摘要:

Summary(1) The aetiology of one of the most striking physiological changes occurring during space‐flight, the loss of red blood cells, remains unknown, and its precise time‐pattern in flight has not yet been studied.(2) It is suggested that the changes during space‐flight responsible for loss of red blood cells in man are (a) loss of plasma volume resulting from disappearance of hydrostatic pressure in the circulation during weightlessness and (b) reduced energy expended in maintenance of form, posture and locomotion resulting from elimination of the usual gravitational load on the muscles. Quadrupeds, like rats, would be expected to suffer minimal blood shifts in weightlessness and therefore have an unchanged plasma volume. However, since in weightlessness the activity‐related energy expenditure by the muscles is reduced, the accompanying reduced oxygen demand by the tissues would cause a reduction in erythropoietin levels and so in the production of red blood cells, and a progressive lowering of the total red blood cell mass toward a new steady‐state level.(3) Loss of plasma volume alone does not explain the observed loss of red blood cells in astronauts because, in the three manned Skylab missions, as the duration of the missions increased, loss of red blood cell mass decreased, whereas loss of plasma volume increased. This discrepancy is, however, well accounted for by the above hypothesis by taking into consideration the increased level of exercise of the astronauts as the duration of the mission increased.(4) Though water submersion of human subjects does mimic the effects of weightlessness, such effects were overriden in sea mammals because of adaptation to other factors associated with a life in the sea.(5) From the presented analysis of haemopoietic changes observed in spaceflight, an experiment can be designed for a future flight to uncover the causes and mechanisms of these changes and provide a basis for developing protective measures. Thus, the space environment can be used as an investigative tool to enhance the knowledge of the function of the haemopoietic system, which is a major homeostatic system of man and other ve

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00344.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Summary1. All heavy metals, including those that are essential micronutrients (e.g. copper, zinc, etc.), are toxic to algae at high concentrations.2. One characteristic feature of heavy‐metal toxicity is the poisoning and inactivation of enzyme systems. Many of the physiological and biochemical processes, viz., photosynthesis, respiration, protein synthesis and chlorophyll synthesis, etc., are severely affected at high metal concentrations.3. Some algae inhabit waters chronically polluted with heavy‐metal‐laden wastes from mining and smelting operations;Nodulariasp.,Oscillatoriasp.,Cladophorasp.,Hormidiumsp.,Fucussp. andLaminariasp., etc., occur in metal‐rich waters. These algal forms are probably more capable of combating the toxic levels of heavy metals and this attribute is a result of physiological and/or genetic adaptations. The sensitivity or tolerance to heavy metals varies amongst different algae. The phenomena of multiple tolerance and co‐tolerance may be exhibited by some algae.4. Heavy‐metal pollution causes reduction in species diversity leading to the dominance of a few tolerant algal forms. The primary productivity also decreases after metal supplementation.5. The uptake and accumulation of heavy metals can be active (energy‐dependent), passive (energy‐independent), or both.6. Heavy metals can be safely stored as intranuclear complexes by some algae. Notwithstanding this, some changes in the cell wall can enable the algae to tolerate heavy metals by checking the entry of the metals (exclusion mechanism).7. The metal content of algae growing in a waterbody may yield valuable information for simulating heavy metal pollution: several species ofCladophoraandFucushave been extensively used for this purpose.8. Several factors affect and determine toxicity of heavy metals to algae. At low pH, the availability of heavy metals to algae is greatly increased, as a consequence of which pronounced toxicity is evident. Hard waters decrease metal toxicity. Some ions, e.g., calcium, magnesium and phosphorus, can alleviate toxicity of metals.9. The presence of other metals can influence toxicity of a heavy metal through simple additive effect or by synergistic and antagonistic interactions. Similarly, other pollutants can influence heavy‐metal toxicity.10. The toxicity of heavy metals depends upon their chemical speciation. Various ionic forms of a metal characterized by different valency states, may be differentially toxic to a test alga.11. Amino acids, organic matter, humic acids, fulvic acid, EDTA, NTA, etc. can complex with heavy metals and render them unavailable. This may eventually lead to less toxicity.12. Heavy‐metal toxicity largely depends upon algal population density: the denser the population the more numerous the cellular sites available, leading to

