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.