T cell outputs from the thymus

Because of the combined requirements for positive and negative selection, the total numbers of mature thymocytes exported to the peripheral lymphoid tissues is very limited. The vast majority of these cells are mature SP cells; DP cells are not exported. At present, quantitative information on the number of T cells released from the thymus is limited to mice. In this species, the rate of T cell export from the thymus amounts to about 1 % of the total cellularity of the thymus. Thus, the large thymus of 4-week-old mice, which contains about 2 x10s cells, releases in the order of 2 x 106 cells day-1 into the periphery. With the onset of thymic atrophy, thymic outputs drop dramatically. Thus, by 6 months of age the release of T cells from the thymus is reduced to about 1 x 105 day-1. Since small amounts of thymic tissue are detectable even in old age, production of new T cells from the thymus probably continues throughout life, but at a very limited rate.

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