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An immature or developing lymphocyte in the thymus that is the forerunner of a T cell. Progenitor cells enter the thymus from bone marrow to become thymocytes, which proliferate and undergo distinct developmental stages marked by changes in their CD surface proteins (see CD). During maturation they express on their surface a precursor T-cell receptor (pre-T-cell receptor) and both CD8 and CD4 proteins (double-positive cells). At the same time they slowly migrate from the outer cortex of the thymus to the inner medullary region. Some 98% of thymocytes express surface receptors that fail to recognize self antigen, and these cells undergo apoptosis and die; only the 2% of cells capable of distinguishing self antigen survive to maturity in the medulla. Here they become single-positive cells, expressing exclusively CD4 or CD8 surface proteins, and are exported from the thymus in the bloodstream to become helper T cells and cytotoxic T cells, respectively.

Subjects: Biological Sciences.

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