Figure 1.
Early lymphoid development in bone marrow and thymus. This schematic shows early differentiation routes of B‐ and T‐lineage cells from hematopoietic stem cells (HSCs). In the bone marrow, the most primitive progenitors with lymphoid specification are contained within the c‐KITHigh FMS‐like tyrosine kinase‐3 (FLT3)+ fraction, termed lymphoid‐primed multipotent progenitors (LMPPs). The LMPP population overlaps multipotent progenitors (MPPs) and early lymphoid progenitors (ELPs), and is thought to be a major source for thymus‐immigrating progenitors, including early T‐lineage progenitors (ETPs). In the thymus, ETPs differentiate to double negative (DN)‐2 and DN‐3 cells with the activation of NOTCH signaling. ELPs differentiate to common lymphoid progenitor (CLP) and ProB cells in the bone marrow with the activation of interleukin‐7 receptor (IL7R) signaling. Transcription factors are indicated near the stages where they are most active. Note that these data were taken from mouse studies. Human counterparts to each progenitor category remain to be identified. EBF, early B cell factor; RAG, recombination‐activating gene.