Initial Cluster Formation Research Articles

Emergence and refinement of clustered horizontal connections in cat striate cortex

Pyramidal cells in layer 2/3 of adult cat striate cortex have long, intrinsic horizontal axon collaterals within both layer 2/3 and layer 5. These collaterals form periodic "clusters" of finer axon branches that link columns of similar orientation selectivity. We have investigated the sequence of events and possible mechanisms underlying the development of these clustered intrinsic horizontal connections using a combination of neuronal tracers and intracellular staining. Small injections of fluorescent latex microspheres made during the first postnatal week (at P4-6), when examined in tangential sections, produced an even, unclustered distribution of retrogradely labeled cells up to 2 mm from the injection site. At P8, retrograde labeling extended over a larger area and clustering was discernible, primarily among the most distant labeled cells. At both P6 and P8, labeling was similar in layers 2/3 and 5, indicating that the transition from clustered to unclustered connections occurred simultaneously for cells in superficial and deep laminae. By the end of the second postnatal week (P12-15), retrogradely labeled cells were far more clustered both within and beyond the extent of P6 label; the density of labeled cells was high throughout the labeled region, but much higher within clusters. The periodicity of these nascent clusters was similar to that in the adult. Despite obvious clustering, the pattern of retrograde label observed following injections at 2-3 weeks (P12-21) differed markedly from the adult, in that the regions between clusters contained many labeled cells. Over the next 3 weeks, the connections were refined, so that by the sixth postnatal week (P36-38), regions between clusters contained very few retrogradely labeled cells and the overall pattern of retrograde label was indistinguishable from that in adults. Despite differences in postmigratory ages of neurons from the superficial and deep laminae, clustering of retrogradely labeled cells from these 2 populations was similar at all ages. Experiments in which 2-3 weeks elapsed between the time microsphere injections were made and animals were killed demonstrated that neither the initial formation of crude clusters nor their refinement was due to cell death. Instead, cluster refinement resulted from specific process elimination. When a red microsphere injection at P15 was followed by a green microsphere injection at exactly the same location on P29, the earlier injection resulted in crude clustering, as expected. Virtually all of the cells double-labeled by the later injection were within the densest clusters of label from the early injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesizing Statistical Knowledge from Incomplete Mixed-Mode Data

The difficulties in analyzing and clustering (synthesizing) multivariate data of the mixed type (discrete and continuous) are largely due to: 1) nonuniform scaling in different coordinates, 2) the lack of order in nominal data, and 3) the lack of a suitable similarity measure. This paper presents a new approach which bypasses these difficulties and can acquire statistical knowledge from incomplete mixed-mode data. The proposed method adopts an event-covering approach which covers a subset of statistically relevant outcomes in the outcome space of variable-pairs. And once the covered event patterns are acquired, subsequent analysis tasks such as probabilistic inference, cluster analysis, and detection of event patterns for each cluster based on the incomplete probability scheme can be performed. There are four phases in our method: 1) the discretization of the continuous components based on a maximum entropy criterion so that the data can be treated as n-tuples of discrete-valued features; 2) the estimation of the missing values using our newly developed inference procedure; 3) the initial formation of clusters by analyzing the nearest-neighbor distance on subsets of selected samples; and 4) the reclassification of the n-tuples into more reliable clusters based on the detected interdependence relationships. For performance evaluation, experiments have been conducted using both simulated and real life data.

Ultraviolet irradiation of canine dendritic cells prevents mitogen-induced cluster formation and lymphocyte proliferation.

The objectives of this study were to characterize canine peripheral blood dendritic cells (DC), to compare their functional role in lectin (Con-A) induced and oxidative (NaIO4-induced) mitogenesis, and to investigate the effect of ultraviolet (UV) irradiation on DC function. We used, sequentially, Ficoll-Hypaque fractionation, discontinuous percoll gradients, rosette, sedimentation with human red blood cells, adherence to plastic and cytofluorometric sorting after labeling with an anti-Ia monoclonal antibody (7.2) to obtain cells of low buoyant density that were rosette-negative, plastic-non-adherent, Ia-positive, nonspecific-esterase-negative, and nonphagocytic. By transmission and scanning electron microscopy these cells had large irregular nuclei, numerous spherical mitochondria, elongated cytoplasmatic processes, and lacked phagosomes. These features clearly separate these cells from canine monocytes and characterize them as DC. These cells were capable of reconstituting the proliferative responses of accessory-cell-depleted autologous lymphocytes to both Con-A and NaIO4. The responses to both mitogens showed a similar time course: an initial cluster formation of lymphocytes with DC was followed by lymphocyte blastogenesis (48-72 hr) and proliferation (72-96 hr). UV irradiation of DC before culture completely abrogated accessory activity for the response to both Con-A and NaIO4, and neither cluster formation nor blast transformation or proliferation occurred. These data provide evidence that a UV-sensitive step of direct interaction of lymphocytes and DC is involved in the mechanism of T cell activation and proliferation.

Surface topography of (100)-type electro-faceted platinum from scanning tunnelling microscopy and electrochemistry

The surface of a number of metals such as platinum, gold, rhodium and palladium, which are of particular interest in catalytic and electrocatalytic processes, either as polycrystalline or single-crystal specimens, can be preferentially orientated by a procedure known as electrochemical faceting1–3. Here we report the first real-space observation in the nanometric range of electrochemically faceted (100)-type Pt electrodes, using scanning tunnelling microscopy (STM)4. Our findings confirm the existence of a faceted surface consisting of flat parts surrounded by ridge-type structures. At the initial stages cluster formation is observed, which subsequently develops in a preferential orientation. This constitutes direct evidence that faceting occurs through a selective electrodissolution/electrodeposition process, which probably involves a fast diffusion of metallic species either in the electrolyte or on the surface.

The strength, fracture toughness, and low cycle fatigue behavior of 17-4 PH stainless steel

The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria, the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix interfaces.

Precipitation in the Fe-Mo and Fe-Au systems

Abstract A general hypothesis of atom size effects for G.P. zone formation is discussed in this paper and results are presented of precipitation in the systems Fe-Au and Fe-Mo. Techniques used are resistivity measurements and electron microscopy. In the Fe-Mo system it is shown that after initial cluster formation during the early stages of ageing after the quench, further growth ceases and vacancies anneal out into dislocation loops. The activation energy for the initial clustering was 1·3 ev whilst the excess vacancy concentration estimated from the loop area gave a vacancy formation energy of ∼1·5 ev. These results confirm recent quenching results on pure iron. The refusal of Mo zones to grow confirms the general hypothesis of size effects. In the Fe-Au system precipitation of gold platelets after quenching is shown to be associated with the high density of dislocations formed during quenching through the phase transformation of the iron matrix. The effect of the dislocations is shown by the low value ...