bluuebunny/crossref_metadata_2025_split huggingface dataset:
SIALIC ACID AND THE SOCIAL BEEIAVIOUR OF CELLS
Authors: LLOYD CLIVE W. 
Published: January 2008

Summary1. It is suggested that specific carbohydrate side‐chains of membrane glycoproteins are the sites for cell recognition or adhesion when the terminal sugar, sialic acid, is absent.2. It is suggested that sialic acid plays a ‘protective’ or ‘blocking’ role in cell interactions so that addition of sialic acid to asialo side‐chains converts them to forms inactive for recognition. This principle of ‘blocking’ by sialic acid has been observed in other situations as in covering tumour antigens and in protecting glycoproteins from uptake by the liver. It is here extended to cell‐cell adhesions.3. It is to be expected that specific ‘protective’ actions of sialic acid in membrane‐bound glycoproteins will be difficult to detect. As a charged residue, sialic acid is likely to have a strong influence both on the glycoproteins on which it is borne and on their interactions with each other at the cell surface. Removal of sialic acid by enzymes could therefore perturb the structure of the cell surface in several ways and so obscure the ‘protective’ effects of sialic acid. Sialic acid is therefore suggested to have a structural role also.4. Evidence is assembled in favour of a model in which sialysation of specific adhesive receptors affects the social behaviour of cells. This may be an effect associated with growing cells since the contact properties of mitotic cells (and populations rich in dividing cells) are decreased by the increased sialysation of receptors. One of the factors associated with malignant behaviour could be that adhesive receptors are permanently blocked by sialic acid.5. A schematic representation of some of the points is given in Fig. 4.
GENETIC STUDIES OF NITRATE ASSIMILATION IN ASPERGILLUS NIDULANS
Authors: COVE D. J. 
Published: January 2008

Summary(1) In Aspergillus nidulans, at least 16 genes can mutate to affect the reduction of nitrate to ammonium, a process requiring two enzymes, nitrate reductase and nitrite reductase.(2) niaD is the only gene whose effects on enzyme structure are confined to nitrate reductase alone. It specifies a core polypeptide, one or more of which form the basic subunit of nitrate reductase, molecular weight 50000.(3) At least five cnx genes together specify a molybdenum co‐factor, necessary for the activity of nitrate reductase, and of xanthine dehydrogenases I and II. The cnxH gene specifies a polypeptide component of this co‐factor, and the cnxE and F gene products are involved in co‐factor elaboration, The role of the remaining cnx genes is at present unknown.(4) Functional nitrate reductase has a molecular weight of 200000 and is likely to consist of four subunits, together with one or more molecules of the cnx‐specified co‐factor.(5) The co‐factor plays a catalytic role in the aggregation of nitrate‐reductase subunits.(6) The niiA gene is the structural gene for nitrite reductase.(7) Other genes affecting nitrate assimilation are either regulatory or bring about their effects indirectly.(8) Of the genes affecting nitrate assimilation, close linkage is found only between the niiA and niaD genes.(9) Nitrate and nitrite reductases are subject to control by nitrate induction and ammonium repression.(10) Nitrate induction is mediated by the nirA gene whose product must be active for the niiA and niaD genes to be expressed. Since most niaD mutants produce nitrite reductase constitutively, it is likely that the nirA gene product is normally inactivated by nitrate reductase, but only when the latter is not complexed with nitrate,(11) Ammonium repression is mediated by the areA gene, whose product must be active for the expression of the niiA and niaD genes, and which is inactive in the presence of ammonium.(12) The tamA gene may function similarly to the areA gene, both gene products being necessary for the expression of the niiA and niaD genes.(13) Although the niiA and niiD genes are probably contiguous, they are not likely to be organized into a structure equivalent to a bacterial operon.(14) Whereas the areA and nirA genes regulate the synthesis of nitrate and nitrite reductases, it is probable that at least nitrate reductase is also subject to post‐translational control, the presence of active enzyme being correlated with high levels of NADPH.(15) The regulation of the pentose‐phosphate pathway, of mannitol‐I‐phosphate dehydrogenase and of certain activities required for the catabolism of some nitrogen‐containing compounds appears to be connected with that of nitrate assimilation. In all cases, it is probable that the nirA gene and nitrate reductase itself are involved.
MYCORRHIZAS OF AUTOTROPHIC HIGHER PLANTS
Authors: SMITH SALLY S. E. 
Published: January 2008

