Embryonic stromal clones reveal developmental regulators of definitive hematopoietic stem cells

Surface antigen phenotypes of hematopoietic stem cells from embryos and murine embryonic stem cells

Surface antigens on hematopoietic stem cells (HSCs) enable prospective isolation and characterization. Here, we compare the cell-surface phenotype of hematopoietic repopulating cells from murine yolk sac, aorta-gonad-mesonephros, placenta, fetal liver, and bone marrow with that of HSCs derived from the in vitro differentiation of murine embryonic stem cells (ESC-HSCs). Whereas c-Kit marks all HSC populations, CD41, CD45, CD34, and CD150 were developmentally regulated: the earliest embryonic HSCs express CD41 and CD34 and lack CD45 and CD150, whereas more mature HSCs lack CD41 and CD34 and express CD45 and CD150. ESC-HSCs express CD41 and CD150, lack CD34, and are heterogeneous for CD45. Finally, although CD48 was absent from all in vivo HSCs examined, ESC-HSCs were heterogeneous for the expression of this molecule. This unique phenotype signifies a developmentally immature population of cells with features of both primitive and mature HSC. The prospective fractionation of ESC-HSCs will facilitate studies of HSC maturation essential for normal functional engraftment in irradiated adults.

Lessons from the Niche for Generation and Expansion of Hematopoietic Stem Cells

Hematopoietic Stem Cells (HSCs) are used clinically to treat human blood-related genetic diseases and leukemias, but the availability of this therapy is constrained by the limiting number of transplantable HSCs. Previous strategies to expand HSCs in vitro resulted in precocious differentiation and reduced transplant efficiency. In vivo analysis of the mechanisms utilized by fetal and adult niches to control HSCs can be mimicked for therapeutic use. This review summarizes the latest research on the in vivo HSC niche and the clinical applications of this knowledge to promote ex vivo expansion and direct de novo generation of HSCs.

Fine-tuning of hematopoietic stem cell homeostasis: novel role for ubiquitin ligase

Homeostasis of hematopoietic stem cells (HSCs) is a tightly regulated process, controlled by intrinsic and extrinsic signals. Although a variety of molecules involved in HSC maintenance and self-renewal are known, it remains unclear how robust HSC homeostasis is achieved. In this issue of Genes & Development, Rathinam and colleagues (992-997) report a new player in HSC homeostasis, c-Cbl ubiquitin ligase. They show that this E3 ubiquitin ligase acts as a negative regulator of cytokine signaling.

Stem cell researchers find their niche

The EuroSTELLS Workshop `Stem Cell Niches', organised by Anna Bigas, Ernest Arenas and Pasqualino Loi, took place in January 2008 in Barcelona, Spain. The goal of the conference was to promote scientific collaboration and synergy between stem cell researchers worldwide and those in the EuroSTELLS consortia(an initiative of the European Science Foundation EUROCORES Programme), and to stimulate discussion of the latest results in the field of stem cell niches.

Of lineage and legacy: the development of mammalian hematopoietic stem cells

The hematopoietic system is one of the first complex tissues to develop in the mammalian conceptus. Of particular interest in the field of developmental hematopoiesis is the origin of adult bone marrow hematopoietic stem cells. Tracing their origin is complicated because blood is a mobile tissue and because hematopoietic cells emerge from many embryonic sites. The origin of the adult mammalian blood system remains a topic of lively discussion and intense research. Interest is also focused on developmental signals that induce the adult hematopoietic stem cell program, as these may prove useful for generating and expanding these clinically important cell populations ex vivo. This review presents a historical overview of and the most recent data on the developmental origins of hematopoiesis.