The thymus is located between the lobes of the lungs in the upper chest cavity.
It is a double-lobed lymphoid organ located between the sternum and the heart and well supplied with blood vessels.
In a newborn infant the thymus is relatively huge and often extends from the base of the neck to the upper border of the heart. As the child grows, the thymus continues to enlarge slowly.
Reaching its maximum size just before puberty, the thymus gland begins to atrophy, starting ~20, gradually decreasing in size. In adults, its largest weight is about 37 g and only 6 g in 75-year-olds.
Chronic involution of the thymus gland is thought to be one of the major contributing factors to loss of immune function with increasing age.
In adults, lymphocytes continue to be produced by the spleen and lymph glands.
Thymus gland structure and function
The thymus has two lobes and each lobe is divided into a central medulla and a peripheral cortex.
The thymus is a crucial component that is responsible for maintaining proper immune system function in the body and especially active in young children.
The thymus secretes hormones that regulate the immune system.
T cell (T-lymphocyte, white blood cell) that develops in the bone marrow, matures in the thymus, and expresses what appear to be antibody molecules on its surface.
Quality control mechanisms built into the thymus and other lymphoid organs weed out T cells and B cells with receptors that recognize one’s own body proteins. This mechanism prevents the development of autoimmune diseases.
Thymic hormones, in general, inhibit the changes provoked by aging, and helps to maintain the learning and memory capacities in the aging organism.
The two – lymphoid and endocrine – parts (function) of the thymus are in close connection directly or with the help of mediators.
The state and function of the organ is influenced by sympathetic and parasympathetic nerves as well as by the hormones of endocrine glands.
Outer cortex has immature T cells.
Immature T cells:
- screened for functional capacity
- stimulated to proliferation
- stimulated to maturation
Functions of inner medulla:
- to degenerate defective T cells
- to move mature T cells into blood
Thymus produces paracrine hormones, which are participating in the regulation of immune cell transformation and selection.
It also synthesizes hormones similar to the other endocrine glands such as:
They are transported by the immune cells to the sites of requests (packed transport).
Thymic (epithelial and immune) cells also have receptors for hormones which regulate them.
There are 4 well defined thymic paracrine peptides:
- thymosin alpha 1,
- thymic humoral factor.
These thymic hormones are synthesized and secreted by the epithelial cells.
The epithelial cells also synthesizes such cytokines as IL-1 and 6, G-CSF, GM-CSF, which also play a role in thymocyte differentiation.
Thymosin hormone function
The thymus produces several hormones, collectively known as thymosins.
Thymosins were prepared in 1966 by Goldstein and White from a calf thymus, as functionally and biochemically distinct polypeptides called thymosin fraction 5 (TF5).
Thymosin alpha 1 was isolated and purified from a complex mixture of polypeptides and shown to be secreted by subcapsular, single or grouped epithelial cells.
Thymosin alpha 1 has a pleiotropic action, enhancing T cell, dendritic cell, and antibody responses and inhibits steroid induced apoptosis in thymus.
TF5, thymosin alpha 1, and thymosin beta influence the lymphoid elements of the thymus.
Other thymosin fractions were also isolated, such as:
- polypeptide beta 1,
- prothymosin alpha,
- thymosin beta 4.
However, they were not confirmed to be real thymic hormones because other organs or cells synthesized them and did not influence thymus specific functions.
Thymulin, first extracted from porcine thymus, is exclusively synthesized by the thymic epithelial cells.
For its action, it is necessary the presence of zinc. It acts to the T cell differentiation.
Thymulin is able to induce T-cell markers and normalizes the ratio between the helper and the suppressor T-cells.
The pentapeptide is produced by the epithelial cells of the thymus and is participating in the immune regulation as well as T cell differentiation and influences neuromuscular transmission.
It was primarily recognised for its detrimental effects on neuromuscular transmission. Thymopoietin bind with high affinity to the acetylcholine binding region of the acetylcholine receptor.
Thymic humoral factor (THF)
THF is essential for induction of clonal expansion, differentiation and maturation of T cell subsets. It also enhances most T cell functions.
- Csaba G. The Immunoendocrine Thymus as a Pacemaker of Lifespan. Acta microbiologica et immunologica Hungarica. 2016; 63(2):139-58. [pubmed]
- Kendall MD. Functional anatomy of the thymic microenvironment. Journal of anatomy. 1991; 177:1-29. [pubmed]