The thymus is a two-lobed structure that is positioned in the upper chest cavity. It partially extends into the neck region. The thymus is situated above the pericardium of the heart, in front of the aorta, between the lungs, below the thyroid, and behind the breastbone. The thymus has a thin outer covering called a capsule and consists of three types of cells. Thymic cell types include epithelial cells, lymphocytes, and Kulchitsky cells, or neuroendocrine cells.
- Epithelial cells – tightly packed cells that give shape and structure to the thymus.
- Lymphocytes – immune cells that protect against infection and stimulate an immune response.
- Kulchitsky cells – hormone-releasing cells.
Each lobe of the thymus contains many smaller divisions called lobules. A lobule consists of an inner area called the medulla and an outer region called the cortex. The cortex region contains immature T lymphocytes. These cells have not yet developed the ability to distinguish cells of the body from foreign cells. The medulla region contains the larger, mature T-lymphocytes. These cells have the ability to identify self and have differentiated into specialized T lymphocytes. While T lymphocytes mature in the thymus, they originate from bone marrow stem cells. Immature T-cells migrate from the bone marrow to the thymus via the blood. The “T ” in T lymphocyte stands for thymus-derived.