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. It is worth noting the variability of the anatomical structure: sometimes an additional intercalated 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 topographical 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 ascends higher into the superior mediastinum, where it is located posterior to the manubrium and the upper part of the body of the sternum. This area is known in topographical anatomy as the thymic triangle. In newborns, the upper parts of the organ may extend beyond the superior mediastinum, reaching the thyroid gland. The lower border of the thymus is usually projected at the level of the 3rd rib cartilage.
Syntopy (relations with neighboring organs):
- Anteriorly: the gland is adjacent to the attachment site of the sternothyroid muscles.
- Posteriorly (upper parts): adjacent to the trachea.
- Posteriorly (the rest of the gland): borders the major structures of the mediastinum—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. Septa extend from this capsule deep into the organ, dividing the gland parenchyma into structural and functional units—lobules (lobuli thymi).
Each lobule has a clear zonal division:
- Cortex (cortex thymi) is 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 formation of the body's defenses. Its endocrine function consists in the production of thymosins, which are hormone-like substances.
Effects of thymosins:
- Increase in the total number of lymphocytes;
- Enhancement of immune responses;
- Participation in growth and skeletal formation processes;
- Provision of adaptive reactions of the organism.
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 tissue of the gland turns yellow due to the gradual accumulation of fat within it.
Briefly: answers to topical questions
Which vessels and nerves supply blood to and innervate the thymus?
The blood supply to the thymus is provided by thymic branches, venous outflow occurs into the unpaired thyroid plexus, and innervation is carried out by the vagus nerves, phrenic nerves, and sympathetic ganglia.
Blood supply and venous outflow:
- Thymic branches (rr. thymici) branch off from the internal thoracic artery (a. thoracica interna) and the inferior thyroid artery (a. thyroidea inferior).
- Pericardiophrenic artery (a. pericardiacophrenica) and mediastinal branches (rr. mediastinales) also participate in blood supply.
- Unpaired venous plexus (plexus thyroideus impar) collects venous blood from the organ.
Innervation:
- The vagus nerves (nn. vagi).
- The phrenic nerves (nn. phrenici).
- Cervical sympathetic ganglia give off branches to the gland.
To avoid confusing the arrangement of layers within the lobule, imagine a regular tree: the cortex (cortical substance, cortex thymi) is always on the outside, at the periphery, while the wood/core (medulla, medulla thymi) is hidden in the center.
Class and exam questions
What lobes does the thymus gland consist of?
Normally, it consists of two asymmetric lobes—right and left, connected by loose connective tissue. Intercalated lobes are less common, increasing their total number to three or four.
Where is the main mass of the thymus located?
The bulk of the organ is located in the anterior mediastinum, and its apex is situated in the superior mediastinum (posterior to the manubrium of the sternum).
What are Hassall's corpuscles and where are they located?
These are special concentric structures (corpuscula thymica). They are located in the center of the thymic lobules—in its medulla, along with epithelial cells.
Why does the thymus change color with age?
In young people, the gland is grayish-pink, but with age it turns yellow as its functional tissue is gradually replaced by adipose tissue (age-related involution).
What else to study on this topic
- Topography of the thymic triangle in the superior mediastinum
- Syntopy of the thymus with the aortic arch and brachiocephalic veins
- Mechanism of action of thymosins on lymphocyte differentiation
- Differences in thymic topography in newborns
- Histological structure of Hassall's corpuscles (corpuscula thymica)
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