Models of the hematopoietic hierarchy. (A) Classical model of the hematopoietic hierarchy with a strict separation between the myeloid and lymphoid branches as the first step in lineage commitment downstream of the hematopoietic stem cell. (B) Alternative model as proposed by Adolfsson et al,32 incorporating the identification of LMPPs. The relationship between the LMPP and the CMP has not yet been resolved, but they are placed at similar positions in the hierarchy based on their lineage potential. (C) Recently proposed model, in which immunophenotypically defined HSC populations or their immediate committed progeny have restricted megakaryocytic potential. In this model, there is heterogeneity within the HSC population with respect to lineage potential, including a megakaryocyte-biased HSC that directly gives rise to megakaryocyte progenitors and bypasses classical intermediate commitment stages, including the CMP and MEP. Note: Nearly all of the studies described were performed in mice, and therefore the model shown should be considered to apply only to the murine hematopoietic system at this time. EryP, erythroid progenitor.
