Thymus Gland

Thymus Gland

It is a single, bilobed, flattened, pyramidal gland, situated in the mediastinal space just in front of the heart. It is also called Throne of immunity.

Grows in early age and becomes fully developed at 16 years but starts degenerating after 18 years and atrophies after the age of 25 years. Castration (surgical removal of gonads) prolongs the period of persistence of thymus.

At birth it weighs 10-12 gms, during childhood & adolescence 20-30 gms, and during old age 3-6 gms.

It is partly endocrine and partly lymphoid, covered by a connective tissue capsule and consists of two zones: (i) Outer cortex with lymphocyte-like cells; and (ii) Inner medulla comprising reticular cells, few lymphocytes and Hassal’s corpuscles. These are phagocytic.

Surgical removal of thymus is called Thymectomy.

Immunological role of thymus :

Thymus provides an environment favourable to lymphopoiesis. It receives precursors from the bone marrow which after development in thymus pass on to the lymph.

The reticulo-epithelial tissue in thymus secretes a hormone called Thymosin which stimulates lymphopoiesis within the thymus and in peripheral lymphoid tissue. Thus, thymus promotes the development of immunological competent T-hymphocytes (maturation of T- lymphocytes).

Thymus also secretes a hormone Thymopoietin (Thymin) which inhibits acetylcholine release at motor nerve endings in myasthenia gravis.

In autoimmune diseases like myasthenia gravis, Hashimoto’s disease there is hyperplasia of thymus.