Defects in osteoblast function but no changes in long-term repopulating potential of hematopoietic stem cells in a mouse chronic inflammatory arthritis model.

Recent studies support the notion that there is an intricate relationship between hematopoiesis and bone homeostasis in normal steady states. Using mice undergoing chronic inflammatory arthritis, we investigated the relationship between hematopoiesis and bone homeostasis in pathologic conditions. We demonstrate that mice undergoing chronic inflammatory arthritis displayed osteoporosis resulting from a severe defect in osteoblast function. Despite the defective osteoblast function, however, the hematopoietic stem cells from these mice exhibited normal properties in either long-term repopulation or cell cycling. Therefore, the bone-forming capacity of osteoblasts is distinct from their ability to maintain hematopoietic stem cells in chronic inflammatory conditions.

Both primitive and definitive blood cells are derived from Flk-1+ mesoderm.

Emerging evidence suggests that all hematopoietic and endothelial cells originate from Flk-1(+) mesoderm in the mouse. However, this concept has not been completely proven, especially for the origin of blood cells. Using either Flk1(+/Cre);Rosa26R-EYFP or Flk1(+/Cre);Rosa26R-LacZ mice, we permanently marked Flk-1(+) cells and their progenies to determine the relationship between hematopoietic tissues and cells that express Flk-1. In embryos, all blood cells within the yolk sac and aorta were of Flk-1(+) origin. In addition, nearly all CD45(+) cells in bone marrow and circulating blood in adults were of Flk-1(+) origin. These results provide clear evidence that all blood cells, primitive and definitive, in mice are derived from Flk-1(+) mesodermal cells.

Differentiation of mouse embryonic stem cells into blood.

Embryonic stem (ES) cells can be maintained as pluripotent stem cells or induced to differentiate into many different somatic cell types. As ES-derived somatic cells can potentially be used for cell transplantation or cell-based therapy, ES cells have gained much scientific and general public attention. Successful derivation of blood from ES cells for tissue engineering will require a comprehensive understanding of inductive signals and downstream effectors involved in blood lineage development. Ideally, directed differentiation of ES cells into blood and isolation of pure hematopoietic progenitors will enhance our ability to utilize ES-derived blood cells for future clinical applications. The protocols provided in this unit describe methods of maintaining and differentiating mouse ES cells as well as identifying and isolating hematopoietic progenitors by utilizing flow cytometry and progenitor assays.

Evidence for the hemangioblast.

In recent years, numerous cellular and genetic studies have led to a better understanding of the developmental relationship between hematopoietic and endothelial cell lineages. Specifically, tracing cells expressing various genes such as Brachyury, Flk-1, or Scl has delineated the cellular sequence leading to hematopoietic and endothelial cell development from mesoderm. Moreover, in vitro as well as in vivo studies of invertebrate and vertebrate systems have established that hematopoietic and endothelial cells develop from a common progenitor, the hemangioblast. Finally, the presence of the hemangioblast has been confirmed in postnatal mice and humans. Further characterization of the hemangioblast, both embryo and postnatal, will be critical for a better understanding of the molecular events involved in hematopoietic and endothelial cell differentiation as well as for utilizing this cell population for clinical applications.