What is the thymus gland?
The thymus gland is a flask-shaped organ lying in the lower neck and upper part of the mediastinum (superior mediastinum) in the chest cavity. It is an important part of the immune system in that it plays a major role in cell-mediated immunity. However, surgical removal of the thymus gland, even in childhood, does not result in significant immune dysfunction.
Thymus Gland Anatomy
The thymus gland is most active in early life and this is reflected in its weight – 20 to 30 grams at birth, 30 to 50 grams up until puberty and 5 to 15 grams in old age. Its size at birth is approximately 50 millimeters in length and 40 millimeters in width with a variable thickness of about 6 millimeters.
Location of the Thymus Gland
The thymus gland lies in the anterior part of the thoracic cavity. It extends from the inferior part of the neck to the superior mediastinum. It lies posterior to the manubrium of the sternum (breastbone) and variations in size may see it extend into the upper part of the inferior mediastinum (below the sternal angle). This puts the thymus gland approximately at the level of the T4 to T6 vertebrae. It is attached to the anterior chest wall and behind it lie the aorta, airway and esophagus.
Parts of the Thymus Gland
The prolific activity of the thymus gland starts in the fetus, continues after birth and peaks around puberty. This however, may be disrupted in children living with HIV and other severe illnesses where the thymus may involute at an early stage. In childhood developmental disorders characterized by thymic hypoplasia or apoplasia, there is a profound effect on T-cell mediated immunity.
When it is fully developed, the thymus is an encapsulated, lobar organ. After puberty, the thymus undergoes gradual involution and is largely replaced by fat tissue. Each of the two lobes are subdivided into several lobes by the fibrous extensions of the thymus capsule.
The thymus has two parts :
- Outer cortex
- Inner medulla
The cortex is abundant with immature T-cells which migrate to the medulla in order to mature. It is also here where the T-cells learn to differentiate between foreign invaders and structures of the body so that inappropriate immune responses are prevented. Other types of cells found in the thymus includes small populations of neutrophils, eosinophils, macrophages and B-lymphocytes.
Blood and Lympathic Supply of the Thymus Gland
Oxygen-rich blood reaches the thymus gland through arteries derived from the anterior intercostal and anterior mediastinal branches of the internal thoracic arteries.
Venous blood drains into the left brachiocephalic, internal thyroid and internal thoracic veins.
The brachiocephalic, parasternal and tracheobronchial lymph nodes receive the lympathic vessels from the thymus gland.
Functions of the Thymus Gland
The thymus gland is responsible for the selection and maturation of T-lymphocytes. Partially differentiated stem cells, which are to become lymphocytes, originate in the bone marrow and migrate to the the thymus gland. Here these cells divide rapidly and also develop antigen-specific reactivity so that it can act against specific antigens.
At the same time, the T-cells are programmed so as not to react against the body’s own tissues – self-antigens. The T-lymphocytes are then either destroyed if the programming is ‘wrong’ thereby reducing the risk of these cells attacking the body’s own cells or it is released where it can play a crucial role in the body’s immune defenses. This T-cell processing ensures that the circulating lymphocytes are equipped to fight invading pathogens without damaging the body’s cells.
Most of this preprocessing occurs before birth (fetus) and immediately after birth (neonates). Removal of the thymus after this period will reduce but not eliminate the T-lymphocyte immune system.