Thymus gland

Thymus gland

1. Introduction : it is a specialized primary lymphoid organ of the immune system.

2. Situation : the thymus gland lies in the upper part of the mediastinum behind the sternum and

extends upwards into the roof of the neck.

3. Weight :

10 -15gm during birth

30-40 gm during puberty

4. Organs associated with the thymus:

Anteriorly : sternum and upper four costal cartilages.

Posteriorly :aortic arch and its branches,brachiocephalic veins,trachea

Laterally: lungs

Superiorly :structures in the root of the neck

Inferiorly:heart

5. Structure:

The thymus gland consists of two lobes joined by areolar tissue.

The lobes are enclosed by a fibrous capsule which dips into their substance,dividing them into lobules that consist of an irregular branching framework of epithelial cells and lymphocytes.

6. Function:

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Lymphocytes originate from pluripotent stem cells in red bone marrow,those that enter the thymus develop into activated T-lymphocytes.

Thymic processing produces mature T-lymphocytes that can distinguish self tissue from foreign tissue,and also provides each T-lymphocyte with the ability to react to only one specific antigen from the millions it will encounter.

T-lymphocytes then leave the thymus and enter the blood.

Some enter lymphoid tissues and others circulate in the bloodstream.

T-lymphocytes production,although most prolific in youth,probably continues throught life from a resident population of thymic stem cells.

The maturation of the thymus and other lymphoid tissue is stimulated by THYMOSIN.

Thymosin is a harmone secreted by the epithelial cella that form the framework of the thymus gland.

Involution of the gland begins in adolescence and, with increasing age, the effectiveness of the T-lymphocyte response to antigens declines.