The biology of primordial germ cells

Primordial germ cells are the founder cells of the gametes. They arise at the earliest stages of development, and migrate to the genital ridges, where they join the somatic cells of the future gonad. The factors that determine their formation and migration are largely unknown. Primary culture of PGCs isolated at different times during their migration has allowed the analysis of some aspects of the control of their behaviour. The effects of culture medium conditioned by genital ridges suggested that both proliferation and migration of PGCs may be controlled in part by diffusible factors. Several purified growth factors are now known which affect PGC numbers in culture. These include positive and negative regulators of proliferation, survival factors, and chemotropic factors. During migration, PGCS must change their affinities for surrounding cells, and some aspects of this can be analysed in culture. For example, PGCs isolated at different stages in their migration show different capacities to adhere to fibronectin.     

Rapid and long-lasting suppression of the ATF-2 transcription factor is a common response to neuronal injury

To determine if intestinal stromal cells secrete diffusible factors such as insulin-like growth factors (IGFs) capable of regulating epithelial cell growth in vitro, stromal cells were isolated by enzymatic digestion of rat intestine. Incorporation of [3H]thymidine into DNA and [14C]leucine into protein of IEC-6 cells, a model intestinal epithelial cell line, was significantly increased (two- to threefold) when the IEC-6 cells were co-cultured with stromal cells, relative to IEC-6 cells grown alone. Medium conditioned by stromal cells stimulated DNA synthesis of IEC-6 cells in a dose-dependent manner. Analysis of the conditioned medium revealed that intestinal stromal cells secreted IGF-I, but little IGF-II, in addition to an M(r) 32,000 IGF-binding protein (IGFBP-2) and an IGFBP having M(r) approximately 24,000. We conclude that rat intestinal stromal cells secrete one or more diffusible factors, which may include IGF-I and IGFBPs, capable of stimulating proliferation of IEC-6 cells in vitro.     

Microglial ramification requires nondiffusible factors derived from astrocytes

It is generally accepted that process-bearing microglial cells origina te from ameboid macrophage-like mesodermal cells. This transformation, often called ramification, accompanies down regulation of macrophage-like properties, but the mechanisms involved in ramification have not been clarified. We investigated factors to promote ramification in culture. Isolated ameboid microglial cells were seeded on living or paraformaldehyde-fixed astrocyte monolayers. About 80% of the cells ramified on the fixed astrocytes in astrocyte-conditioned medium as well as on the living astrocytes. In fresh culture medium, 50% of the cells on the fixed astrocytes ramified. On the other hand, ameboid cells rarely ramified on noncoated glass coverslips even in the conditioned medium. Ameboid cells cultured on extracellular matrices dervied from astrocytes ramified more than on those coated with plasma fibronectin or collagen type I. A synthetic peptide containing Arg-Gly-Asp sequence or a tyrosine kinase inhibitor genistein partially reversed the ramification induced on the fixed astrocyte monolayers. These results show that some nondiffusible factors derived from astrocytes are essential for microglial ramification. A part of the nondiffusible factors are present in the extracellular matrices, and the effects might be mediated by integrins. Some diffusible factors secreted by astrocytes seem to promote ramification, if the nondiffusible factors are present. The experiments using the fixed astrocyte monolayers may be useful to identify the diffusible factors responsibile for ramification.     

Diet and risk of colon cancer in a large prospective study of older women: an analysis stratified on family history (Iowa, United States)

The aim of this study was to determine if diffusible angiogenic growth factors were released in human dental pulp during orthodontic tooth movement. These factors, if diffusible, could induce angiogenesis in other tissues, and may then be isolated and identified. The pulps from 14 premolar teeth treated with straight wire fixed orthodontic appliances for 2 weeks were compared with those of 14 untreated control premolar teeth from the same subjects. Following tooth extraction and sectioning, 1-mm horizontal sections of pulp tissue were embedded in collagen with 1-mm sections of rat aorta and co-cultured in growth media for up to 4 weeks. Sections of rat aorta alone were also cultured. Angiogenic changes in the form of microvessel growth were observed by light microscopy. Microvessel identification was confirmed by electron microscopy and by immunohistochemistry using staining for factor VIII-related antigen marker for endothelial cells. When compared at days 5, 10, and 14 of co-culture, the number of microvessels was significantly greater in the pulps from orthodontically moved teeth than in those from the control teeth. The number of rat aorta microvessels was also significantly greater when co-cultured with pulp from orthodontically moved teeth than with pulp from control teeth and when compared with control cultures of rat aorta alone. There were no significant differences in microvessel numbers between the rat aorta co-cultured with pulp from control teeth and control cultures of rat aorta alone. These results indicate an increase in angiogenic growth factors in the pulp of orthodontically moved teeth, and the enhanced response of the rat aorta when co-cultured with this pulp shows that these factors appear to be diffusible.     

