Overview
The thymus gland is an asymmetrical, flat, bi-lobed lymphoid organ located posterior to the manubrium in the superior mediastinum. 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). In the adolescent, it is involved in the development of the immune system and involutes following puberty, slowly becoming replaced by adipose tissue as it becomes functionally less active.
Structure
The thymus lobules are composed of a series of follicles, which have a medullary and cortical component:
- Cortical portion – Located peripherally within each follicle. It is largely composed of lymphocytes, supported epithelial reticular cells.
- 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.
Arterial Supply
- Inferior thyroid – branch of the thyrocervical trunk, derived from first part of the subclavian artery
- Internal thoracic artery – direct branch of the first part of the subclavian artery
- Pericardiacophrenic – branch of the internal thoracic artery
- Anterior intercostal arteries – branch of the internal thoracic artery
Venous Drainage
- Drained by left brachiocephalic and internal thoracic tributary veins
- Thymic venous supply runs in the interlobular septa, into the thymic capsule, and leaves the cortex via a plexus on the posterior side of the organ. These veins then join together and drain each lobe separately
Innervation
- Sympathetic – superior cervical and stellate ganglion. These fibres travel in a plexus along large blood vessels until they enter the thymic capsule.
- Parasympathetic – vagus nerve, recurrent laryngeal nerve and phrenic nerve
Relations
Posterior
Anatomical Variants
- Extension to thyroid gland
- Ectopic thymus – occurs during descent of the thymus, typically located in the anterior cervical region, deep to the sternocleidomastoid muscle and anterior to the carotid sheath. It can expand into the retropharyngeal space.
- Variability in arterial supply:
- Middle thyroid artery
- Accessory thymic arteries
- Posterior thymic arteries derived from the brachiocephalic artery and aorta