Stromal cell-dependent terminal maturation of K562 erythroleukaemia cells

K562 cells were cultured over monolayers of various cell types for four days and the characteristics of those cells which became firmly adherent to such monolayers studied. The monolayers used were composed of stromal cells derived from human bone marrow, two human fibroblast lines, human umbilical cord-derived endothelial cells, bone marrow macrophages, blood-monocyte-derived macrophages, a neuroglial cell line (U-251 MG) or mixtures of some of the above. The data showed that K562 cells adhered much more frequently to fibroblasts than to other cell types. In the case of some marrow-derived stromal cell layers grown in human group AB serum, the firmly adherent K562 cells showed morphological evidence of nuclear and cytoplasmic maturation and, especially in the presence of haemin, sometimes developed into erythrocyte-like cells. The maturing cells were weakly or moderately benzidine-positive. By contrast, K562 cells which adhered to monolayers of fibroblasts, endothelial cells or macrophages showed little or no maturation and were either benzidine-negative or only very weakly positive. In experiments employing monolayers made up by mixing skin fibroblasts and endothelial cells or skin fibroblasts, endothelial cells and macrophages, some adherent K562 cells developed into cells resembling early and late polychromatic normoblasts and erythrocytes. The maturing cells were moderately or strongly benzidine-positive. Maturation was most marked in K562 cells adherent to regions of the monolayer containing all three of the stromal cell types. The results indicate that the optimal microenvironmental niche for the development of K562 cells into erythroblasts and erythrocytes may consist of a cluster of closely associated fibroblasts, macrophages and endothelial cells.