Summary(1) The range of mycorrhizal types is briefly compared, with respect to structure and nutritional mode of the symbionts. Ectotrophic, vesicular‐arbuscular and erica‐ceous mycorrhizas of autotrophic plants are selected for further consideration, both because the symbionts have nutritional similarities, and because recent experimental work provides a basis for useful comparisons.(2) A generalized, qualitative model of interactions between the symbionts is presented, with the aim of providing a framework for discussion of the similarities and differences between the mycorrhizas of autotrophic plants. The model describes the distribution of biomass and the flow of carbon and mineral nutrients, together with the effects of distribution of fungal inoculum and environmental conditions.(3) Experimental work pertaining to the model is discussed with emphasis on experimental problems, growth depressions, changes in root: shoot ratio and nitrogen nutrition, as well as the more frequently discussed increases in growth and improved phosphorus nutrition.(4) Nitrogen is considered with respect not only to its uptake, but also in relation to the possible involvement of mycorrhizas in pH regulation and the absorption of cations by plants.(5) The importance of mycorrhizas in forestry and agriculture is briefly discussed.(6) It is concluded that more research into the physiology and ecology of mycor‐rhizal associations is required, in order to provide a basis for effective management of the symbioses in agricultural and natural ecosystems.
REGULATION OF HAEMOPOIESIS IN ALTERED GRAVITY
Authors: Economos Ancelos C. 
Published: January 2008

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 vertebrates.
PROPERTIES OF AVIAN EGG SHELLS AND THEIR ADAPTIVE VALUE
Authors: BOARD R. G. 
Published: January 2008

SummaryI. An arbitrary classification of avian eggshells is proposed.2. The role of the eggshell in conserving the water in eggs at oviposition is discussed. There is as yet no correlation between this property and the pore systems in avian eggshells.3. The pore systems may act as diffusion pathways and the hypothesis has been advanced that in many eggs the shells are adapted so that restriction of gaseous diffusion by mud, preening oils and nest debris is prevented.4. The mechanical properties of the shell are considered in the novel context of defence against (a) attrition that could lead to the pores being blocked with dust, and (b) cracking that would destroy the diffusion pathways noted in 3.5. The overall objective of the review was to discuss the concept that avian eggshell are adapted to fit an egg to the nest environment.
CALCIUM BALANCE AND MOULTING IN THE CRUSTACEA
Authors: GREENAWAY PETER 
Published: January 2008

Summary1. Crustaceans have a high content of calcium, which is chiefly located in the skeleton as calcium carbonate. Calcium is generally the most abundant cation in the body.2. During intermoult, the exoskeleton is usually fully calcified and the animal is in calcium equilibrium with its environment.3. In the premoult stages calcium is resorbed from the skeleton and may be lost to the environment or stored within the body. Typically, losses are high and storage is small in aquatic species, whilst most terrestrial forms store much larger amounts of calcium and losses are reduced. Loss of calcium in soluble form by aquatic species must be by outward transport across the gills.4. Calcium is stored in a variety of different ways, usually with a common taxonomic theme. The main forms are as calcium phosphate granules in cells of the midgut gland (Brachyura), gastroliths (Astacidea and some Brachyura), the haemocoel (some Brachyura) the posterior midgut caeca (Amphipoda) and the ventral portion of the body generally in the Isopoda.5. At ecdysis, the skeleton is shed and the calcium remaining in it is lost from the body.6. Recalcification begins immediately, or shortly after, ecdysis using calcium mobilized from the stores. Simultaneously, or when the stores are exhausted, other sources of calcium are utilized. These are calcium in the water (aquatic species), the food (aquatic and terrestrial species) and the exuviae (chiefly terrestrial species).7. Marine species store little calcium and must obtain the bulk of their requirement (ca. 95%) from the water. Fresh‐water species also store little calcium but have, seemingly, adapted to the lower availability of calcium by increasing the affinity of the calcium‐absorbing mechanism. The rates of uptake of calcium are consequently similar in marine and fresh‐water species.8. A high degree of storage is essential for terrestrial crustaceans as they do not have access to a large aquatic reservoir of calcium. These large reserves enable the animals to reach an advanced stage of calcification, allowing the resumption of foraging and feeding necessary for completion of calcification.9. The control of calcium metabolism during the intermoult cycle is poorly under stood. β Ecdysone appears to control the resorption of calcium and the formation of calcium stores during premoult, but the mechanism of control of calcium metabolism during postmoult and intermoult is unknown.10. The concentration of calcium in the haemolymph of most species is high, but a large proportion of this is in non‐ionized form. In premoult, total calcium levels rise as a result of calcium resorption but little change occurs in the concentration of ionized calcium. Postmoult generally sees a fall in blood calcium, sometimes below the intermoult value.
CLOTTING PROCESSES IN CRUSTACEA DECAPODA
Authors: DURLIAT MICHÈLE 
Published: January 2008

