Left lobe of thymus

At a glance

English

Left lobe of thymus

Latin (TA2)

Lobus sinister thymi

System

Lymphoid organ

Region

Trunk

Side

Single, on the midline

About the left lobe of thymus

The thymus is a soft, pinkish-grey lymphoid organ in the upper chest, made of two lobes, left and right, joined in the midline by connective tissue rather than by a true bridge of gland. The left lobe lies behind the manubrium and upper body of the sternum, in the superior mediastinum, and often extends down into the anterior mediastinum in front of the pericardium. The two lobes are seldom equal, and either may be the larger. Each is long and narrow, with a pointed upper end that may reach into the lower neck towards the thyroid gland.

Behind the left lobe lie the left brachiocephalic vein, the arch of the aorta and its branches, and below these the pericardium over the heart. In front lie the sternum, the sternothyroid and sternohyoid muscles near the top, and the edges of the pleura and lungs. The gland is wrapped in a thin capsule from which septa pass inwards, dividing each lobe into many small lobules. Every lobule has a dark outer cortex packed with developing T lymphocytes and a paler inner medulla that contains epithelial cells and the concentric thymic (Hassall) corpuscles.

The thymus is where T lymphocytes mature. Precursor cells arrive from the bone marrow, multiply in the cortex and are tested as they move inwards: cells that cannot recognise the body's own markers, or that react too strongly against the body's own tissues, are removed, and the survivors pass into the blood. This shapes the T-cell repertoire for life. The gland is relatively largest in infancy, keeps growing in absolute size into the early teens, and then slowly involutes, its tissue being replaced by fat while some function persists.

In a child the lobe can be broad enough to widen the upper mediastinal shadow on a chest X-ray. Thymic tumours (thymomas) are associated with myasthenia gravis, and removing the gland can help that condition. Failure of the thymus to develop, as in DiGeorge syndrome, leaves a severe lack of T cells.

Function and supply

Function

Provides the setting in which T lymphocytes mature and are selected before entering the circulation; secretes thymic hormones that support this.

Nerve supply

Branches of the vagus nerves and sympathetic fibres from the cervicothoracic ganglia, mainly to its vessels.

Blood supply

Thymic branches of the internal thoracic artery, with twigs from the inferior thyroid and pericardiacophrenic arteries; thymic veins drain mostly into the left brachiocephalic vein.