In this note, we are going to discuss the anatomy and function of the thymus.
The thymus is a primary lymphoid organ, located in the mediastinum, behind the sternum.
It is responsible for producing T-cells by maturing thymocytes. Embryologically, it is derived from the third pharyngeal pouch. The thymus is most active during childhood, but after puberty, it gradually decreases in size and is replaced by adipose tissue.
The thymus is located within the superior mediastinum and the anterior part of the inferior mediastinum, extends between the thyroid gland superiorly and the fourth costal cartilage inferiorly.
It is composed of two lobes (left and right) connected by an isthmus.
Enclosed by a connective tissue capsule, the thymus is divided into lobules by penetrating septa. Histologically, each lobule has a peripheral dark zone known as the cortex and a central lighter zone called the medulla.
The cortex contains numerous T-lymphocyte precursors, known as thymocytes, which have migrated from the bone marrow. The cortex is where the initial stages of thymocyte development take place. It also houses epithelial reticular cells and macrophages.
The medulla, located centrally within each lobule, contains fewer lymphocytes compared to the cortex but has a higher concentration of epithelial cells.
One of the key functions of the epithelial cells is their role in forming the blood-thymus barrier. This selective barrier separates the cortex from the bloodstream, preventing exposure of lymphoblasts to circulating antigens.
Additionally, the medulla contains structures known as Hassall’s corpuscles, which are concentric arrangements of epithelial reticular cells.
Thymocytes eventually undergo positive and negative selection. Once this process is complete, the mature T-cells enter the circulation and become a crucial part of the adaptive immune system.
Thymus
- Lymphatic system
- systema lymphaticum
- Lymphatic capillaries
- vasa lymphocapillaria
- Lymphatic plexus
- plexus lymphaticus
- Superficial lymphatic vessels
- vasa lymphatica superficialia
- Deep lymphatic vessels
- vasa lymphatica profunda
- Lymphatic valves
- valvulae lymphaticae
- Lymph nodes
- nodi lymphatici
- Thymus
- thymus
