Enriched Clusters Research Articles

Influence of small Cu addition on the crystallization of amorphous alloy Fe−Zr−B

The behavior of Cu during the crystallization processes in Fe−Zr−B amorphous alloy was studied by atom probe field ion microscopy and transmission electron microscopy. It was found that Cu atoms formed clusters prior to the crystallization reaction. During the crystallization, Cu enriched clusters provided nucleation sites for α-Fe phase so that the nucleation density of α-Fe phase is increased.

ANALYSIS OF PASSENGER CELL COMPOSITION OF HUMAN FETAL PANCREAS: IMPLICATIONS FOR TRANSPLANTATION

169 Human fetal pancreas (HFP) is a potential source of transplantable islets for the treatment of insulin dependent diabetes mellitus (IDDM). Transplantation of either HFP explants or HFP proislets (endocrine enriched cells generated by limited collagenase digestion) beneath the kidney capsule of streptozotocin (STZ)-induced diabetic nude mice restores euglycemia within 3 months post-transplant. HFP proislets have previously been reported to be less immunogenic than nondigested HFP tissue. The aim of this study is to understand the passenger leukocyte composition of HFP and to identify a preparation of HFP with decreased immunogenicity suitable for clinical transplantation. Single cell suspensions of exocrine enriched cells were obtained following a 3 minute collagenase digestion of 1-2 mm3 HFP explants. The remaining endocrine enriched proislets clusters were collected and further digested with dispase to single cells. The single cell suspensions of either exocrine or endocrine enriched cells were analyzed for the presence of monocytes/macrophages using the CD14 determinant and T cells using the CD3, CD4 or CD8 determinates by flow cytometry. The exocrine cell population contained a significant number of CD14+ cells (4.9 ± 2% of total cells, n=9), whereas there were significantly fewer CD14+ cells in the endocrine enriched population (0.8 ± 0.4% of total cells, n=9). The CD14 determinant was unaffected by the dispase digestion as determined using human PBMC. CD3+ T cells (both CD4 and CD8 phenotypes) were observed in varying amounts ranging from 0.2% to 2.5% of the total population of cells, and were present in both the exocrine- and endocrine-enriched cell populations. MLR cultures were established at a responder to stimulator ratio of 5:1 using either irradiated exocrine-enriched or endocrine-enriched cells as stimulators and adult human PBMC as responders. Both IFNγ and IL10 production was significantly greater in MLR cultures stimulated with exocrine enriched HFP cells compared to MLR cultures stimulated with endocrine enriched HFP cells (IFNγ: p=0.003 on day 4, p=0.02 on day 7; IL10: p=0.02 on day 4 and p=0.007 on day 7, n=5 different HFP/PBMC combinations). Comparative CTL activity is being evaluated. It is now recognized that both direct and indirect pathways of allorecognition can contribute to graft loss. Direct T cell recognition is the result of host T cells interacting directly with allo-MHC molecules on donor APC's whereas in the indirect pathway host T cells recognize allopeptides processed and presented by self APC's. The decreased numbers of CD14+ cells may account in part for the decreased immunogenicity of the endocrine enriched HFP cell population via a decreased ability to stimulate via the direct pathway. The ability to selectively deplete passenger cells from the transplanted population using simple digestion protocols may significantly enhance the ability to successfully transplant HFP islets.

Solid state13C NMR spectra of metal carbonyl clusters

The solid state13C NMR spectra of four13CO enriched carbonyl clusters having a tri-iron metallic core have been analyzed to provide structural and dynamic information. In Fe3(CO)12 (1), the high temperature spectra suggest the occurrence of large amplitude motions of the CO groups around their position at the vertexes of the coordination polyhedron in addition to the motion involving the Fe3-triangle previously detected in the VT-13C MAS spectra.13C and31P NMR data of Fe3(CO)11PPh3 (2) indicates the presence of one molecule in the asymmetric unit in apparent disagreement with the previously reported X-ray data. Furthermore, we show that structural information can be obtained from the chemical shift tensor components readily available from the analysis of the spinning sideband manifold.

