Soon after transplantation of wild-type thymi into immunodeficient mice lacking functional T cell receptors, productive T cell development in the donor thymus ceases. This observation underlies one of the central dogmas of T cell biology: because thymocytes are seemingly short-lived, intrathymic T cell development depends on continuous import of lymphoid progenitors from the bone marrow. New work reinterprets the outcome of this classical experiment as being the result of competition for intrathymic niches specifically supporting the DN3 stage of early T cell development. Surprisingly, when this niche space is uncontested by immigrating host progenitors, development of T cells in the thymus grafts continues. These new findings suggest that early thymocytes do indeed have substantial self-renewing potential
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