8,761 research outputs found
Self-renewal of colony forming units (cfu) in serial bone marrow transplantation experiments
The capacity of stem cells (CFU) for selfârenewal was tested by transplanting normal bone marrow (primary transplantation) and bone marrow which had been subjected to one or two earlier transplantations (secondary and tertiary transplantation) into lethally irradiated syngeneic recipients. It was found that the capacity for selfârenewal is diminished within the first weeks after one or more previous transplantations. This ability of stem cells recovered after a longer interval after the previous transplantation. The time required for this recovery depended upon the number of previous transplantations and amounted to more than 1 or 2 months after one or two transplantations respectively. Shortly after transplantation the CFU/nucleated cell ratio in bone marrow was below normal and its decrease was more pronounced when the bone marrow had been transplanted more often. An increase of the ratio towards normal values was observed in the course of one month after the last transplantation. Measurements of the spleen colony size after transplantation of normal and reâtransplanted bone marrow indicated that CFUs from reâtransplanted marrow gave slightly smaller spleen colonies than those of normal marrow. It is concluded that the decreased selfârenewal of stem cells shortly after previous transplantations is probably not due to a limitation in the number of normal mitoses they can perform, but to a loss of stem cells by transfer to the compartment of differentiating cells. Copyrigh
Geiger-Mode Avalanche Photodiodes in Particle Detection
It is well known that avalanche photodiodes operated in the Geiger mode above
the breakdown voltage offer a virtually infinite sensitivity and time accuracy
in the picosecond range that can be used for single photon detection. However,
their performance in particle detection remains still unexplored. In this
contribution, we are going to expose the different steps that we have taken in
order to prove the efficiency of Geiger mode avalanche photodiodes in the
aforementioned field. In particular, we will present an array of pixels of
1mmx1mm fabricated with a standard CMOS technology for characterization in a
test beam.Comment: 7 pages, 2 figures, Proceedings of LCWS1
Mobilization of haemopoietic stem cells (cfu) into the peripheral blood of the mouse; effects of endotoxin and other compounds
Factors affecting the circulation of haemopoietic stem cells (CFU) in the peripheral blood of mice were investigated. I.v. injection of sublethal doses of endotoxin, trypsin and proteinase appeared to raise the number of CFU per ml blood from about 30â40 to about 300â400 or more within 10 min. The effect was smaller when smaller doses of the substances were injected. After this initial rise the number of circulating cells returned to normal in a few hours. Following endotoxin there was a second rise which started 2â3 days after injection and attained a peak on the 6thâ7th day. The first rise is explained as a mobilization of stem cells from their normal microenvironments into the blood stream; the second rise is considered to reflect proliferation of CFUs in the haemopoietic tissues. The spleen seems to be acting as an organ capturing CFUs from the blood and not as a source adding stem cells to the blood. The early mobilization of CFU after endotoxin injection did not coincide with a mobilization of neutrophils. The number of circulating band cells was increased during the first hours. The importance of âopen sitesâin the haemopoietic tissue for capturing CFUs was studied by emptying these sites through a lethal Xâirradiation and injecting normal bone marrow cells. When a greater number of syngeneic bone marrow cells was injected intravenously, the level of circulating CFU in irradiated mice was slightly lower than the level in unirradiated mice during the first hours. Copyrigh
The perception of stroke-to-stroke turn boundaries in signed conversation
Speaker transitions in conversation are often brief, with minimal vocal overlap. Signed languages appear to defy this pattern with frequent, long spans of simultaneous signing. But recent evidence suggests that turn boundaries in signed language may only include the content-bearing parts of the turn (from the first stroke to the last), and not all turn-related movement (from first preparation to final retraction). We tested whether signers were able to anticipate âstroke-to-strokeâ turn boundaries with only minimal conversational context. We found that, indeed, signers anticipated turn boundaries at the ends of turn-final strokes. Signers often responded early, especially when the turn was long or contained multiple possible end points. Early responses for long turns were especially apparent for interrogativesâlong interrogative turns showed much greater anticipation compared to short ones
Inhibited spontaneous emission of quantum dots observed in a 3D photonic band gap
We present time-resolved emission experiments of semiconductor quantum dots
in silicon 3D inverse-woodpile photonic band gap crystals. A systematic study
is made of crystals with a range of pore radii to tune the band gap relative to
the emission frequency. The decay rates averaged over all dipole orientations
are inhibited by a factor of 10 in the photonic band gap and enhanced up to 2?
outside the gap, in agreement with theory. We discuss the effects of spatial
inhomogeneity, nonradiative decay, and transition dipole orientations on the
observed inhibition in the band gap.Comment: 5 figures, update author lis
Charge asymmetries of top quarks: a window to new physics at hadron colliders
With the next start of LHC, a huge production of top quarks is expected.
There are several models that predict the existence of heavy colored resonances
decaying to top quarks in the TeV energy range. A peak in the differential
cross section could reveal the existence of such a resonance, but this is
experimentally challenging, because it requires selecting data samples where
top and antitop quarks are highly boosted. Nonetheless, the production of such
resonances might generate a sizable charge asymmetry of top versus antitop
quarks. We consider a toy model with general flavour independent couplings of
the resonance to quarks, of both vector and axial-vector kind. The charge
asymmetry turns out to be a more powerful observable to detect new physics than
the differential cross section, because its highest statistical significance is
achieved with data samples of top-antitop quark pairs of low invariant masses
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