The classic dichotomy model of hematopoiesis postulates that the first step of differentiation beyond the multipotent hematopoietic stem cell generates the common myelo-erythroid progenitors and common lymphoid progenitors (CLPs). Previous studies in fetal mice showed, however, that myeloid potential persists in the T- and B-cell branches even after these lineages have diverged, indicating that the simple dichotomy model is invalid, at least for fetal hematopoiesis. Nevertheless, CLPs have persisted in models of adult hematopoiesis; results from several groups support the presence of CLPs in bone marrow. Recent evidence challenges the dichotomy model in the adult, and it is proposed here that the alternative myeloid-based model is applicable to both fetal and adult hematopoiesis.
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