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Thymus

Thymus

For medical students2 min readUpdated 2026-10-10

The thymus is a primary lymphoid organ located predominantly in the anterior mediastinum. With age, it undergoes fatty involution, changing its color from gray-pink to yellow. It has a lobular structure and is abundantly vascularized during childhood.

Inferior borderReaches the level of the 3rd rib
HistologyEpithelial network with lymphocytes and concentric corpuscles
ArteriesBranches from 4 major arterial sources (abundant in children)
In newbornsMay extend as high as the thyroid gland

Topography and Skeletotopy

The bulk of the thymus lies within the anterior mediastinum. Its superior parts are located in the superior mediastinum—posterior to the manubrium and the upper part of the body of the sternum, forming the so-called "thymic triangle".

The inferior border of the organ is clearly projected at the level of the 3rd rib. An interesting topographical feature in newborns is that the thymus may extend beyond the superior thoracic aperture, ascending along the pretracheal space all the way to the thyroid gland.

Syntopy

The thymus has close spatial relationships with major thoracic structures:

Morphology and Internal Structure

Normally, the thymic tissue has a gray-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 anterior mediastinal lymph nodes.

Externally, the thymus is covered by a thin connective tissue capsule. Trabeculae extend inward from the capsule, dividing the tissue into thymic lobules (lobuli thymi). Accessory lobules (lobuli thymici accessorii) may occasionally be present as developmental variants.

Each lobule consists of two distinct zones:

  1. Cortex (cortex thymi) — located at the periphery of the lobule.
  2. Medulla (medulla thymi) — occupies the center of the lobule.

Histologically, the tissue consists of a network of epithelial reticular cells filled with lymphocytes. A specific feature of the medulla is the presence of specialized 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 supply is derived from thymic branches (rr. thymici). A rich blood supply is characteristic of childhood. Arteries arise from the following vessels:

Venous drainage can occur via two main pathways. In the first, blood collects into a single venous trunk that empties into the left brachiocephalic vein (v. brachiocephalica sinistra). In the second, drainage occurs via smaller veins into the internal thoracic, inferior thyroid, pericardiacophrenic, and superior phrenic veins (occasionally into the jugular veins). Lymphatic drainage is directed to the mediastinal lymph nodes.

Mnemonic

To remember topography: the upper parts of the thymus hide behind the sternal manubrium, while its lower margin 'rests' precisely on the 3rd rib.

Frequently asked questions

What are the specific concentric corpuscles in the thymic medulla called eponymously?

The specific concentric corpuscles in the thymic medulla are called Hassall's corpuscles (or thymic corpuscles). They are layered epithelial structures formed by concentrically arranged, partially keratinized reticuloepithelial cells (epithelioreticulocytes). These structures can reach up to 100 µm in size. While their exact function is not fully understood, they serve as a definitive morphological marker of the thymic lobular medulla.

Into which specific regional lymph node groups does lymph drain from the thymus?

Lymph from the thymus drains into the mediastinal lymph nodes. The adipose and loose connective tissue surrounding the organ contains the anterior mediastinal lymph nodes (nodi lymphoidei mediastinales anteriores).

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 begins to form in the early second month of embryonic development. Its stromal epithelium originates from the endoderm of the 3rd pharyngeal pouch. Subsequently, the epithelial cord connecting the gland to the pharyngeal wall degenerates, and by the end of the second month, the organ is colonized by lymphoid progenitor cells.

What hormones does the thymus secrete?

The thymus secretes peptide hormones and factors produced by secretory epithelioreticular cells. These include:

  • Thymosins (primarily thymosin α1) — promote lymphocyte maturation and enhance immune function.
  • Thymopoietins.
  • Thymulin — a zinc-binding nonapeptide produced exclusively in the thymus.
  • Thymic humoral factor.
  • Parathymosin.

These factors induce the expression of mature T-lymphocyte markers and support T-cell proliferation and interleukin-2 secretion.

What vertebral or skeletal level does the inferior border of the thymus reach?

The inferior border of the thymus normally lies at the level of the 3rd rib.

Which pleura is adjacent to the thymus laterally?

Laterally, the thymus is adjacent to the mediastinal pleura, as well as the pericardium.

What is the key histological feature of the thymic medulla?

In the center of the lobule (the medulla), alongside the network of epithelial cells and lymphocytes, characteristic concentric corpuscles are found.

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