Role of the microenvironment of the embryonic aorta-gonad-mesonephros region in hematopoiesis.

Although various cytokines, growth factors, and chemokines are known to regulate hematopoiesis, expansion of hematopoietic stem cells (HSCs) in vitro with the use of such agents has proved problematic. Stromal cells are major components of the microenvironment that surrounds hematopoietic cells and are thought to play an important role in hematopoiesis in vivo. Co-culture of HSCs with stromal cells promotes hematopoiesis and self-renewal of HSCs. Definitive hematopoietic cells first appear during mammalian embryonic development in the aorta-gonad-mesonephros (AGM) region, and it is therefore thought that the microenvironment of this region plays an important role in HSC ontogeny. We have adopted two approaches to studying the contribution of the AGM microenvironment to hematopoiesis. In the first approach, we have developed an in vitro culture system for mouse AGM explants. Hematopoiesis is enhanced in such cultures by the presence of the combination of stem cell factor (SCF), basic fibroblast growth factor, leukemia inhibitory factor, and oncostatin M (SFLO culture). However, transplantation assays revealed that HSCs capable of long-term reconstitution of the hematopoietic compartment of irradiated mice (LTR-HSCs) do not expand in AGM-SFLO cultures; rather, these cultures appear to provide a favorable microenvironment for hematogenic angioblasts that are precursors of both endothelial and hematopoietic cells. In our second approach, we have established various stromal cell lines from the mouse AGM region. The AGM-S3 cell line supports human and mouse primitive hematopoietic cells as well as mouse LTR-HSCs. Maintenance of LTR-HSCs is mediated by a mechanism other than SCF signaling through its receptor (c-Kit). These two in vitro approaches should prove useful for further elucidation of the mechanisms that underlie hematopoiesis and HSC self-renewal.