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Structure

Thymus

Not medically reviewed · Content last updated · 1 min read ·

This is the date the draft was written, not a medical review date.

The thymus is a primary lymphoid organ in the anterior superior mediastinum where T lymphocytes mature. It is largest relative to body size in childhood and involutes after puberty, being progressively replaced by fat.

Terminologia Anatomica (Latin): Thymus · Open in the 3D atlas

Location

Behind the manubrium and upper body of the sternum, in front of the great vessels (left brachiocephalic vein, aortic arch) and the pericardium; it may extend upward into the neck.

Structure

Two asymmetrical lobes, each divided into lobules with a dense cortex of immature thymocytes and a paler medulla containing epithelial reticular cells and Hassall corpuscles. A blood-thymus barrier protects developing cells in the cortex.

Blood supply

Arterial supply is from branches of the inferior thyroid and internal thoracic arteries (including anterior mediastinal branches). Veins drain to the left brachiocephalic, internal thoracic and inferior thyroid veins.

Innervation

Mainly sympathetic fibres from the cervical ganglia and branches of the vagus nerve, with minor twigs from the phrenic nerve to the capsule; the nerves are largely vasomotor.

Function

Precursors from bone marrow are educated here: T cells undergo positive selection for self-MHC recognition and negative selection against strongly self-reactive receptors, then leave as naive CD4 or CD8 cells. The thymus is essential for establishing T-cell immunity in early life.

Clinical correlations

Thymic aplasia in DiGeorge syndrome (22q11.2 deletion, third and fourth pharyngeal pouch anomaly) causes T-cell deficiency. Thymoma, an anterior mediastinal mass, is associated with myasthenia gravis, and thymectomy is part of its management in selected patients. Anterior mediastinal masses also include lymphoma, germ-cell tumours and retrosternal thyroid.

Common exam points

  • Anterior superior mediastinum; large in children and fatty in adults.
  • Derived from the third pharyngeal pouch (with the inferior parathyroid glands).
  • Site of T-cell maturation and selection; cortex immature, medulla mature with Hassall corpuscles.
  • Thymoma is associated with myasthenia gravis; DiGeorge syndrome gives T-cell deficiency.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease. Elsevier, 10th edition (2020) — Tier 3 (textbook)

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