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Pax‐5 and EBF are Expressed in Committed B‐Cell Progenitors Prior to the Colonization of the Embryonic Bursa of Fabricius*

The committed B‐cell precursors developing from hemopoietic stem cells have been considered to differentiate through a common lymphoid progenitor stage in the mouse. In the chicken B‐cell system, however, the committed B‐cell progenitors burst as a single wave prior to the bursal colonization and most likely as direct descendants of hemopoietic stem cells. In the present report we show that prebursally committed B‐cell progenitors specifically express early B‐cell factor (EBF) and Pax‐5 transcription factors. In addition we show that the expression of these and other B‐lineage‐associated transcription factors starts early in the chicken ontogeny. Altogether our findings strongly support the model of early delineation of B‐ and T‐cell development and do not support the existence of common lymphoid stem cells.