topic 9: The definition and structures of the superior mediastinum. Topography, developmental stages and histology of the thymus.

Question 1

definition of sup. mediastinum

Answer 1

The superior mediastinum is a region of the thoracic cavity located above the plane of the sternal angle (T4-T5 vertebral level) and below the superior thoracic aperture.

Question 2

contents of sup. mediastinum

Answer 2

Thymus Gland: A primary lymphoid organ involved in the development and maturation of T lymphocytes.

Superior Vena Cava (SVC): Large vein that returns deoxygenated blood from the upper body to the right atrium of the heart.

Brachiocephalic Veins: Right and left veins formed by the confluence of the internal jugular and subclavian veins.

Arch of Aorta: Curved segment of the aorta that gives rise to major branches supplying the head, neck, and upper limbs.

Trachea: Tubular structure that connects the larynx to the bronchi, facilitating air passage to and from the lungs.

Esophagus: Muscular tube responsible for transporting food from the pharynx to the stomach.

Thoracic Duct: Largest lymphatic vessel in the body, responsible for draining lymph from the lower body and left upper body into the venous circulation.

Question 3

thymus gland & its topography

Answer 3

Topography

Location: Situated in the anterior mediastinum, extending from the level of the lower border of the thyroid gland to the upper border of the pericardium.

Shape: Bilobed structure with a variable size, typically larger in infants and children, and gradually involutes with age.

Relations: Anterior to the trachea and brachiocephalic veins, posterior to the sternum, and superior to the pericardium.

Question 4

development stages of thymus

Answer 4

Fetal Stage: The thymus begins development from the third pharyngeal pouches during embryogenesis and continues to grow until birth.

Childhood: The thymus reaches its maximum size during childhood, with active lymphopoiesis and T cell maturation.

Adulthood: In adulthood, the thymus undergoes involution, decreasing in size and activity, although it remains functionally significant throughout life.

Question 5

histology

Answer 5

Cortex: Outer region containing densely packed lymphocytes arranged in cortical lobules, where immature T cells undergo positive and negative selection processes.

Medulla: Inner region containing fewer lymphocytes and epithelial reticular cells (ERCs) that promote T cell maturation and induce self-tolerance.

Hassall’s Corpuscles: Concentrically arranged clusters of ERCs in the medulla, believed to be involved in regulatory and apoptotic processes.