The Hematopoietic Microenvironment: Stromal Cell Types: Characterization and Function In Situ and In Vitro.
The murine hematopoietic stroma is essential for the homing and the continued replication of hematopoietic stem cells following transplantation and appears to control their lineage proliferation. In our studies it has been found to consist of 3 cell types with differing antigenic displays, growth factor responses and interactions when placed in culture. These characteristics are in keeping with their classifications as myofibroblasts, endothelial like cells and macrophages, respectively. Cell adhesion molecules (CAMs) and early acting cytokines have been found to be associated with these cell types. Only the myofibroblast type has been found to support proliferation of the descendants of long term repopulating stem cells (LTRC) plated over their confluent layer. The endothelial like cell type, and possibly the macrophage, were found to suppress not only the growth of LTRC but, also, that of the myofibroblasts obtained from the same marrow samples. Culture medium obtained from near-confluent growth of the endothelial like cells contained a growth inhibiting molecule of less than 3000 da. capable of inhibiting cells of several types, including the stromal myofibroblasts. The endothelial like cells were shown to unilaterally express VCAM-1 in culture, as well as vWf and endothelial cell specific antigens. Tissue sections of femurs from normal irradiated recipients taken 1 hour post-transplantation revealed Lac-Z marked donor stem cells lodged just external to the endothelial lining of thin walled vessels and in contact with myofibroblasts. Previous studies had shown that some stem cells enter replication at these sites within 48 hours of lodgment. It is suggested that a balance of growth stimulation versus suppression may be maintained between the myofibroblasts and the endothelial like cell types, respectively, in the functioning marrow stroma. Both the myofibroblastoid and the endothelial like stromal cells of old mice displayed a reduced replicative rate in vivo and a reduced replicative capacity in vitro. This suggests that treatments that induce damage to the marrow microenvironment may be especially destructive in aged individuals. In recent experiments it has become clear that the stromal cell population that includes cells identifiable as alkaline phosphatase positive myofibroblasts also contains cells that produce osteocalcin, osteonectin and bone sialoprotein, and deposit von Kossa positive calcium. Whether this results from a single cell lineage or two lineages of similar morphology and alkaline phosphatase expression is under investigation.
Wolf
NORMAN S.
NS
Department of Pathology, University of Washington.