Stages in human B-cell development.

Six stages beginning with the CLPs and culminating with immature B cells are shown as one model of B-cell development in human BM. The letters in parentheses represent an approximation of the counterpart stages in murine B-cell development, using the nomenclature of Hardy and colleagues.19 20 Numbers on the cell surface indicate CD antigens frequently used to define the individual stages in human B-cell development. Patterns of gene expression inside the cells have been determined by RT-PCR and/or flow cytometry. Dashed arrows indicate possible cellular targets for positive (+) and negative (−) growth regulators/chemotactic factors produced by BM stromal cells. Cells in a particular developmental stage are not necessarily uniform in the expression of a specific receptor. For example, only 10% to 20% of the pre-BI plus the pre-BII cells express the μ-ψLC pre-BCR.