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00345.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Summary1. Three classes of cytotoxic lymphocyte are discussed: thymus‐derived T cells, antibody‐dependent K cells and NK (natural killer) cells. Each of these cytotoxic lymphocytes has receptors allowing the formation of adhesions (contact) with a target cell (the cell to be killed). The type of receptor and the corresponding ligand on the target cell is different in each class. Cytotoxic T cells (and probably NK cells) react with a target cell antigen, in a manner rather like antibody‐antigen reactions (although not involving classical serum antibody). K cells have a receptor for the Fc part of immunoglobulin (IgG) and hence can make contact with antibody‐coated target cells.2. It seems likely that all three classes of cytotoxic lymphocyte have a similar basic mechanism of killing, which is different from the membrane leakage occurring in complement‐mediated lysis. Much more information is available on cytotoxic T cells than on the other types of cell.3. Cytotoxic T cell killing can be divided into two phases. A reversible phase in which the T cell is in contact with the target cell, but causes no apparent damage. This phase can vary from a few minutes up to several hours, when a single T cell interacts with a single target cell. If the T cell detaches or is inactivated the target cell survives. The second phase is irreversible, once the lethal event has occurred, and the target cell will progress to eventual lysis in the absence of the Tc cells.4. The lethal event initiates a period of zeiosis (membrane blebbing) in the target cell, which is accompanied by increased effiux of86rubidium. Cell lysis occurs at a variable time after the initiation of zeiosis, when the soluble contents of the cytoplasm burst out of the target cell. The fact that both these phases are of variable length leads to the accumulation of cytoplasmic markers (such as51chromium) in the medium in an approximately linear fashion.5. The nature of the lethal event is unknown, but it is suggested that it involves changes inside the target cell rather than at the target cell membrane. Remarkable long projections from the T cell (and also seen from K cells and NK cells), apparently arising as a result of the receptor‐ligand interaction, may be involved in the delivery of the l

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00346.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00347.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Summary1. At the behavioural level, cell adhesion is generally non‐specific. The search for molecular mechanisms of adhesion should be conducted on this basis.2. Cells in general, be they from slime moulds or vertebrate epithelia, possess multiple molecular adhesive mechanisms. In epithelial cells this is shown by the number of their different ultrastructurally recognizable intercellular junctions. Elucidation of the structure and composition of such intercellular junctions will make a valuable contribution to the understanding of cell adhesion.3. The measurement of cell adhesion is fraught with difficulties. Commonly used assays by aggregation cannot give a true representation of the normal adhesive interactions of cells in tissues, and the results they yield must be interpreted with caution. This is because it takes dissociated tissue cells up to 24 h to develop their full adhesiveness after making initial contact.4. Cell‐cell adhesion probably depends largely upon the interaction of complementary molecules on adjacent cell surfaces. Glycoproteins seem the most likely candidates but, as yet, there is no compelling evidence in any individual case and mechanisms of cell adhesion still remain obsc

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00348.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00349.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Summary(1) Ciliated protozoa are viewed as unicellular organisms structured in a hierarchy of organizational levels that include the macromolecular, suborganellar, unit organellar, organellar complex, and organellar system.(2) The ciliate cortex is divided into two major functional regions, the somatic region and the oral region. The fundamental component of the cortex is an organellar complex, the kinetid, whose organizing centre is the kinetosome with which are associated three fibrillar associates diagnostic of ciliates. These three fibrillar associates are the periodically striated kinetodesmal fibril and two microtubular ribbons, the transverse and postciliary ribbons.(3) Somatic and oral kinetids are found to be of three major types: monokinetids are composed of one kinetosome and its fibrillar associates; dikinetids are composed of two kinetosomes and their fibrillar associates; polykinetids are composed of more than two kinetosomes and their fibrillar associptes.(4) The mechanisms underlying kinetid function and development remain largely unexplored. Research into the molecular biology and ultrastructure, especially of mutant forms, should provide basic insights in the near future.(5) The conservation of kinetid structure across major phyla of organisms suggests that this subcellular structure should be useful in phylogenetic analysis despite the concepts of ‘chemical identity’ and ‘organic design’.(6) The evolutionary rate of change of oral features is greater than that of somatic features, probably due to developmental and ecological factors. Nevertheless both cortical regions are constrained by the phenomenon of structural conservatism; that is, the conservation of structure through time is inversely related to the level of biological organization.(7) Eight major groupings of ciliate species are recognized, based on ultrastruc‐tural features of the cortex. Several examples of differences between these eight groups and the groups presently recognized are

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00350.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1981.tb00351.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY