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Category : 11th Class

Origin and position : The thymus is bilobed gland, is located in the upper part of the thorax near the heart in the mediastinum. It is endodermal in origin, arising in the embryo from the epithelium of outer part of third branchial pouches.

Structure : Structurally, it is like lymph gland enveloped by a thin, loose and fibrous connective tissue capsule. Septa, or trabeculae extending inwards from the capsule, divide the two lobes of the gland into a number of small lobules. Each lobule is distinguished into a cortical parenchyma containing numerous lymphocytes, and a medullary mass of large, irregularly branched and interconnected epithelial cells (reticular cells), a few lymphocytes and some phagocytic cells called macrophages or Hassal's corpuscles.

Function of thymus glands

(1) Thymus is haemopoietic, as well as, an endocrine gland. Thymus is the "seedbed" of "thymic lymphocytes (T-lymphocytes). Certain "stem cells", originating in yolk sac and liver in early embryo, but only in bone marrow in late embryo, migrate into the thymus and proliferate to form a large number of lymphocytes.

(2) The major function of thymus is to secrete thymosin hormone, thymic humoral factor (THF), thymic factor (TF), thymopoietin. These compounds induce, not only the proliferation of lymphocytes, but also their differentiation into a variety of clones differently specialized to destroy different specific categories of antigens and pathogens likely to get into the body. This is called maturation of lymphocytes.

(3) As is clear from above account, thymus is essential in neonatal (newly born) infant and postnatal child for normal development of lymphoid organs and cellular immunity. That is why, the thymus, small at birth, progressively grows in size about three or four-folds upto about the age of puberty. By this time lymphoid organs and tissues are well-developed. The thymus, therefore, starts gradually diminishing in size and its tissue is progressively infiltrated by yellowish adipose tissue. This is known as the "immunity theory of ageing". By the old age, the thymus is reduced to quite a thin, yet functional chord of tissue.