Organs - Thymus Gland

Question 1

What is the thymus gland?

Embryological origin?

Location?

Answer 1

It is a pink, lobulated lymphoid organ, located in the thoracic cavity and neck. In the adolescent, it is involved in the development of the immune system. After puberty, it decreases in size and is slowly replaced by fat.

Embryologically, the thymus gland is derived from the third pharyngeal pouch.

Question 2

Anatomical structure and position?

Answer 2

The thymus gland has an asymmeetrical flat shape, with a lobular structure. The lobules are comprised of series of follicles, which have a medullary and cortical component:

1) Cortical component - located peripherally within each follicle. It is largely composed of lymphocytes, supported epithelial reticular cells.
2) Medullary portion - located centrally within each follicle. It contains fewer lymphocytes than the cortex, and an increased number of epithelial cells. Hassall’s corpuscles are also present - these are concentric arrangements of epithelial reticular cells. Their function is unclear.

The gland is mainly located within the thoracic superior mediastinum, posterior to the manubrium of the sternum. However, in some individuals, it can extend superiorly into the neck (reaching the thyroid gland), and inferiorly into the anterior mediastinum (lying in front of the fibrous pericardium).

Question 3

Vasculature?

Answer 3

The arterial supply to the thymus gland is via the anterior intercostal arteries and small branches from the internal thoracic arteries. Venous blood drains into the left brachiocephalic and internal thoracic veins.

Question 4

`Clinical relevance - DiGeorge Syndrome?

Answer 4

DiGeorge syndrome is a genetic syndrome caused by the deletion of part of chromosome 22. The clinical findings vary greatly between individuals. The most common features of the syndrome can be memorised using the mnemonic ‘CATCH’:

C - congenital heart defects

A - abnormal facies

T - thymic aplasia

C - cleft palate

H - hypoparathyroidism

Individuals with an absent or asplastic thymus are susceptible to recurrent infections due to an underdeveloped immune system.