Bone marrow cells give rise to distinct cell clones within the thymus

Abstract

The thymus is the major, if not the sole site of maturation of T lymphocytes from their haematopoietic precursors¹. During embryonic life (at a few well-defined intervals, at least in birds²) the thymus receives thymus-homing haematopoietic precursors that give rise to antigen-specific functional T lymphocytes^3–10. Although the number and thymic location of distinct T-cell lineages destined to form the peripheral T-cell pool are not yet well defined, at least two independent pathways have been proposed. First, thymic sub-capsular lymphoblasts divide and differentiate to give rise to small deep cortical thymic lymphocytes, medullary lymphocytes^11,12 and thymus emigrants (I.W., unpublished data) and second, the medulla contains an independent self-renewing population that contains the precursors of the peripheral T-cell pool¹³. Following irradiation the thymus may be repopulated by injected haematopoietic cells presumably related to the thymus-homing haematopoietic cells of the embryo^14,15. Here we have reconstituted irradiated mice with limiting numbers of bone marrow cells from Thy-1 congeneic donors and have found distinct clones of cells within the thymus. The pattern of reconstitution by the precursor cells indicates that two independent thymus lineages exist: cortex plus medulla, and medulla alone.