releases a hormonal substance that stimulates further growth of lymphoidal tissue, although such a substance has not yet been isolated.
a lobed endocrine gland in vertebrates and man. The thymus gland develops from the endodermal epithelium of the branchial pouches.
In man the thymus gland is formed in the sixth week of development. Its rudiments originally consisted solely of epithelial tissue. In the process of evolution its structure be-came more complex and lobed. In man the thymus is located in the thoracic cavity in the region of the superior interpleural space of the anterior mediastinum. Well developed in the newborn, it is the largest lymphoid organ at birth, and its tissue produces lymphocytes more actively than all the other tissues of the organism. The gland continues to grow until sexual maturity, when it weighs 30-40 g; subsequently, it undergoes involution. The thymus is covered with a connective-tissue capsule from which septa go into the gland, dividing it into lobules, each of which has a cortex and medulla. The cortex consists of reticular epithelial tissue containing a great quantity of lymphocytes in its loops. Thus, the thymus gland is classified as a lymphoepithelial formation. There are fewer lymphocytes in the medulla, which is similar in structure to the cortex. In the center of the medulla there are stratified epithelial bodies about 50 microns in diameter—Hassall’s bodies, the most characteristic structures of the thymus. They are made up of concentrically stratified epithelial cells. By age 15 the number of stratified epithelial bodies reaches a maximum, after which it quickly decreases. However, Hassall’s bodies continue to be formed even in old age. The cortex of the lobules gradually loses lymphocytes with age. The cortex shrinks more rapidly than the medulla, but remnants of it remain even after the glandular substance is completely replaced by fatty tissue.
The functional significance of the thymus gland has not been conclusively established. Some data indicate that in lower vertebrates it functions on a seasonal basis, helps regulate growth and mineral metabolism, and is involved in the formation of specific immunity. In birds it is considered a depot for labile nucleoproteins that are intensively consumed during periods of sexual activity. The thymus functions in close relationship with other endocrine glands (adrenals, pituitary, and gonads). It is very sensitive to external factors—physical (radiation), chemical (many carcinogens), and hormonal (hormones of the adrenal cortex, thyroid, gonads, and other glands)—to which it reacts by involution and atrophy. The gland is supplied with blood from the internal mammary and inferior thyroid arteries. It is innervated by branches of the vagus, sympathetic, and phrenic nerves.
Developmental abnormalities of the thymus include aplasia (complete absence of the gland), which is usually combined with other developmental defects in the organism, and hypoplasia (incomplete development) combined with hypoplasia of the thyroid and mental retardation. In some cases there are accessory glands located in the neck. Hyperplasia (marked enlargement) of the thymus may prevent normal development of adjacent organs or cause impairment of respiration and sudden death, and it is often a manifestation of status thymicoly mphaticus.
REFERENCESMiller, J., and P. Dukor. Biologiia timusa. Moscow, 1967. (Translated from German.)
Galustian, Sh. D. Stroenie zobnoi zhelezy v svete eksperimental’nogo analiza. Moscow, 1949.
Tesseraux, H. Physiologie und Pathologie des Thymus. Leipzig, 1959.
Tesseraux, H. Physiologie und Pathologie des Thymus. Leipzig-Munich, 1959.
IU. I. DENISOV-NIKOL’SKII