Anatomy, Location, and Functions of the Thymus

The thymus gland sits beneath the sternum in the upper-middle section of the chest, stretching up towards the neck. The two lobes that make up the thymus can stretch from below the thyroid in the neck to as low as the fourth rib. The thymus rests on the pericardium, the membrane that surrounds the heart.

At birth, the thymus is one to two inches long, one to two inches wide, and less than a half-inch thick. A child’s thymus has a pink-gray color, is soft to the touch, and has a lobulated surface. It keeps growing until puberty, where it typically reaches a weight of 40 to 50 grams. As a person ages, the process of involution causes the thymus to decrease in size to around five grams in elderly adults.

Two lobes make up the thymus and meet in the middle. A capsule surrounds the lobes, which are both made up of two sub-components: cortex and medulla. The outer cortex is rich with cells, while the inner medulla is less dense. Thymocytes and epithelial cells account for most of the cells of the cortex. A network of so-called epithelial reticular cells supports the thymocytes. The medulla has a similar network, though it is much coarser. As the epithelial cells gather in the medulla, they form Hassall’s corpuscles.

Blood reaches the thymus from the internal thoracic artery and the superior and inferior thyroid arteries. It drains through the left innominate vein, as well as the inferior, middle, and superior thyroid veins. Several arteries enter the thymus, forming the blood-thymus barrier in the cortex. The thymus’ nerve supply comes from the vagus nerves and the sympathetic nervous system.

Every thymus is slightly different and there are a number of common variations, including

• Different lobe sizes
• Varying height between the left and right lobes
• Varied lengths in children — the organ could stretch up to the thyroid
• Thymic tissue in the thyroid
• Thymic tissue on and around the gland

Some people have ectopic thymuses: the thymus implants into other tissue, usually into the thyroid gland.

During embryonic development, the epithelium of the thymus is the first to develop. It appears as two growths, one on either side of the third pharyngeal pouch. Over time, these growths extend into the surrounding mesoderm in front of the ventral aorta. This is where the thymocytes and the epithelium meet and then join with connective tissue.

After birth, the thymus continues to grow. It is most active in infants and young children and reaches its maximum size during puberty. Fat and connective tissue begin to fill the thymus as T cell production slows. This is the process of thymic involution and it continues well into adulthood. Some experts believe that thymic involution is the result of higher sex hormone levels during puberty.

Perhaps the most important function of the thymus is its facilitation of T cell maturation. T cells start as hematopoietic precursors in the bone marrow, and they migrate to the thymus, where they are called “thymocytes.” In the thymus, the thymocytes mature and then travel to other parts of the body to help the immune system. The blood-thymus barrier provides a safe environment for T cell development by protecting against dangerous antigens that would destroy immature T cells.

Because T cells mature in the thymus, it has direct links to immunodeficiency and autoimmune diseases. Additionally, tumors may develop from the thymic epithelial cells. Known as thymomas, these tumors most often occur in people over 40. In some instances, thymomas cause the immune system to attack its own cells and tissues. Cysts can also develop in or around the thymus, though these are usually asymptomatic.

The thymus is quite difficult to operate on because of its non-uniform size and blood supply. Additionally, since the thymus becomes smaller as a person ages, it may not appear on medical imaging. Should surgeons need to remove the thymus, ultrasounds and CT scans are the most helpful tools. Doctors need to properly differentiate ectopic thymuses from tumors or similar growths.