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The thymus (pl.: thymuses or thymi) is a specialized primary lymphoid organ of the immune system.Within the thymus, thymus cell lymphocytes or T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders.
The final anatomical location of the thymic gland is reached at 6 weeks in the fetus. TECs originate from non-hematopoietic cells that are characterized by negative expression of CD45 and positive expression of EpCAM. Then TECs are divided into two phenotypically and functionally different groups that have distinct location, cytokeratin ...
The thymus contributes fewer cells as a person ages. As the thymus shrinks by about 3% [ 15 ] a year throughout middle age, a corresponding fall in the thymic production of naive T cells occurs, leaving peripheral T cell expansion and regeneration to play a greater role in protecting older people.
Molecules known to be important for thymus entry include P-selectin (CD62P) and the chemokine receptors CCR7 and CCR9. [5] Following thymus entry, progenitors proliferate to generate the ETP population. This step is followed by the generation of DN2 thymocytes which migrate from the cortico-medullary junction toward the thymus capsule.
Known as "Thymosin Fraction 5", this was able to restore some aspects of immune function in animals lacking thymus gland. Fraction 5 was found to contain over 40 small peptides (molecular weights ranging from 1000 to 15,000 Da.), [ 4 ] which were named "thymosins" and classified as α, β and γ thymosins on the basis of their behaviour in an ...
The blood–thymus barrier regulates exchange of substances between the circulatory system and thymus, providing a sequestered environment for immature T cells to develop. The barrier also prevents the immature T cells from contacting foreign antigens (since contact with antigens at this stage will cause the T cells to die by apoptosis ).
The process of removing auto-reactive T lymphocytes occurs in the thymus. [2] [8] [12] The thymus contains two zones: the outer region called the thymic cortex, and the inner region called the thymic medulla. Within these regions T lymphocytes will undergo a series of positive or negative selection. [12] [13]
Stage One: Thymus Migration Multi-potent lymphoid progenitors (MLP) enter the T cell pathway as they immigrate to the thymus. The most primitive cells in the thymus are the early thymocyte progenitors (ETP), which retain all lymphoid and myeloid potential but exist only transiently, rapidly differentiating into T and NK lineages.