[Hematopoiesis: I. Cytokines involved in its regulation]

The study of the direct involvement of colony stimulating factors, interleukins, and other purified factors in distinct steps of hematopoiesis has been complicated by the in vitro presence of non-hematopoietic cells which can intermediate the effects observed on hematopoietic precursors. The review covers the recent finding that the CD34 antigen is expressed on the membranes of essentially all pluripotent stem cells, but is lacking in the majority of the differentiated blood and stromal bone marrow cells. This finding allowed in vitro experiments with selected CD34+ hematopoietic precursors, and a consequent reevaluation of the participation of different factors in their differentiation. The role of interferons, tumor necrosis factors, and transforming growth factors beta in the negative regulation of hematopoiesis is also analysed.