焊缝金属中扩散氢的形成及控制研究进展

 随着焊接技术的广泛应用,焊接接头冷裂纹的形成也备受关注。焊缝中的扩散氢是导致焊缝区产生氢致裂纹的重要原因之一。详细综述了近年来焊缝金属中扩散氢的来源、产生和扩散氢质量分数测定方法的研究进展,总结了影响扩散氢质量分数的有关因素。着重分析了氢在焊接接头的动态扩散行为,对扩散氢的危害和预防措施进行总结,并对相关研究存在的问题和进一步的研究方向进行概括。     

Mathematical guidance for axons

Axon guidance by gradients plays an important role in wiring up the developing nervous system. Growth cones seem to sense a concentration difference across their spatial extent, and convert this into a signal to move up or down a gradient. In this article, a simple mathematical framework is developed to understand when and where gradient detection can occur as a function of gradient shape. This framework is applied to two examples:the guidance of axons by target-derived diffusible factors in vivo and in collagen gels, and guidance by substrate-bound gradients of optimal shape, as might be relevant in the retinotectal system.Two distinct spatial limits on guidance emerge: I mm for a target-derived diffusible gradient, and I cm for a substrate-bound gradient.     

Effects of 1-year treatment with ipriflavone on bone in postmenopausal women with low bone mass

Genetic mosaic maize plants that contained dwarf1 sectors in otherwise normal plants were analyzed. The analysis of dwarf1 sector growth relative to the surrounding wild-type tissue should test the hypothesis that Dwarf1 controls the production of a diffusible factor. Maize dwarf1 sectors did not show altered growth relative to the surrounding tissue, indicating that Dwarf1 controls the production of a diffusible factor. As Dwarf1 has been proposed to control the conversion of GA20 to biologically active GA1, these results suggest that GA, itself is the diffusible factor.     

Time courses of calcium and calcium-bound buffers following calcium influx in a model cell

Fixed and diffusible calcium (Ca) buffers shape the spatial and temporal distribution of free Ca following Ca entry through voltage-gated ion channels. This modeling study explores intracellular Ca levels achieved near the membrane and in deeper locations following typical Ca currents obtained with patch clamp experiments. Ca ion diffusion sets an upper limit on the maximal average Ca concentration achieved near the membrane. Fixed buffers restrict Ca elevation spatially to the outermost areas of the cell and slow Ca equilibration. Fixed buffer bound with Ca near the membrane can act as Ca source after termination of Ca influx. The relative contribution of fixed versus diffusible buffers to shaping the Ca transient is determined to a large extent by the binding rate of each buffer, with diffusible buffer dominating at equal binding rates. In the presence of fixed buffers, diffusible buffers speed Ca equilibration throughout the cell. The concentration profile of Ca-bound diffusible buffer differs from the concentration profile of free Ca, reflecting theoretical limits on the temporal resolution which can be achieved with commonly used diffusible Ca indicators. A Ca indicator which is fixed to an intracellular component might more accurately report local Ca concentrations.     

Induction of axon-like processes from axotomized retinal ganglion cells of adult hamsters after intravitreal injection of sciatic nerve exudate

We have shown previously that intravitreous transplantation of a peripheral nerve (PN) graft stimulates a subpopulation of retinal ganglion cells (RGCs) to sprout axon-like processes. The present study examined whether the effect of PN graft was due to diffusible factors released from the graft. Injection of sciatic nerve exudate into vitreous of the eye induced the formation of axon-like processes. Significantly more RGCs with axon-like processes were stained after injection of exudate from the distal stump than the proximal stump of the transected sciatic nerve. Thus, our results showed that trophic factors released from the intravitreous PN can induce the formation of axon-like processes and that more trophic factors are secreted from the distal than the proximal stump of the transected sciatic nerve.     

