Thymus

• The thymus is an important lymphoid organ, 
• situated in the anterior and superior mediastina of the thorax, 
• extending above into the lower part of the neck. 
• It is well developed at birth, continues to grow up to puberty, and thereafter, undergoes gradual atrophy and replacement by fat.
• The thymus is a bilobed structure, made up of two pyramidal lobes of unequal size which are connected together by areolar tissue.
• Each lobe develops from the endoderm of the third pharyngeal pouch.
• It lies on the pericardium, the great vessels of the superior mediastinum, and the trachea.
• The thymus weighs 10-15 g at birth, 30-40 g at puberty, and only 10 g after mid-adult life. 
• Thus after puberty, it becomes inconspicuous due to replacement by fat.

Blood Supply

• The thymus is supplied by branches from the internal thoracic and inferior thyroid arteries. 
• Its veins drain into the left brachiocephalic, internal thoracic and inferior thyroid veins.

Nerve Supply

• Vasomotor nerves are derived from the stellate ganglion. 
• The capsule is supplied by the phrenic nerve and by the descendens cervicalis.

Functions

1. The thymus controls lymphopoiesis, and maintains an effective pool of circulating lymphocytes, competent to react to innumerable antigenic stimuli.

2. It controls development of the peripheral lymphoid tissues of the body during the neonatal period. By puberty, the main lymphoid tissues are fully developed.

3. The cortical lymphocytes of the thymus arise from stem cells of bone marrow origin. 

Most (95%) of the lymphocytes (T lymphocytes) produced are autoallergic (act against the host or 'self' antigens), short-lived (3-5 days) and never move out of the organ. 

They are destroyed within the thymus by phagocytes. 

Their remnants are seen as Hassall’s corpuscles. 

The remaining 5% of the T lymphocytes are long lived (3 months or more), and move out of the thymus to join the circulating pool of lymphocytes where they act as immunologically competent but uncommitted cells, i.e. they can react to any unfamiliar, new antigen. 

On the other hand, the other circulating lymphocytes (from lymph nodes, spleen, etc.) are committed cells, i.e. they can mount an immune response only when exposed to a particular antigen. 

Thymic lymphopoiesis, lympholysis and involution are all intrinsically controlled.

4. The medullary epithelial cells of the thymus are thought to secrete:

a. Lymphopoietin, which stimulates lymphocyte production both in the cortex of the thymus and in peripheral lymphoid organs.

b. The competence-inducing factor, which may be responsible for making new lymphocytes competent to react to antigenic stimuli.

5.Normally there are no germinal centres in the thymic cortex. Such centres appear in autoimmune diseases.

This may indicate a defect in the normal function of the thymus.

Clinical Anatomy

• Involution of the thymus is enhanced by hypertrophy of the adrenal cortex, injection of cortisone or of androgenic hormone. 
• The involution is delayed by castration and adrenalectomy.
• Thymic hyperplasia or tumours are often associated with myasthenia gravis, characterized by excessive fatiguability of voluntary muscles.
• Thymic tumours may press on the trachea, oesophagus and the large veins of the neck, causing hoarseness, cough, dysphagia and cyanosis.

Watch the lecture on YouTube:

Thymus | Situation |Relations | Microscopic Structure |Haemo-thymic Barrier |Function |Clinical Anat