Topography and Skeletotopy
The bulk of the thymus lies in the anterior mediastinum. The upper parts of the organ are located in the superior mediastinum—they lie posterior to the manubrium and the upper part of the body of the sternum, forming the so-called "thymic triangle".
The lower border of the organ clearly projects to the level of the 3rd rib. An interesting topographic feature is observed in newborns: their thymus may extend beyond the sternum, ascending along the pretracheal space up to the thyroid gland.
Syntopy of the organ
The thymus has close spatial relationships with major thoracic organs:
- Posteriorly (in the upper parts): adjacent to the trachea.
- Anteriorly (in the upper parts): this is the attachment site of the sternothyroid muscles.
- Posteriorly (remaining parts): the parenchyma is closely adjacent to major vessels. These include the superior vena cava, brachiocephalic veins, as well as the aortic arch and its branches.
- Laterally: the organ is bounded by the pericardium and mediastinal pleura.
Morphology and internal structure
Normally, the gland tissue has a grayish-pink color, but with age it undergoes fatty degeneration and turns yellow. The organ is surrounded by a layer of adipose and loose connective tissue containing the anterior mediastinal lymph nodes.
Externally, the thymus is covered by a thin connective tissue capsule. Septa extend inward from it, dividing the tissue into thymic lobules (lobuli thymi). Accessory lobules (lobuli thymici accessorii) may occur as a developmental variant.
Each lobule consists of two zones:
- Cortex (cortex thymi) — located at the periphery of the lobule.
- Medulla (medulla thymi) — occupies the center of the lobule.
Histologically, the tissue is a network of epitheliocytes filled with lymphocytes. A specific feature of the medulla is the presence of special concentric corpuscles.
Blood supply and innervation
Innervation of the thymus is provided by branches of the vagus nerves (nn. vagi), cervical sympathetic ganglia, and phrenic nerves (nn. phrenici).
Arterial blood supply is carried out by thymic branches (rr. thymici). Childhood is characterized by an abundance of blood sources. Arteries branch from the following vessels:
- Internal thoracic artery (a. thoracica interna)
- Inferior thyroid artery (a. thyroidea inferior)
- Pericardiacophrenic artery (a. pericardiacophrenica)
- Superior phrenic artery (a. phrenica superior)
Venous outflow can occur via two pathways. In the first case, blood collects into a single venous trunk that empties into the left brachiocephalic vein (v. brachiocephalica sinistra). In the second case, outflow occurs via small veins into the system of internal thoracic, inferior thyroid, pericardiacophrenic, and superior phrenic veins (sometimes into the jugular veins). Lymphatic drainage is directed to the mediastinal lymph nodes.
Briefly: answers to topical questions
What are the eponyms for the specific concentric corpuscles in the medulla of the thymus?
The specific concentric corpuscles in the medulla of the thymus are called Hassall's corpuscles. They are layered epithelial structures formed by concentric layers of partially keratinized reticuloepitheliocytes (epithelioid reticular cells) overlapping each other. The size of these structures can reach up to 100 µm. The function of Hassall's corpuscles is not yet fully understood, but they serve as an absolute morphological marker of the medulla of thymic lobules.
To which specific groups of regional lymph nodes does lymph drain from the thymus?
Lymph from the thymus drains into the mediastinal lymph nodes. Anterior mediastinal lymph nodes (nodi lymphoidei mediastinales anteriores) are located in the adipose and loose connective tissue surrounding the organ, although a more detailed subdivision into specific regional drainage groups for the thymus is not provided in the materials.
From which pharyngeal pouches does the thymus develop during embryogenesis?
During embryogenesis, the thymus develops from the epithelium of the 3rd and 4th pharyngeal pouches. The organ is laid down at the beginning of the second month of embryonic development as a branchiogenic exocrine gland. Its development originates from the endoderm (the region of the 3rd pharyngeal pouch), which forms the stromal epithelium of the thymus. Subsequently, the cord connecting the gland to the epithelium of the pharyngeal pouches undergoes regression, and by the end of the second month, the organ is populated by lymphocytes.
What hormones does the thymus secrete?
The thymus secretes peptide hormones and factors produced by secretory epithelioreticular cells. These include:
- Thymosins (primarily alpha-1-thymosin) — increase the number of lymphocytes and enhance immunity.
- Thymopoietins.
- Thymulin — a Zn-binding nonapeptide produced exclusively in the thymus.
- Thymic serum factor (thymic humoral factor) — a complex of thymic peptides.
- Parathymosin.
These factors induce the expression of mature T-lymphocyte markers and support the ability of T-lymphocytes to secrete interleukin-2 and proliferate.
To remember the topography: the upper parts of the thymus hide behind the manubrium of the sternum, while the lower edge "rests" precisely on the 3rd rib.
Class and exam questions
To what level does the lower border of the thymus reach?
The lower border of the thymus is normally located at the level of the 3rd rib.
Which pleura is adjacent to the thymus laterally?
Laterally, the mediastinal pleura and pericardium are adjacent to the thymus.
What is the histological feature of the medulla?
In the center of the lobule (medulla), in addition to the network of epitheliocytes with lymphocytes, there are specific concentric corpuscles.
What else to study on this topic
- Age-related involution of the thymus and the process of fatty degeneration
- Variant anatomy: accessory lobules of the thymus
- Anatomy and syntopy of the spleen (gastric, renal, and colic surfaces)
- Skeletotopy of the spleen and the localization of its hilum relative to the peritoneum
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