How it works
The first drawing shows where the thymus sits: in the upper chest, behind the sternum and in front of the top of the heart. The second enlarges its two lobes and cuts one open.
Read the full guide
Find it in the chest
Numbers 1 to 4 place the thymus among its neighbors: the trachea behind its upper end, the heart below it and a lung on each side. The sternum, which lies in front of the thymus, is left out so that it can be seen.
Then look inside
Numbers 5 to 10 are on the enlarged thymus. The whole lobe shows the lobules on its surface. The cut lobe shows the capsule and, in each lobule, a cortex round a medulla. The round inset enlarges a piece of cortex.
- Cortex is the outer part of each lobule and medulla the inner part.
Questions and answers
Where is the thymus located?
In the upper front part of the chest, behind the sternum and in front of the heart, in the space between the sternum and the aorta. Its upper end reaches toward the neck, in front of the trachea.
What are the parts of the thymus?
Two lobes wrapped in a connective tissue capsule. Extensions of the capsule, called trabeculae or septa, divide each lobe into lobules. Each lobule has a densely packed outer cortex and a less dense inner medulla.
What is the difference between the cortex and the medulla of the thymus?
The cortex is the outer region of a lobule, densely packed with thymocytes. The medulla is the inner region, with fewer thymocytes. Thymocytes move into the medulla before they leave the thymus.
What are thymic corpuscles?
Concentric, nest-like bodies in the medulla, formed by clusters of epithelial cells. They are also called Hassall’s corpuscles.
What does the thymus do?
It is where T cells mature. Immature T cells, called thymocytes, develop in the thymus before they leave it. The thymus grows until puberty and then slowly shrinks.