LearningObjective:Describe the structure, embryologic origin, and clinical significance of the thymus, and identify related pathologies seen on imaging and in immunodeficiency disorders.
Overview
| Feature | Description |
|---|---|
| Location | Anterosuperior mediastinum |
| Function | Site of T-cell differentiation and maturation |
| Capsule | Fully encapsulated lymphoid organ |
| Mnemonic | T cells = Thymus, B cells = Bone marrow |
Embryologic Origin
| Component | Embryologic Source |
|---|---|
| Thymic epithelium | Derived from third pharyngeal pouch (endoderm) |
| Thymic lymphocytes | Derived from mesoderm |
Key Point:
The thymus has a dual embryologic origin — epithelial and lymphoid components develop separately and combine to form the mature organ.
Histology & Structure
| Region | Features |
|---|---|
| Cortex | Dense with immature T cells (thymocytes) |
| Medulla | Pale, contains mature T cells and Hassall corpuscles (epithelial reticular cells) |
Mnemonic Tip:
“Cortex = Crowded (immature), Medulla = Mature”
Radiologic Appearance
- Normal neonatal thymus: “Sail-shaped” shadow on CXR
- Involution: Begins by age 3 years, continues into adulthood
Clinical correlation:
- A visible thymic shadow in an infant is normal, not a mediastinal mass.
Clinical Relevance
| Condition | Key Feature / Association |
|---|---|
| Thymic hypoplasia / absence | Seen in DiGeorge syndrome (22q11 deletion) and SCID |
| Thymoma | Neoplasm of thymic epithelial cells |
| Associated with: Myasthenia gravis, Pure red cell aplasia, Good syndrome, Superior vena cava syndrome |
Key Points for Step 1
- Thymus = primary lymphoid organ (no afferent lymphatics)
- Site of positive and negative selection of T cells
- Dual embryologic origin is high-yield
- Thymic shadow absence suggests immunodeficiency
- Thymoma has paraneoplastic syndromes — remember myasthenia gravis
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