Learning Objective
By the end of this lesson, students should be able to describe the embryological origin, anatomical structure, location, and vasculature of the thymus gland, and explain its role in T-cell maturation and its clinical relevance in DiGeorge syndrome.
The thymus gland is a pink, lobulated lymphoid organ located primarily in the superior mediastinum, posterior to the manubrium of the sternum. It plays a critical role in the development of the adaptive immune system during childhood.
After puberty, the thymus gradually undergoes involution (replacement by fat).
👌Fun Fact: The thymus is relatively large in infants and children but shrinks significantly in adults.
Embryological Origin
The thymus develops from the THIRD PHARYNGEAL POUCH. It descends from the neck into the thoracic cavity during development.
Clinical Importance: Failure of development of the third pharyngeal pouch leads to thymic aplasia in DiGeorge syndrome.
Anatomical Structure
The thymus has an asymmetrical, flattened shape and is divided into lobules. Each lobule contains two regions:
Cortex
- Located peripherally
- Contains dense lymphocytes
- Supported by epithelial reticular cells
- Site of T-cell proliferation
Medulla
- Located centrally
- Contains fewer lymphocytes
- Contains Hassall’s corpuscles
Hassall’s corpuscles are concentric arrangements of epithelial reticular cells.
Note: The exact function of Hassall’s corpuscles is not fully understood.
Position
The thymus is located in the SUPERIOR MEDIASTINUM.
- Posterior to the manubrium
- Anterior to the great vessels
- May extend into the neck
- May extend into the anterior mediastinum
Vasculature
Arterial Supply
- Anterior intercostal arteries
- Branches of the internal thoracic arteries
Venous Drainage
- Left brachiocephalic vein
- Internal thoracic veins
Clinical Relevance – DiGeorge Syndrome
DiGeorge syndrome is caused by a
22q11.2 deletion.
Mnemonic: CATCH
- C – Congenital heart defects
- A – Abnormal facies
- T – Thymic aplasia
- C – Cleft palate
- H – Hypoparathyroidism
Clinical Outcome: Thymic aplasia leads to T-cell deficiency and recurrent infections.
High-Yield Summary
- Derived from the third pharyngeal pouch
- Located in the superior mediastinum
- Responsible for T-cell maturation
- Involution after puberty
- Absent in DiGeorge syndrome → T-cell immunodeficiency
