Developmental relationship between hematopoietic stem cells (HSCs), common lymphoid progenitors (CLPs), and putative early-B or T/NK/dendritic cell (DC) progenitors.

HSCs include all primitive CD34⁺/lineage⁻hematopoietic developmental stages prior to the CLP, shown schematically as 3 cells. Arrows with solid lines indicate developmental flow culminating in increased lineage restriction. Dashed arrows indicate possible cellular targets of IL-7 signaling or an unknown (?) ligand. Numbers on the cell surface indicate CD antigens useful in distinguishing the developmental compartments. Although not shown in this figure, the 3 reports that described the cell surface phenotype of CD19⁻ lymphoid progenitors revealed considerable heterogeneity.7,8,13 For example, CD7 and CD33 were detected on a minority of the lymphoid progenitors in each study.7,8 13 There is no known surface marker that distinguishes the CLP from the early-B cell. It is also likely that IL-7R expression and signaling vary both within and between the lymphoid progenitor compartments.