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排序方式: 共有590条查询结果,搜索用时 15 毫秒
1.
Anulus fibrosus in bulging intervertebral disks 总被引:1,自引:0,他引:1
In this investigation the association of radial tears of the anulus fibrosus and bulging of the intervertebral disk was studied. An index of disk bulging was measured in sagittal anatomic sections in 149 lumbar disks from 31 cadavers. The indexes of disk bulging were correlated with stages of disk development and the presence of an annular tear. The largest disk-bulging indexes were always associated with radial tears of the anulus. Eighty-four percent of the disks with radial tears had disk-bulging indexes greater than 2.5 mm. Most normal adult disks had an index of less than 2.5 mm. The results challenge the concept that the anulus fibrosus is intact in bulging disks, although ruptured in herniated disks. 相似文献
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Physical characteristics of the ECAT EXACT3D positron tomograph 总被引:9,自引:0,他引:9
Spinks TJ Jones T Bloomfield PM Bailey DL Miller M Hogg D Jones WF Vaigneur K Reed J Young J Newport D Moyers C Casey ME Nutt R 《Physics in medicine and biology》2000,45(9):2601-2618
The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. A single time frame of data without any compression is very large (> 300 Mbyte) and two approaches are employed to overcome this difficulty: (a) adjacent sinograms can be summed automatically into different combinations and (b) listmode (event-by-event) acquisition has been instituted, which is both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional side shielding. The mean spatial resolution is 4.8 +/- 0.2 mm FWHM (transaxial, 1 cm off-axis) and 5.6 +/- 0.5 mm (axial, on-axis). Its absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively. The rate of random events in blood flow studies in the brain and body, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding. 相似文献
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CDK2 activity is regulated by phosphorylation/dephosphorylation, subcellular localization, cyclin levels, and cyclin dependent kinase inhibitors (CKIs). Using Xenopus egg extracts, we find that degradation of Xic1, a Xenopus p21(cip1)/p27(kip1) family member, is coupled to initiation of DNA replication. Xic1 turnover requires the formation of a prereplication complex (pre-RC). Additionally, downstream initiation factors including CDK2, Cdc7, and Cdc45, but not RPA or DNA polymerase alpha, are necessary for activating the degradation system. Xic1 degradation is attenuated following completion of DNA replication. Unlike degradation of p27(kip1) in mammalian cells, CDK2 activity is not directly involved in Xic1 degradation and interactions between Xic1 and CDK2/cyclin E are dispensable for Xic1 turnover. Interestingly, a C-terminal region (162-192) of Xic1 is essential and apparently sufficient for triggering Xic1 ubiquitination prior to degradation. These observations demonstrate that a direct link exists between DNA replication and CKI degradation. 相似文献
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Two natural populations ofDrosophila melanogaster selected for fast and slow female remating (Gromko and Newport,Behav. Genet.18: 621–632, 1988) were analyzed for correlated responses to selection. Several of the correlated responses differed between the lines derived from the two different source populations. In particular, the degree to which female receptivity to remating was dependent on the number of sperm in storage ceptivity to remating was dependent on the number of sperm in storage remained high in one pair of fast and slow selected lines but not in the other. In the line which retained sperm dependence (JEFFERS), fast and slow remating was achieved by changes in the threshold at which remating occurred. In the other line (COMP) changes in the temporal pattern of sperm use were seen. Thus both the level of the receptivity threshold and the existence of a sperm effect appeared to be selectable features of this species. Mated female attractiveness and receptivity were shown to be genetically distinct from virgin female attractiveness and receptivity. Effects of the genotype of the female on copulation duration were found.This work was supported by NSF Grants 82-01183 and 84-18934. 相似文献
10.
B. Somasundaram A. Newport R. Tregear 《Journal of muscle research and cell motility》1989,10(5):360-368
Summary Single glycerol-extracted rabbit psoas muscle fibres have been slowly extended either in rigor or in the unhydrolysable ATP analogue AMPPNP, and their sarcomere length, sarcomere structure and tension measured. The length of regularly arrayed sarcomeres, measured by optical diffraction, increased continuously as the muscle was stretched; the maximum sarcomere extension seen was approximately 6%. In the electron microscope sarcomeres from extended muscle fixed in rigor or AMPPNP remained regular in their internal structure, without rupture or obvious lengthening around the Z line. During steady extension at 0.024% per min the tension in the muscle fibre rose until it reached a limiting value [T
m
] when the sarcomeres had stretched by 0.8–1.6% and then remained constant with continued extension, while the sarcomeres continued to stretch. Provided that a novel form of preparation of the glycerol-extracted fibres was employed,T
m
in rigor was a large fraction of the tension expected from an active isometric muscle fibre. In the presence of AMPPNPT
m
was reduced by a factor of 2 to 3. Step extension by 0.08% at 5-min intervals gave the same pattern of mechanical response with similar values ofT
m
. The isometric tension decay in the interval between the steps was very rapid at first and slowed continuously until the next step. The average speed of tension fall between 30 and 300 s after stretch was measured at each step and plotted relative to the tension in the muscle. The relationship approached linearity, although with a significant upward curvature at high tension. The proportionality constant of the rate of tension fall to tension was 4.5×10–4 s–1 in rigor and 9×10–4 s–1 in AMPPNP. These values are less than the apparent dissociation rate constants for acto-subfragment-1 or acto-heavy meromyosin under similar conditions (Marston, 1982). These results indicate that interfilament slip does occur in rabbit skeletal muscle both in rigor and in AMPPNP, but that it is much slower that that predicted from the behaviour of the isolated proteins, as if the myosin heads interacted so as to obstruct their detachment. 相似文献