Primary monolayer cultures of marrow stromal cells, as they are referred to in the hemopoietic field, contain monocytes, macrophages, and endothelial cells (1 ,2 ). In addition, they contain precursor cells that can differentiate into multiple mesenchymal cell types (3 –5 ). In vitro, the highly proliferative adherent fibroblast-like cells of marrow form colonies, which Friedenstein termed colony-forming units-fibroblast (CFU-f) (3 ). These cells are cultured as stromal support cells for hematopoietic cell differentiation, but the use of these cells for the investigation of mesenchymal tissue cell differentiation has become common. In recent years, there has been interest in using these cells for regeneration/repair of mesenchymal tissues (6 ,7 ).
