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Anatomy · Organs & viscera

Thymus

Largest in childhood, shrinks after puberty

Anterior mediastinum · lymphatic system

The thymus is a soft, two-lobed lymphoid organ lying in the upper front part of the chest, in the anterior mediastinum behind the sternum and in front of the great vessels and pericardium. It is where T lymphocytes mature; the T stands for thymus. It is relatively largest in infancy, reaches its greatest absolute weight around puberty and then shrinks (involutes), gradually turning to fat.

How to spot them

  • ▸A pinkish-grey two-lobed organ behind the upper sternum, in front of the aortic arch, the great veins and the top of the heart
  • ▸Each lobe is made of many lobules with an outer cortex and inner medulla, and the medulla holds Hassall's corpuscles
  • ▸Prominent in a child's chest, sometimes visible on an infant's X-ray as a 'sail sign'; replaced by fat in adults
  • ▸Its upper poles may extend into the neck, towards the thyroid gland

What to know

A two-lobed organ behind the top of the breastbone, large in a baby and mostly replaced by fat in an adult

Thymus — it involutes after puberty.

The organ where lymphocytes mature before being released, giving their letter to the name 'T cells'

The thymus, where T lymphocytes develop.

Concentric whorls of flattened epithelial cells in its inner zone, named after a British physician

Hassall's corpuscles, in the medulla of the thymus.

Its absence at birth, together with lack of the parathyroids, occurs in a chromosome 22 deletion syndrome

DiGeorge syndrome — failure of development of the third and fourth pharyngeal pouches, giving thymic aplasia.

The organ sometimes removed to help patients with myasthenia gravis, and the site of a tumour that may accompany that disease

The thymus, and thymoma.

On an infant's chest film it can produce a triangular shadow like a ship's sail

The thymic 'sail sign'.

Together with the pancreas it is sold in butchers as 'sweetbread'

The thymus (calf's sweetbread).

Key ideas

  • In the cortex of the thymus, developing T cells that fail to recognise the body's own MHC molecules die, and those that react too strongly against self are eliminated, so the T-cell repertoire is 'educated'.
  • The thymus involutes with age, and the production of new T cells declines, one reason immune responses weaken in the elderly.

Quick-fire facts

  • •It lies in the anterior superior mediastinum, in front of the aortic arch, the left brachiocephalic vein and the trachea, and behind the manubrium.
  • •It weighs about 10 to 15 g at birth and reaches about 30 to 40 g at puberty, then shrinks to a few grams of thymic tissue within fat.
  • •Its blood supply comes mainly from the inferior thyroid and internal thoracic arteries.
  • •Each lobe has a cortex packed with immature lymphocytes and a paler medulla with Hassall's corpuscles.
  • •The organ derives from the third pharyngeal pouch, which is why it can be found in the neck.
  • •Jacques Miller showed in 1961 that removing the thymus of newborn mice crippled their immune systems, revealing its role.
  • •The name may come from the Greek thymos, variously 'soul', 'life force' or the thyme-flower-like bud.

Drill it — 8 cards

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Sources