Evidence that Involucrin Is a Covalently Linked Constituent of Highly Purified Cultured Keratinocyte Cornified Envelopes

The cornified envelop, the terminal product of keratinocyte differentiation, is composed of a variety of covalently cross-linked proteins that form a rigid three-dimensional structure. Our present studies show that preparations of intact envelopes prepared from cultured human keratinocytes contain soluble involucrin, keratin, and filaggrin. Sequential extraction with sodium dodecyl sulfate (SDS) and urea followed by sonication produces envelope fragments that are largely free of soluble proteins, including involucrin. Digestion of these highly purified envelope fragments with cyanogen bromide (CNBr) releases a smear of anti-involucrin immunoreactive material (40-180 kDa) with two enriched clusters of bands at approximately 52 and 70 kDa. The 52-kDa band cluster co-migrates with products released by CNBr digestion of purified involucrin. The CNBr-mediated release of discrete (52- and 70-kDa) involucrin-immunoreactive bands suggests that many involucrin molecules may be cross-linked at relatively few glutamyl residues/molecule in envelopes prepared from cultured keratinocytes. Moreover, what appears to be cross-linked involucrin can be localized on highly purified sonicated envelope fragments using colloidal gold electron microscopy. These results provide evidence that involucrin is a cross-linked component of the keratinocyte marginal band.

Electronic theory for screw dislocation motion in dilute b.c.c. transition metal alloys

Abstract Screw dislocation motion in dilute b.c.c. transition metal alloys has been investigated using a tight-binding type electronic theory coupled to the moments approach. The repulsive cort—core interaction energies are simulated by a Born—Mayer potential. Assuming existence of solute enriched clusters (SEC), the Peierls stress of the dilute alloys is calculated and compared with the available experimental data, in relation to the alloy softening. It is found that the Peierls stress of the SEC could be reduced significantly compared with that of the solvent (α-Fe) metal, provided that diagonal disorder due to alloy (SEC) formation is sufficiently large.

An X-Ray Diffraction Study of the Aging Reaction in Two Austenitic Alloys

AbstractAging processes exhibiting cluster to precipitate transitions were studied in polycrystalline line austenitic iron-base alloys with a Siemens' Guinier camera. This camera combines the Seemann-Bohlin focusing geometry with a curved-crystal monochromator arid thus maximizes the resolution of observed sidebands and the weak precipitate lines. Growth studies encompassing a cluster-size range of 15 to 70 unit cells were followed. For the systems of interest, this coincided with a variation from detectable hardness increase to a stage of maximum hardness immediately preceding precipitation, Cluster sizes were calculated on the basis of the Guinier model; variation with time and temperature permitted calculations of an apparent activation energy in the one system where decomposition was spontaneous. An iron-nickeltitanium alloy was used to study aging in a ternary system. Behavior was classic in that the cluster size present on quenching grew with aging coincident with a simultaneous hardness increase. Calculation of activation energies indicated strongly that transportation of nickel to, or iron from, the cluster was rate determining. Upon overaging, the nickel-titanium enriched clusters gave way to the hexagonal Ni2Ti phase. An iron-nickel-chromium-niobium quaternary, in addition to presenting a clustering system similar to the above ternary, showed two rather interesting phenomena. First, chromium was necessary for precipitation; the ternary ironnickel-niobium did not age. Secondly, a stable Pe2Nb Laves phase present upon quenching from 2200°F disappeared on aging in favor of nickel-niobium clusters; an incubation time for the formation of these clusters existed, and its duration was about 4 hr. An asymmetry was noted in the diffraction intensities about the (311)γ line in both systems. In the iron-nickel-titanium case, the asymmetry was only in intensity, whereas, with the iron-nickel-chromium-niobium alloys, the asymmetry existed in both intensity and position. Interpretation of these observations is made on the basis of anticipated variations in scattering factors, lattice spacings, and cluster sizes.

Rate of clustering in Al-Cu alloys at low temperatures

It is found that substantial resistance increases and release of energy occur in rapidly cooled Al-Cu (2 at. % Cu) alloys at temperatures at least as low as −45°C. These property changes are attributed to the formation of copper enriched clusters which are presumed to be Guinier-Preston zones or their precursors. The kinetics of this clustering in the temperature range 25°C to −45°C were investigated. It was found that the isothermal rate of clustering continuously decreases with increasing time and is greatly decreased (by factors of 10–100) by interruption of the quench from the homogenization temperature at 200°C for a few seconds. The activation energy for low temperature clustering is about 12 kcal/g atom which is near that for the annealing out of the quenched-in point defects in pure aluminum. The results are consistent with the hypothesis of Zener and Seitz that the clustering is effected by point defects retained in quenching from a high temperature. However, the dislocation glide mechanism is not yet ruled out.