A close developmental relationship between the lymphoid and myeloid lineages.

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.