Summary1. In Limulidae, all the factors involved in the coagulation processes are located inside the amoebocytes. The cellular coagulogen is a single 20,000‐polypeptide‐chain protein. It is converted into a non‐covalently crosslinked gel by a serine protease enzyme which cleaves a single peptide bond, releasing peptice C.2. Pro‐clotting enzyme can be activated by two independent pathways: coagulation is induced by either LPS or 1,3‐β‐D‐glucan, both of which result in gel formation. The two pathways comprise a complex enzyme cascade with several limited protein proteolyses.3. In Decapoda, clotting factors are found in both the cell‐free plasma and haemocyte compartments. Analogous factors are present in Insecta.4. Plasma coagulogen is a 400,000 molecular weight protein with both lipid and carbohydrate moieties. Its soluble polymers are converted into covalently crosslinked polymers of coagulin by Ca2+‐dependent transglutaminase. In crayfish, it is also found in other tissues such as soft integument and calcified cuticle. Its concentration varies greatly with the species investigated. It seems to possess many diversified functions such as plasma coagulation, protein transport of tanning agents, lipid and sugar transport and protein storage, and resembles fibronectin.5. A type of cellular coagulogen seems to be present in the haemocytes of Decapoda. It can be converted to a gel by a serine protease pro‐clotting enzyme. This pro‐enzyme can be activated by either LPS or 1,3‐β‐D‐glucans. The mechanism of LPS action is not entirely clear. 1,3‐β‐D‐glucans also activate the prophenoloxidase system and cause phenoloxidase attachment to foreign surfaces of haemocyte lysates. The latter system is restricted to semi‐granular and granular haemocytes, and plays an important part in host‐defence reactions.6. The evolutin of clotting processes throughout the phylogenetic tree is discussed.
THE AVIAN SALT GLAND
Authors: HOLMES W. N. , PHILLIPS J. G. 
Published: January 2008

Summary1. Since the electrolyte concentrating capacity of the avian nephron is insufficient to excrete all of the ingested electrolyte when the bird consumes hyperosmotic drinking water, an extra‐renal pathway is necessary to maintain homeostasis.2. In birds living in coastal and estuarine environments, and in terrestrial environments where the available drinking water is also hyperosmotic with respect to their body fluids, paired salt glands situated in the orbits have evolved as extra‐renal excretory organs.3. These excretory organs wax and wane functionally in species that migrate between these environments where there is an abundance of freshwater.4. This functional development of the glands involves the synthesis of new protein and the differentiation of additional excretory tissue that is reflected in the overall growth of the gland.5. When activated, the glands secrete a fluid containing high concentrations of Na+, Cl‐ and K+ and during prolonged periods of exposure to hypertonic drinking water the birds are able to excrete sufficient quantities of these ingested ions to permit them to remain in positive water balance.6. The mechanisms responsible for producing the characteristic hyperoosmotic fluid discharged by the activated salt glands appear to be located at two sites: (a) in the principal cells of the secretory tubules, where there is a ouabain‐sensitive Na+‐K+‐dependent ion transporting system located in the basolateral membranes and a furosemide‐sensitive Na+ and Cl‐ co‐transport mechanism located in the basal membrane, and (b) in the complex system of collecting ducts into which the secretory tubules discharge their fluid; the cells lining the duct also possess a ouabain‐sensitive ion transporting system.7. In the activated gland the Na+‐transporting system operates intermittently to produce excretory fluid only during periods immediately following the ingestion of hyperosmotic drinking water, and secretion ceases when the bird has gained sufficient osmotically‐free water to maintain homeostasis.8. The initiation and continuation of salt gland secretion occurs in response to the stimulation of an osmoreceptor, probably located in the cardiac vasculature.9. Sensory impulses arriving in the central nervous system are relayed via parasympathetic motor neurones to muscarinic receptors in the cells of the secretory tubules; the Na+‐K+ dependent ATP‐ase system located in the basolateral membranes is then activated by a cyclic GMP‐mediated mechanism.10. Sustained salt gland secretion is also dependent upon activation of the pituitary‐adrenal axis. The corticosteriod hormones secreted as a result of this activation interact with specific cytosolic and nuclear receptor macromolecules in the cells of the secretory tubule. Transcriptional changes are presumed to be necessary for sustained extrarenal excretion.11. Thyroid hormones and prolactin may also be involved indirectly to support continued extrarenal excretion via the salt glands.
THE CORTICAL MAGNIFICATION FACTOR AND PHOTOPIC VISION
Authors: POINTER JONATHAN S. 
Published: January 2008