焊缝中氢的扩散行为及影响因素

 扩散氢会在焊缝中引起氢脆、延迟裂纹等，导致结构产生低应力断裂。为了研究氢的扩散行为，采用水银法和气相色谱法测定了逸出的扩散氢量，并采用真空抽取法测试了不同温度下残余氢的释放量。试验表明，扩散氢量不受焊道数量的影响，它的逸出时间随焊道数的增多而增长，逸出速度随合金含量的增多而降低。随着焊后冷速的降低，冷却过程中逸出的氢增多，测定出的扩散氢量减少；测氢试样在100~200℃保温时，逸出氢的总量变化不大，但逸出时间随温度的升高而明显缩短。残余氢量与扩散氢量的多少无关，它与焊缝的含氧量、组织和硬度等有关系。     

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.     

Patterning of mammalian somites by surface ectoderm and notochord: evidence for sclerotome induction by a hedgehog homolog

An early step in the development of vertebrae, ribs, muscle, and dermis is the differentiation of the somitic mesoderm into dermomyotome dorsally and sclerotome ventrally. To analyze this process, we have developed an in vitro assay for somitic mesoderm differentiation. We show that sclerotomal markers can be induced by a diffusible factor secreted by notochord and floor plate and that heterologous cells expressing Sonic hedgehog (shh/vhh-1) mimic this effect. In contrast, expression of dermomyotomal markers can be caused by a contact-dependent signal from surface ectoderm and a diffusible signal from dorsal neural tube. Our results extend previous studies by suggesting that dorsoventral patterning of somites involves the coordinate action of multiple dorsalizing and ventralizing signals and that a diffusible form of Shh/Vhh-1 mediates sclerotome induction.     

A study on cold cracking susceptibility in high strength steel fillet weld joints

Controlled thermal severity (CTS) test which simulates the conditions in a single-pass fillet welding was carried out to determine the suitable minimum preheating temperature for the cold-crack-free welding of high strength steel ASTM A 516-70. The dependence of this minimum preheating temperature on diffusible hydrogen contents in weld metal was clarified. Then, the effect of climatic conditions on cold cracking susceptibility as a function of diffusible hydrogen contents in weld metal was studied. It is found that the cold cracking susceptibility of high strength steel is primarily related to the microsturcture of the HAZ which, in turn, is related to the preheating temperature. A lower preheating level, which resulted in harder microstructure, led to increased susceptibility. Suitable minimum preheating temperature required to prevent cold cracking increased with the increase in diffusible hydrogen contents of weld metal as a function of climatic conditions.     

The effects of thrombocytopenia on the activated coagulation time

Cortical efferents grow from deep cortical layers to innervate numerous subcortical structures late in embryogenesis. The mechanisms that control their development are poorly understood. We co-cultured organotypic embryonic cortical explants with other tissues, maintaining a distance between them to avoid contact-mediated interactions. At embryonic day 15, when the cortical plate comprises only cells of the deep cortical layers, outgrowth from cultured cortex was stimulated by co-cultured subcortical structures, but not by additional cortex or liver. These data support the hypothesis that diffusible factors from subcortical structures, and not from the cortex itself, enhance cortical efferent growth.     

Patterning of cortical efferent projections by semaphorin-neuropilin interactions

Cortical neurons communicate with various cortical and subcortical targets by way of stereotyped axon projections through the white matter. Slice overlay experiments indicate that the initial growth of cortical axons toward the white matter is regulated by a diffusible chemorepulsive signal localized near the marginal zone. Semaphorin III is a major component of this diffusible signal, and cortical neurons transduce this signal by way of the neuropilin-1 receptor. These observations indicate that semaphorin-neuropilin interactions play a critical role in the initial patterning of projections in the developing cortex.     

