Overview of the Thymus Gland

Thymus Anatomy

The thymus is a two-lobed structure in the upper chest cavity that partially extends into the neck. The thymus is 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: epithelial cells, lymphocytes, and Kulchitsky, 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 contains immature T lymphocytes. These cells haven't developed the ability to distinguish cells of the body from foreign cells. The medulla contains the larger, mature T lymphocytes, which 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.

Thymus Function

The thymus functions chiefly to develop T lymphocytes. Once mature, these cells leave the thymus and are transported via blood vessels to the lymph nodes and spleen. T lymphocytes are responsible for cell-mediated immunity, an immune response that involves the activation of certain immune cells to fight infection. T-cells contain proteins called T-cell receptors that populate the T-cell membrane and are capable of recognizing various types of antigens (substances that provoke an immune response). T lymphocytes differentiate into three major classes in the thymus:

• Cytotoxic T cells: Directly terminate antigens
• Helper T cells: Precipitate the production of antibodies by B-cells and also produce substances that activate other T-cells
• Regulatory T cells: Also called suppressor T cells; suppress the response of B-cells and other T-cells to antigens

The thymus produces hormone-like proteins that help T lymphocytes mature and differentiate. Some thymic hormones include thympoieitin, thymulin, thymosin, and thymic humoral factor (THF). Thympoieitin and thymulin induce differentiation in T lymphocytes and enhance T-cell function. Thymosin increases immune responses and stimulates certain pituitary gland hormones (growth hormone, luteinizing hormone, prolactin, gonadotropin-releasing hormone, and adrenocorticotropic hormone (ACTH)). Thymic humoral factor increases immune responses to viruses.

Summary

The thymus gland regulates the immune system through the development of immune cells responsible for cell-mediated immunity. In addition to immune function, the thymus also produces hormones that promote growth and maturation. Thymic hormones influence structures of the endocrine system, including the pituitary gland and adrenal glands, to assist in the growth and sexual development. The thymus and its hormones influence other organs and organ systems, including the kidneys, spleen, reproductive system, and central nervous system.

Sources

SEER Training Modules, Thymus. U. S. National Institutes of Health, National Cancer Institute. Accessed 26 June 2013 (http://training.seer.cancer.gov/)

Thymus Cancer. American Cancer Society. Updated 11/16/12 (http://www.cancer.org/cancer/thymuscancer/detailedguide/thymus-cancer-what-is-thymus-cancer)