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.
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