External Structure and Shape
Morphologically, the thymus is an asymmetric, elongated organ. In most cases, it consists of two closely apposed parts:
- Right lobe (lobus dexter);
- Left lobe (lobus sinister).
Both lobes are connected by a layer of loose connective tissue. The anterior surface of the organ is convex, while the posterior surface is concave. Anatomical variations exist: sometimes an additional intermediate part is located between the main lobes, and in rare cases, the gland may consist of three or even four lobes.
Topography and Syntopy
The topographic anatomy of the thymus is closely related to the fascial spaces of the thoracic cavity. The bulk of the organ lies in the anterior mediastinum.
The apex of the gland extends higher into the superior mediastinum, where it lies posterior to the manubrium and the upper part of the body of the sternum. This area is known in topographic anatomy as the thymic triangle. In newborns, the upper parts of the organ can extend beyond the superior mediastinum, reaching the thyroid gland. The inferior border of the thymus is usually projected at the level of the cartilage of the 3rd rib.
Syntopy (relations with neighboring organs):
- Anteriorly: The gland lies against the attachment site of the sternothyroid muscles.
- Posteriorly (upper parts): Adjacent to the trachea.
- Posteriorly (remainder): Borders major mediastinal structures—the superior vena cava, brachiocephalic veins, aortic arch and its branches, as well as the pericardium and mediastinal pleura.
Internal Structure (Microanatomy)
Externally, the thymus is covered by a thin connective tissue capsule. Trabeculae extend from this capsule deep into the organ, dividing the parenchyma into structural and functional units called lobules (lobuli thymi).
Each lobule has a clear zonation:
- Cortex (cortex thymi) — located at the periphery of the lobule. This zone is extremely rich in lymphocytes.
- Medulla (medulla thymi) — occupies the central part of the lobule. It contains epithelial cells and specific concentric structures known as thymic corpuscles (corpuscula thymica, or Hassall's corpuscles).
Functions and Age-Related Features
As a primary lymphoid organ, the thymus plays a key role in the development of the body's immune defenses. Its endocrine function involves the production of thymosins, which are hormone-like substances.
Effects of thymosins:
- Increase in the total lymphocyte count;
- Enhancement of immune responses;
- Participation in growth and skeletal development;
- Facilitation of adaptive responses of the organism.
臓 The color of the gland changes throughout life. Initially, it has a grayish-pink hue, but with age, a natural process of involution occurs: the thymic tissue turns yellow due to the gradual accumulation of adipose tissue.