Overview
The thymus is located
in the neck and thorax. It is a pink lobulated organ involved in the
development of the immune system before puberty, after which it atrophies and
is replace with fatty tissue.
Gross Anatomy
The thymus is an asymmetrical,
bilobed lymphoid organ. The gland mainly lies in the superior mediastinum, but
the upper portion can extend to the thyroid in the neck and the lower portion
typically lies in the anterior mediastinum, posterior to the manubrium and
anterior to the fibrous pericardium.
In the embryo the
thymus is derived from the third pharyngeal pouch and relative to body size the
thymus is largest in the infant. During adolescence the thymus is involved in
the immune systems' early development; after puberty the thymus atrophies and
the tissue is replaced with fat.
The lobules of the
thymus are made up of follicles, which consist of a central medulla surrounded
by a peripheral cortex. The cortex contains lymphocytes and epithelial
reticular cells, the medulla has fewer lymphocytes but more epithelial
reticular cells.
Branches of the
internal thoracic and inferior thyroid arteries supply the thymus and venous
drainage is mainly into the left brachiocephalic and some into the internal
thoracic veins. There is minimal nervous supply to the thymus and no afferent
lymphatics. Lymphatic drainage from the thymus is into parasternal,
tracheobroncial and brachiocephalic nodes.
Clinical Anatomy
DiGeorge
Syndrome - This is a genetic syndrome
caused by deletions on chromosome 22. Clinical findings include thymic aplasia,
congenital heart defects, abnormal facies, hypoparathyroidism and cleft palate.
Individuals with an aplastic thymus have an underdeveloped immune system and are
therefore susceptible to repeated infections.
Ectopic
parathyroid glands - The thymus and
parathyroid glands both develop embryologically from the third pharyngeal pouch
resulting in the thymus as a common site from ectopic parathyroid glands and
possible ectopic parathyroid hormone production.
Quick Anatomy
Key Facts
Developmental precursor |
Arterial supply |
Venous drainage |
Lymphatic drainage |
Third pharyngeal pouch. |
Branches from the internal thoracic and
inferior thyroid arteries. |
Left brachiocephalic vein, internal
thoracic veins. |
Parasternal, tacheobroncial and
brachiocephalic nodes. |
Aide-Memoire
The 5 T's - These are the most common causes of
anterior mediastinal masses:
1 - Thymus, thymoma.
2 - Thyroid, ectopic thyroid masses.
3 - Thoracic aorta, dilated or anaeurysm of
the ascending aorta.
4 - Terrible lymphoma.
5 - Teratoma and germ cell tumors.
Summary
The thymus is a
lobulated lymphoid organ with an important role in immune development before
puberty. It is located mainly in the superior mediastinum, posterior to the
manubrium. Follicles make up the thymus tissue, which are arranged with a
central medulla and a peripheral cortex consisting of lymphocytes and
epithelial reticular cells.