Anterior Mediastinum

The anterior mediastinum is the most anterior subdivision of the inferior mediastinum. It lies between the sternum and the fibrous pericardium and is the smallest compartment of the mediastinal cavity. Although limited in size and content, it serves as a connective and lymphatic space and holds important structural relevance, particularly in pediatric anatomy.

Location and Boundaries

The anterior mediastinum is a narrow triangular space positioned in the thoracic cavity. It lies anterior to the pericardium and posterior to the sternum, extending vertically between the superior thoracic aperture and the diaphragm.

• Anterior boundary: Body of the sternum and transversus thoracis muscles
• Posterior boundary: Fibrous pericardium covering the heart
• Superior boundary: Continuous with the superior mediastinum at the level of the sternal angle (T4/T5 vertebral level)
• Inferior boundary: Diaphragm
• Lateral boundaries: Mediastinal pleura of the lungs

This space is more prominent in children due to the large size of the thymus, which occupies much of the anterior mediastinum during early development. In adults, the space becomes smaller as the thymus regresses and is replaced by fatty and fibrous tissue.

Contents of the Anterior Mediastinum

The anterior mediastinum contains relatively few vital structures in adults, but its contents include connective, lymphatic, and residual embryological elements. The main components include:

• Loose areolar connective tissue – fills most of the space, providing structural support and cushioning
• Fat – especially prominent in adults, as the thymus undergoes involution after puberty
• Inferior portion of the thymus – remnants may still be present in the anterior mediastinum in adults
• Anterior mediastinal lymph nodes – involved in drainage of the anterior thoracic wall and pericardium
• Small branches of the internal thoracic (mammary) arteries – such as pericardiacophrenic and mediastinal branches
• Sternopericardial ligaments – fibrous bands anchoring the pericardium to the sternum

In infants and children, the anterior mediastinum plays a greater role due to the presence of the thymus gland, which occupies much of the space and functions in T-cell maturation. In adults, this gland involutes and becomes a small fibrofatty structure.

Thymus in the Anterior Mediastinum

The thymus is a bilobed lymphoid organ that extends from the root of the neck into the anterior and superior mediastinum. In newborns and children, it is large and plays a critical role in the immune system by producing T-lymphocytes. With age, it gradually atrophies and is replaced by adipose tissue, although remnants persist into adulthood.

In the anterior mediastinum, the thymus lies posterior to the manubrium and anterior to the pericardium. It may also extend superiorly into the lower neck, anterior to the brachiocephalic veins and great vessels. Its position is variable and may affect imaging and surgical approaches to the anterior thorax.

Lymphatic Structures

The anterior mediastinal lymph nodes are small and scattered within the loose connective tissue. They receive lymphatic drainage from:

• Anterior part of the pericardium
• Diaphragm (especially superior surface)
• Anterior thoracic wall including internal thoracic vessels

These nodes may communicate with parasternal lymph nodes and are part of the broader mediastinal lymphatic network. Though small and inconspicuous in normal anatomy, they can become prominent in pathological conditions.

Vascular Structures

While major arteries do not course through the anterior mediastinum, small branches of the internal thoracic arteries (which descend along the posterior aspect of the anterior chest wall) send off minor mediastinal and pericardial branches into this region. These vessels are usually not visible on imaging unless enlarged or involved in disease.

The internal thoracic artery, itself a branch of the subclavian artery, gives rise to the following vessels that may have relevance to the anterior mediastinum:

• Pericardiacophrenic artery – travels with the phrenic nerve and supplies the pericardium and diaphragm
• Mediastinal branches – supply the thymus, connective tissue, and surrounding fascia

Ligamentous Structures

The sternopericardial ligaments are fibrous bands that extend from the posterior surface of the sternum to the anterior surface of the fibrous pericardium. These ligaments help anchor the pericardium and heart within the thoracic cavity and provide mechanical stability during movement and respiration.

Topographical Relationships

• Anteriorly: Sternum, costal cartilages of ribs 1 to 7, and transversus thoracis muscle
• Posteriorly: Fibrous pericardium enclosing the heart
• Superiorly: Continuous with the superior mediastinum, where the thymus and vessels course into the neck
• Inferiorly: Diaphragmatic surface forming the floor of the anterior mediastinum

The anterior mediastinum is narrow in adults due to the expansion of the lungs, which meet in the midline and displace the mediastinal pleura laterally. It contains no major nerves or airways but remains an important landmark area in thoracic imaging and dissection.

Developmental Considerations

In the fetus and neonate, the anterior mediastinum is dominated by the large thymus gland, which may extend into the superior mediastinum and cervical region. During development, the thymus originates from the third pharyngeal pouch and descends into the anterior mediastinum by the 6th week of gestation.

Postnatally, the gland is active during childhood and reaches its maximum size at puberty. Following this, it undergoes involution and is replaced by adipose and fibrous tissue, leaving only residual thymic tissue in the adult anterior mediastinum.

Size and Shape

The anterior mediastinum is narrowest at its upper portion, where it lies deep to the manubrium and in front of the great vessels. It widens slightly as it descends toward the diaphragm but remains limited in size. On cross-sectional imaging (e.g., CT scans), it appears as a thin triangular space between the sternum and heart/pericardium.

In surgical or diagnostic approaches (such as mediastinoscopy or thoracotomy), the anterior mediastinum is accessed less frequently than other compartments due to its small size and relative lack of major structures. However, its proximity to the sternum makes it relevant in sternotomy procedures and thymectomy operations.