Although the ontogeny of hematopoietic stem cells (HSCs) in vertebrates has been studied intensely, a lineage relationship between the HSCs found in the developmentally successive hematopoietic organs remains to be shown. By using an in situ photoactivatable cell tracer in the transparent zebrafish embryo, we demonstrated that definitive blood precursors appeared between the dorsal aorta and axial vein, validating the homology of this tissue with the AGM (aorta-gonad-mesonephros) of amniotes. These cells first migrated through the blood to a previously undescribed caudal hematopoietic tissue (CHT), where they differentiated, expanded, and further migrated to seed the definitive hematopoietic organs, the thymus and kidney. Immigrants on the way to the thymus expressed c-myb and ikaros but not rag1; they were probably no longer HSCs, however, because they lacked scl and runx1 expression, unlike immigrants to the kidney. The CHT thus has a hematopoietic function similar to that of the mammalian fetal liver.

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Loss of kidney function underlies many renal diseases1. Mammals can partly repair their nephrons (the functional units of the kidney), but cannot form new ones2,3. By contrast, fish add nephrons throughout their lifespan and regenerate nephrons de novo after injury4,5, providing a model for understanding how mammalian renal regeneration may be therapeutically activated. Here we trace the source of new nephrons in the adult zebrafish to small cellular aggregates containing nephron progenitors. Transplantation of single aggregates comprising 10–30 cells is sufficient to engraft adults and generate multiple nephrons. Serial transplantation experiments to test self-renewal revealed that nephron progenitors are long-lived and possess significant replicative potential, consistent with stem-cell activity. Transplantation of mixed nephron progenitors tagged with either green or red fluorescent proteins yielded some mosaic nephrons, indicating that multiple nephron progenitors contribute to a single nephron. Consistent with this, live imaging of nephron formation in transparent larvae showed that nephrogenic aggregates form by the coalescence of multiple cells and then differentiate into nephrons. Taken together, these data demonstrate that the zebrafish kidney probably contains self-renewing nephron stem/progenitor cells. The identification of these cells paves the way to isolating or engineering the equivalent cells in mammals and developing novel renal regenerative therapies.

von Willebrand Factor (vWF) is a multimeric protein that mediates platelet adhesion to exposed subendothelium at sites of vascular injury under conditions of high flow/shear. The A1 domain of vWF (vWF-A1) forms the principal binding site for platelet glycoprotein Ib (GpIb), an interaction that is tightly regulated. We report here the crystal structure of the vWF-A1 domain at 2.3-Å resolution. As expected, the overall fold is similar to that of the vWF-A3 and integrin I domains. However, the structure also contains N-and C-terminal arms that wrap across the lower surface of the domain. Unlike the integrin I domains, vWF-A1 does not contain a metal ion-dependent adhesion site motif. Analysis of the available mutagenesis data suggests that the activator botrocetin binds to the right-hand face of the domain containing helices ␣5 and ␣6. Possible binding sites for GpIb are the front and upper surfaces of the domain. Natural mutations that lead to constitutive GpIb binding (von Willebrand type IIb disease) cluster in a different site, at the interface between the lower surface and the terminal arms, suggesting that they disrupt a regulatory region rather than forming part of the primary GpIb binding site. A possible pathway for propagating structural changes from the regulatory region to the ligand-binding surface is discussed. von Willebrand Factor (vWF) 1 is a multimeric protein that mediates platelet adhesion to exposed subendothelium at sites of vascular injury (1). The adhesive properties of vWF are tightly regulated so that plasma vWF does not normally interact with circulating platelets. vWF, however, will bind to platelets after it is "activated" by poorly understood conformational changes that occur after it binds to the vessel wall. A reduction in the plasma concentration of vWF or mutations that impair binding, activation, or assembly of vWF multimers cause von Willebrand disease (vWD), a common bleeding disorder characterized by decreased platelet adhesion and mucocutaneous bleeding (2).vWF-mediated adhesion of platelets to the vessel wall, under the high flow/shear conditions present in circulating blood, is mediated by sequences within the first (A1 domain) and third (A3 domain) A type repeats of vWF. The A1 domain (residues 479 -717) binds to platelet glycoprotein Ib⅐IX⅐V complex (GpIb), subendothelial heparans, cell surface sulfatides (reviewed in Ref. 3), and the non-fibrillar collagen type VI (4). The vWF-A3 domain contains the principal site for binding the fibrillar collagens types I and III (5, 6).Although initially noted in the primary sequence of vWF, the A domain has been subsequently discovered in a large number of cell matrix-associated or adhesive proteins and receptors (7). For example, varying numbers of A domains are found in several of the atypical, short chain collagens. A single A domain is inserted into the sequence of several integrin receptors, where it is generally referred to as the I domain. A/I domains are frequently involved in either cell adhesion or cell ligand interactions...