The existence of a common precursor for endothelial and hemopoietic cells, termed the hemangioblast, has been postulated since the beginning of the century. Recently, deletion of the endothelial-specific vascular endothelial growth factor receptor 2 (VEGFR2) by gene targeting has shown that both endothelial and hemopoietic cells are absent in homozygous null mice. This observation suggested that VEGFR2 could be expressed by the hemangioblast and essential for its further differentiation along both lineages. However, it was not possible to exclude the hypothesis that hemopoietic failure was a secondary effect resulting from the absence of an endothelial cell microenvironment. To distinguish between these two hypotheses, we have produced a mAb directed against the extracellular domain of avian VEGFR2 and isolated VEGFR2؉ cells from the mesoderm of chicken embryos at the gastrulation stage. We have found that in clonal cultures, a VEGFR2؉ cell gives rise to either a hemopoietic or an endothelial cell colony. The developmental decision appears to be regulated by the binding of two different VEGFR2 ligands. Thus, endothelial differentiation requires VEGF, whereas hemopoietic differentiation occurs in the absence of VEGF and is significantly reduced by soluble VEGFR2, showing that this process could be mediated by a second, yet unidentified, VEGFR2 ligand. These observations thus suggest strongly that in the absence of the VEGFR2 gene product, the precursors of both hemopoietic and vascular endothelial lineages cannot survive. These cells therefore might be the initial targets of the VEGFR2 null mutation.

No abstract

No abstract

Positron trapping into vacancies in semiconductors is studied on the basis of Fermi's golden-rule calculations. The emphasis is put on the comparison of the trapping properties into defects in different charge states. In particular, the temperature dependences are investigated. Important features for vacancy-type defects in semiconductors are the localized electron states within the forbidden energy gap and (in the case of negatively charged defects) the weakly bound Rydberg states for positrons. Compared to vacancy-type defects in metals, these features make possible new kinds of trapping mechanisms with electron-hole and phonon excitations. For charged defects the Coulomb wave character of the delocalized positron states before trapping determines the amplitude of the wave function at the defect and thereby strongly affects the magnitude of the trapping rate. As a result, trapping into positively charged defects is effectively forbidden while negatively charged defects show remarkable properties which differ from the picture established for positron trapping in metals. The trapping rate into negative defects increases strongly with decreasing temperature and at very low temperatures "gigantic" values may result.

In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50-400 K. The sample with smaller pore size exhibits a higher minimal energy (73 ± 5 meV), compared to the sample with bigger pores (48 ± 5 meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.