Emergence of hematopoietic stem cells in the human embryo

Two different pathways for the transport of primitive and definitive blood cells from the yolk sac to the embryo in humans

During the early human embryonic period nutrients and blood cells are temporarily provided by the extraembryonic yolk sac (YS). The YS before week six is involved not only in primitive but also in definitive erythropoiesis. While the destiny of primitive erythroid cells that fill the blood vessels of the YS is well known, the final destination of erythrocytes present in the endodermal vesicular system is unknown. In the present study we have investigated, step by step, the destiny of the erythrocytes present in the endodermal vesicles during the embryonic period. Twelve human YSs and their corresponding yolk stalks were analyzed between weeks 4 and 7 of embryonic age by light and scanning electron microscopy. It is shown that erythrocytes (according to their size and morphological features) located within the endodermal vesicles of the YS wall are pulled out through endodermal pits into the YS cavity, from where they reach the lumen of the primitive gut of the embryo through the vitelline duct, a temporary pathway communicating both compartments. During the study period no erythrocytes were seen within the embryo's vascular network where only primitive erythroblasts were identified. Our results indicate that the vitelline duct plays an important transient role as a pathway for the transport of nutrients and blood cells between the YS and the embryo before week five of embryonic development that ends just at the time when YS-embryo circulation becomes established.

Plasticity after allogeneic hematopoietic stem cell transplantation

The postulated almost unlimited potential of transplanted hematopoietic stem cells (HSCs) to transdifferentiate into cell types that do not belong to the hematopoietic system denotes a complete paradigm shift of the hierarchical hemopoietic tree. In several studies during the last few years, donor cells have been identified in almost all recipient tissues after allogeneic HSC transplantation (HSCT), supporting the theory that any failing organ could be accessible to regenerative cell therapy. However, the putative potential ability of the stem cells to cross beyond lineage barriers has been questioned by other studies which suggest that hematopoietic cells might fuse with non-hematopoietic cells and mimic the appearance of transdifferentiation. Proof that HSCs have preserved the capacity to transdifferentiate into other cell types remains to be demonstrated. In this review, we focus mainly on clinical studies addressing plasticity in humans who underwent allogeneic HSCT. We summarize the published data on non-hematopoietic chimerism, donor cell contribution to tissue repair, the controversies related to the methods used to detect donor-derived non-hematopoietic cells and the functional impact of this phenomenon in diverse specific target tissues and organs.

Cytokines in the differentiation therapy of leukemia: from laboratory investigations to clinical applications

Differentiation therapy of leukemia is the treatment of leukemia cells with biological or chemical agents that induce the terminal differentiation of the cancer cells. It is regarded as a novel and targeted approach to leukemia treatment, based on our better understanding of the hematopoietic process and the mechanisms of its deregulation during leukemogenesis. Clinically, differentiation therapy has been most successful in acute promyelocytic leukemia using all-trans-retinoic acid as the inducer, either alone or in combination with chemotherapy. This review presents evidence that a number of hematopoietic cytokines play important roles in both normal and aberrant hematopoietic processes. In vitro laboratory investigations in the past two decades using well-characterized myeloid leukemic cell lines and primary blast cells from leukemia patients have revealed that many hematopoietic cytokines can trigger lineage-specific differentiation of leukemia cells, which may have important implications in the clinical setting. Moreover, our current understanding of cytokine interactions and the molecular mechanisms of cytokine-induced leukemic cell differentiation will be discussed in the light of recent findings. Finally, ways in which laboratory research on cytokines in the differentiation therapy of leukemia can lead to the improved design of protocols for future clinical applications to leukemia therapy will also be addressed.

Improved Arteriogenesis with Simultaneous Skeletal Muscle Repair in Ischemic Tissue by SCL+ Multipotent Adult Progenitor Cell Clones from Peripheral Blood

BACKGROUND

The CD34(-) murine stem cell line RM26 cloned from peripheral blood mononuclear cells has been shown to generate hematopoietic progeny in lethally irradiated animals. The peripheral blood-derived cell clones expresses a variety of mesodermal and erythroid/myeloid transcription factors suggesting a multipotent differentiation potential like the bone marrow-derived 'multipotent adult progenitor cells' (MAP-C).

METHODS

SCL(+) CD34(-) RM26 cells were transfused intravenously into mice suffering from chronic hind-limb ischemia, evaluating the effect of stem cells on collateral artery growth and simultaneous skeletal muscle repair.

RESULTS

RM26 cells are capable of differentiating in vitro into endothelial cells when cultured on the appropriate collagen matrix. Activation of the SCL stem cell enhancer (SCL(+)) is mediated through the binding to two Ets and one GATA site and cells start to express milieu- and growth condition-dependent levels of the endothelial markers CD31 (PECAM) and Flt-1 (VEGF-R1). Intravenously infused RM26 cells significantly improved the collateral blood flow (arteriogenesis) and neo-angiogenesis formation in a murine hind-limb ischemia transplant model. Although transplanted RM26 cells did not integrate into the growing collateral arteries, cells were found adjacent to local arteriogenesis, but instead integrated into the ischemic skeletal muscle exclusively in the affected limb for simultaneous tissue repair.

CONCLUSION

These data suggest that molecularly primed hem-/mesangioblast-type adult progenitor cells can circulate in the peripheral blood improving perfusion of tissues with chronic ischemia and extending beyond the vascular compartment.

Embryonic origins of mammalian hematopoiesis

Hematopoiesis and vasculogenesis in the mammalian embryo begin in the blood islands of the yolk sac and continue, somewhat later, within the embryo proper. A subset of the first endothelial and hematopoietic cells of the yolk sac arise in close spatial and temporal association, apparently from a common mesodermal progenitor, the "hemangioblast." The mechanisms that control formation of hemangioblast and embryonic hematopoietic and endothelial (angioblastic) stem/progenitor cells are still not well understood. Formation of these cell types from nascent mesoderm requires signals from an adjacent outer layer of primitive (visceral) endoderm. Indian hedgehog (Ihh), a member of the hedgehog family of extracellular morphogens, is secreted by visceral endoderm and alone is sufficient to induce hematopoiesis and vasculogenesis in explanted embryos. While gene targeting studies in mice support a role for hedgehog signaling in these processes in vivo, they also suggest that additional molecules (perhaps, for example, Wnt proteins) are required for induction and patterning of hematopoietic and vascular mesoderm. Indian hedgehog likely functions through upregulation of genes encoding other signaling molecules, such as bone morphogenetic protein (Bmp)-4, in the target tissue. This review will focus on hematopoietic and vascular development in the early mouse embryo and will discuss potential implications of recent studies for stem cell transplantation in humans.