CD34 expression by stromal precursors in normal human adult bone marrow.

Normal bone marrow cells were isolated by fluorescence-activated cell sorting (FACS) on the basis of CD34 antigen expression and then assayed in vitro for colonies of fibroblastic cells (fibroblast colony-forming units [CFU-F]). Greater than 95% of detectable CFU-F were recovered in the CD34+ population, while their numbers were markedly depleted in the CD34- population. Additional experiments showed that the majority of CFU-F exhibited high forward and perpendicular light scatter and low-density CD34 antigen. Growth of sorted cells in medium optimized for long-term marrow culture (LTMC) produced a complex mixture of adherent stromal elements including fibroblasts, adipocytes, smooth muscle cells, and macrophages. Monoclonal antibody STRO-1, which identifies bone marrow stromal cells, reacted with approximately 5% of CD34+ cells, which included all CFU-F and stromal precursors in LTMC. Experiments using soybean agglutinin (SBA) further showed that these stromal elements were restricted to a population of bone marrow cells with the phenotype CD34+/SBA+. These properties of stromal precursors are quite distinct from those of primitive hematopoietic progenitors, showing that although the precursors of the hematopoietic and stromal systems share expression of CD34, they are otherwise phenotypically distinct cell types.