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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 ...
Thymocytes are classified into a number of distinct maturational stages based on the expression of cell surface markers. The earliest thymocyte stage is the double negative stage (negative for both CD4 and CD8), which more recently has been better described as Lineage-negative, and which can be divided into four substages.
Thymus stromal cells are subsets of specialized cells located in different areas of the thymus. [1] They include all non-T-lineage cells, such as thymic epithelial cells (TECs), endothelial cells, mesenchymal cells, dendritic cells, and B lymphocytes, and provide signals essential for thymocyte development and the homeostasis of the thymic stroma.
The hypothalamus, pancreas, and thymus also function as endocrine glands, among other functions. (The hypothalamus and pituitary glands are organs of the neuroendocrine system . One of the most important functions of the hypothalamus—it is located in the brain adjacent to the pituitary gland—is to link the endocrine system to the nervous ...
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.
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 ).
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.