The thymus is a primary lymphoid organ essential for the development of T lymphocytes, which are critical for adaptive immunity. Located in the anterior superior mediastinum, the thymus is most active during childhood and undergoes involution after puberty, resulting in a reduction of thymic tissue. Despite its decreasing size in adults, the thymus continues to play a role in immune function.

Anatomy of the Thymus

The thymus is divided into two lobes, which are further subdivided into smaller lobules. These lobules are surrounded by a connective tissue capsule that extends into the thymus to form septa, separating the lobules. Each lobule consists of two distinct regions:

1. Cortex
2. Medulla

Histological Features of the Thymus

1. The Cortex

The cortex of the thymus is the outer, darker-staining region, composed mainly of densely packed immature T lymphocytes, also known as thymocytes. These thymocytes are undergoing positive selection, a process essential for the maturation of T cells.

• Cell Types:
  • Thymocytes: Immature T cells present in different stages of development.
  • Cortical Epithelial Cells: These cells form a supportive network and secrete cytokines to aid in the differentiation of thymocytes.
  • Macrophages: Located near the corticomedullary junction, they remove apoptotic thymocytes that fail positive selection.
• Key Functions:
  • Positive selection of thymocytes based on their ability to recognize self-MHC molecules.
  • Removal of thymocytes that are non-functional or autoreactive.

2. The Medulla

The medulla of the thymus is the inner, lighter-staining region that contains more mature T cells, which are undergoing negative selection to eliminate autoreactive T cells. The medulla also contains epithelial cells, but the architecture is less dense than in the cortex.

• Cell Types:
  • Medullary Epithelial Cells: These cells contribute to the negative selection process, ensuring that T cells reactive to self-antigens are eliminated.
  • Hassall’s Corpuscles: These are distinctive, concentrically arranged structures formed by epithelial cells that can become keratinized. The function of Hassall’s corpuscles is not fully understood, but they are thought to play a role in the maturation of regulatory T cells.
  • Dendritic Cells and Macrophages: These antigen-presenting cells contribute to the negative selection of T cells.
• Key Functions:
  • Negative selection of T cells that react strongly to self-antigens, ensuring tolerance to self-tissues.
  • Maturation of regulatory T cells that suppress autoreactive immune responses.

Corticomedullary Junction

This area, where the cortex and medulla meet, is critical for thymocyte migration. Immature T cells enter the thymus through the corticomedullary junction, undergo development in the cortex, and migrate to the medulla for final maturation. This junction is rich in macrophages and antigen-presenting cells, playing a key role in both positive and negative selection.

Development and Involution of the Thymus

The thymus reaches its maximum size during childhood and begins to involute after puberty. Involution results in the replacement of thymic tissue with adipose tissue. However, despite the structural changes, the thymus retains some functional activity throughout adulthood.

• Infants and Children: In children, the thymus is large and fully active, with a prominent cortex and medulla.
• Adults: The thymus decreases in size after puberty, with an increasing amount of adipose tissue replacing the functional thymic parenchyma.

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Clinical Significance

1. Thymic Involution: This normal aging process reduces the capacity for new T cell production. Although the involuted thymus retains some function, the reduced production of naïve T cells has implications for immune aging and decreased immunity in older adults.
2. Thymic Hyperplasia: In certain autoimmune conditions like myasthenia gravis, the thymus can become enlarged, containing increased numbers of germinal centers within the medulla. Thymic hyperplasia is associated with immune dysregulation.
3. Thymomas: These are neoplasms of thymic epithelial cells and can range from benign to malignant. Thymomas are often associated with autoimmune disorders and can have significant clinical impact, including paraneoplastic syndromes.
4. DiGeorge Syndrome: A congenital condition where the thymus is absent or underdeveloped, leading to profound immunodeficiency due to the lack of T cells.

TYPES OF THYMOMA – ACCORDING TO WHO 2021

APPROACH BASED COURSE- ADVANCED HISTOPATHOLOGY AND CYTOLOGY