Active efflux and diffusion are involved in transport of Pseudomonas aeruginosa cell-to-cell signals

Many gram-negative bacteria communicate by N-acyl homoserine lactone signals called autoinducers (AIs). In Pseudomonas aeruginosa, cell-to-cell signaling controls expression of extracellular virulence factors, the type II secretion apparatus, a stationary-phase sigma factor (sigmas), and biofilm differentiation. The fact that a similar signal, N-(3-oxohexanoyl) homoserine lactone, freely diffuses through Vibrio fischeri and Escherichia coli cells has led to the assumption that all AIs are freely diffusible. In this work, transport of the two P. aeruginosa AIs, N-(3-oxododecanoyl) homoserine lactone (3OC12-HSL) (formerly called PAI-1) and N-butyryl homoserine lactone (C4-HSL) (formerly called PAI-2), was studied by using tritium-labeled signals. When [3H]C4-HSL was added to cell suspensions of P. aeruginosa, the cellular concentration reached a steady state in less than 30 s and was nearly equal to the external concentration, as expected for a freely diffusible compound. In contrast, [3H]3OC12-HSL required about 5 min to reach a steady state, and the cellular concentration was 3 times higher than the external level. Addition of inhibitors of the cytoplasmic membrane proton gradient, such as azide, led to a strong increase in cellular accumulation of [3H]3OC12-HSL, suggesting the involvement of active efflux. A defined mutant lacking the mexA-mexB-oprM-encoded active-efflux pump accumulated [3H]3OC12-HSL to levels similar to those in the azide-treated wild-type cells. Efflux experiments confirmed these observations. Our results show that in contrast to the case for C4-HSL, P. aeruginosa cells are not freely permeable to 3OC12-HSL. Instead, the mexA-mexB-oprM-encoded efflux pump is involved in active efflux of 3OC12-HSL. Apparently the length and/or degree of substitution of the N-acyl side chain determines whether an AI is freely diffusible or is subject to active efflux by P. aeruginosa.     

水银法测定焊缝扩散氢的影响因素

采用水银法研究了环境湿度、烘干温度、保护气体露点等测试条件对焊缝扩散氢含量的影响,分析了我国碳钢及低合金风焊条中的扩散氢含量,并且与国外焊条的扩散含量进行了对经     

Diffusion in axon guidance

Axon guidance by target-derived diffusible factors plays an important role in the development of the nervous system. This paper considers the constraints imposed on this process by the mathematics of diffusion. A point source continuously producing a factor into an infinite three-dimensional volume is considered as a model for both the in vivo and in vitro situation. Basic constraints for effective guidance are assumed to be that the concentration falls between certain maximum and minimum limits, and that the percentage change in concentration across the width of the growth cone exceeds a certain minimum value. The evolution of the shape of the gradient over time is analysed. Using biologically reasonable parameter values, it is shown that the maximum range over which growth cone guidance by a diffusible factor is possible for large times (several days) after the start of the production of the factor is 500-1000 microm. This maximum distance is independent of the diffusion constant of the diffusing molecule, applies to both chemoattractants and chemorepellents, and agrees with experimental data. At earlier times, however, the constraints may be satisfied for distances up to several millimetres. The time it takes for this maximum guidance distance to fall to the asymptotic value depends on the diffusion constant. This time is a few hours for a small molecule but as much as a few days for a large molecule. The model therefore predicts that guidance over distances larger than 1000 microm is possible if the start of production of the factor is carefully matched to the time when guidance is required.     

Surround repulsion of spinal sensory axons in higher vertebrate embryos

We have tested whether the orientation of axons sprouting from bipolar dorsal root ganglion neurons is influenced by diffusible cues from surrounding tissues. Surface ectoderm, dermomyotome, and notochord exert strong chemorepulsion on axons growing in collagen gels, operating at separations beyond those found in vivo and active in cocultures of chick and mouse tissues. Basal and alar plates of the neural tube are devoid of activity, as is the posterior-half-sclerotome, which repels in a contact-dependent manner. When ganglia are sandwiched between dermomyotome and notochord placed at a distance, axon growth is channeled in a bipolar trajectory. These results show that gradients of diffusible repulsion molecules flanking axon pathways can generate linear patterns of axon growth. We suggest that such "surround repulsion" may function generally, in concert with contact-dependent guidance mechanisms, to guide axons in the developing nervous system.