摘要:

SummaryPast knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged.1. The resorption of bone is the result of successive steps that can be regulated individually.2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone.3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation.4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells.5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO, by carbonic anhydrase and secreted by an ATP‐driven vacuolar H+‐K+‐ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine‐proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone.6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption.7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and end

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb00996.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb00997.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Summary1. Aphids are notorious pests of world agriculture. Even so, uncertainty persists as to their capacity for successful aerial dispersal. Evidence exists that, under some conditions, aphids can be wind‐borne over long distances, i.e. hundreds of kilometers over desert or sea. It has been argued, in the recent past, that this phenomenon may be part of a strategy to locate fresh host plants in new distant areas. However, the proportion of these insects successfully colonizing new hosts is unknown.2. Other work using meteorological backtracking has also indicated long‐distance movement, but the accuracy of such predictions is dubious unless the altitude of transport is known. Mark‐releaseecapture experiments with such small insects have limited potential due to large dilution effects. Static ‘snap‐shots’ of demographic population densities, using suction traps, cannot accurately distinguish local aerial density fluxes and population movements from a distance. However, genetic and physiological markers may provide more direct information on population mixing; for example, some allozyme studies have shown a limited level of inter‐population gene flow.3. Under suitable conditions, aphids take off, maintain flight and alight in response to the appropriate visual and olfactory cues. Undoubtedly successful long‐distance movement occurs from time to time, but its ecological relevance may have been overstated in the past. It may be selectively disadvantageous for aphids to move from areas containing their host plants. In contrast, it is advantageous for aphids to maximise their chances of survival and reproductive success by landing on suitable plant hosts at the earliest opportunity.4. The clonal nature of aphids (a single genotype may comprise vast numbers of individuals) means that there may be advantages to phenotypic variation between individuals in the readiness to move. Recent evidence indicates that such a variation exists in the duration of the behavioural migratory phase, the initial period of maiden flight when host‐plant cues are ignored and when landing is inhibited.5. The relative biological importance of short‐us. long‐distance movements is reassessed with reference to plant virus epidemiology and the spread of new genotypes, e.g. insecticide resistance. It is concluded that the biological relevance of short‐distance movements have a much greater impact on population and genotype distribution than long‐distance movements, which may be

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb00998.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Summary1. Natural populations are exposed to environmental stress of varying intensities. This provides a reference point for extrapolations from the living biota to fossils and vice versa.2. Evolutionary change is likely when there are resources in excess of maintenance and survival needs. It is largely precluded at species borders by the metabolic costs of stress; from this follows climatic tracking by species.3. A relatively small increase in abiotic stress could underlie extinctions of stress‐sensitive endemic species and the spread of stress‐resistant generalist and widespread species. Widespread fossil species appear resistant to extinction under the stress level of normal background extinctions.4. Synergistic interactions among generalized stresses should increase the likelihood of extinctions, especially for stresses with energetic consequences.5. Some marine organisms survived the K‐T mass extinction event because of stress‐evasion mechanisms such as stress‐resistant life‐cycle stages with low metabolic rates.6. In moderately stressed and narrowly fluctuating environments, sufficient genetic variability and metabolic energy should be available to permit adaptation. In these environments phyletic gradualism is expected.7. In highly stressed and widely fluctuating environments, a punctuated evolutionary pattern is expected whereby stasis occurs most of the time.8. Evolutionary patterns therefore can vary depending on the details of the interaction between stress, environmental fluctuations, energy availability and genetic variability.9. Little evolutionary change is expected when the availability of energy is severely restricted. Examples include cave animals in stable but stressed environments and ‘living fossils’ in widely fluctuating but stressed environments.10. Since the primary effect of abiotic stress may be at the level of energy carriers, a reductionist approach permits generalisations in considering extinctions and conditions under which diversific

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb00999.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01000.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY