ABSTRACT The most straightforward view of early T cell development involves the linear, progressive development of bone marrow-resident progenitor cells to a specific stage, at which the cells exit the bone marrow through the circulation and eventually arrive at the thymus for further development. Although this has served as a convenient model, there is growing evidence that it is likely to be too simplistic. Specifically, the developmental stage at which lympho-hematopoietic progenitors are capable of egress from the marrow and thymus seeding may not be as unique as once postulated. A careful examination of both bone marrow-resident progenitor cells and early thymocytes, in terms of lineage potential, transcriptional activity of key developmental genes, and cell surface protein suggest that the thymus may be normally seeded by a range of progenitor cell types, rather that by cells at a specific development stage. Here, we review the evidence supporting this contention and explore how broad the spectrum of thymus-seeding cell is likely to be.
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