U01.02.005 Thymus

The thymus is a primary lymphoid organ crucial for T-cell development. It plays a central role in immune function, particularly in the differentiation and maturation of T-cells, which are essential for adaptive immunity.

Anatomy and Location

Location: The thymus is located in the anterosuperior mediastinum.
Structure:
- It is encapsulated.
- The thymic epithelium is derived from the third pharyngeal pouch (endoderm).
- The thymic lymphocytes originate from the mesoderm.
- Cortex: Dense with immature T-cells.
- Medulla: Contains mature T-cells and Hassall’s corpuscles, which are composed of epithelial reticular cells.

Development and Involution

The thymus is fully developed at birth, showing a “sail-shaped” appearance on neonatal chest X-rays (CXR).
It begins to involute by age 3, with continued atrophy throughout adulthood.

Function

T-cell Development: The thymus is essential for the maturation of T-cells. Immature T-cells enter the thymus and undergo maturation in the cortex, while fully mature T-cells are found in the medulla.
T-cells vs. B-cells:
- T-cells: Mature in the thymus.
- B-cells: Mature in the bone marrow.

Clinical Significance

Immunodeficiencies:
- Absent or hypoplastic thymus: Seen in certain immunodeficiencies, such as Severe Combined Immunodeficiency (SCID) and DiGeorge syndrome.
Thymoma:
- A thymoma is a neoplasm of the thymus.
- Associated conditions:
  - Myasthenia gravis.
  - Superior vena cava syndrome.
  - Pure red cell aplasia.
  - Good syndrome.

Table: Summary of Thymus Features

Feature	Description
Location	Anterosuperior mediastinum
Origin	Epithelium from 3rd pharyngeal pouch (endoderm); lymphocytes from mesoderm
Cortex	Dense with immature T-cells
Medulla	Contains mature T-cells and Hassall corpuscles
Neonatal appearance	Sail-shaped on chest X-ray
Involution	Begins by age 3
Associated disorders	Thymoma (associated with myasthenia gravis, superior vena cava syndrome, pure red cell aplasia, Good syndrome), immunodeficiencies like SCID, DiGeorge syndrome

Table: Summary of Thymus Features

Points to Remember

The thymus is essential for T-cell maturation.
It is derived from the third pharyngeal pouch (endoderm) and mesoderm.
Thymic atrophy starts around age 3.
Thymomas are associated with various clinical syndromes, including myasthenia gravis and superior vena cava syndrome.
Absence or hypoplasia of the thymus is seen in immunodeficiencies like SCID and DiGeorge syndrome.

Bibliography

Abbas, A. K., Lichtman, A. H., & Pillai, S. (2018). Basic Immunology: Functions and Disorders of the Immune System. Elsevier.
Janeway, C. A., Travers, P., Walport, M., & Shlomchik, M. J. (2001). Immunobiology: The Immune System in Health and Disease. Garland Science.
Parham, P. (2020). The Immune System. Garland Science.

USMLE Step 1 Review Series

Curriculum

U01.02.005 Thymus

Anatomy and Location

Development and Involution

Function

Clinical Significance

Table: Summary of Thymus Features

Points to Remember

Bibliography

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