Summary1. The concept of a topographical representation of sensory surfaces on the mammalian cerebral cortex is now well established. Furthermore, there is claimed an expanded representation for those sensory surfaces associated with behavioural specializations.2. In the case of the visual system, the field of view associated with the retina is projected on to the striate cortex, with the central fovea or point of most acute vision occupying a disproportionately large area of representation.3. The term ‘cortical magnification factor’, denoted by M, has been introduced to indicate, for a given eccentricity and meridian, the linear distance in mm along the primary visual cortex concerned with each degree of visual field.4. The quantitative accuracy of this function subsequently has been improved and a relationship established between cortical magnification and visual acuity in man.5. On the basis of spatio‐temporal investigations across the visual field under levels of photopic adaptation, visual scientists have utilized retino‐cortical magnification to reconcile aspects of structure and function in the human visual system.
THE ROLE OF HETEROCHRONY IN THE EVOLUTION OF CAMBRIAN TRILOBITES
Authors: McNAMARA K. J. 
Published: January 2008

Summary1. Renewed interest in the role of changes to developmental regulation in organisms has highlighted the importance of heterochrony in the evolution of the Metazoa.2. Beecher's interpretation of the evolution of the Trilobita as having been principally by peramorphosis is examined, as is the view of later workers, principally Stubblefield and Hupé, that paedomorphosis was a dominant factor in trilobite evolution.3. Both peramorphosis and paedomorphosis are considered to have been important in trilobite evolution.4. The role of paedomorphosis in the evolution of major morphological novelties is critically examined. Its importance in changes to the structure of the glabella is discussed and new terms proposed to describe the ontogenetic and phylogenetic state of the glabella.5. The highly variable nature of early Cambrian trilobites, in particular the large degree of ontogenetic change, is considered, along with possible poor developmental control of the growth and moulting hormonal systems, to have been significant in providing a high degree of intrapopulational morphological variability. Selection of these heterochronic variants was responsible for the rapid diversification of the Trilobita during the Cambrian.
OPERATION OF THE PURINE NUCLEOTIDE CYCLE IN ANIMAL TISSUES
Authors: WAARDE AREN VAN 
Published: January 2008

Summary1. The operation of the purine nucleotide cycle, consisting of the enzymes adenylate deaminase (E.C. 3.5.4.6), adenylosuccinate synthetase (E.C. 6.3.4.4) and adenylosuccinate lyase (E.C. 4.3.2.2), has been reviewed with reference to its metabolic function in animal tissues.2. Abundant evidence, both from in vitro and in vivo studies, suggests that the purine nucleotide cycle serves to stabilize the adenylate ‘energy charge’ (or ‘phosphorylation potential’) in the cytoplasm of vertebrate cells during a temporary imbalance between ATP‐consumption and ATP‐production. This stabilization, however, is absent or much less efficient in tissues of invertebrates.3. The hypothesis that AMP‐deaminase is involved in the regulation of glycolysis is not supported by recent work. In a variety of cell types, including skeletal muscle and blood platelets, blocking of AMP‐deaminase activity (due to a genetic defect or to pharmacological inhibition) is without effect on the glycolytic rate. Detailed kinetic and histochemical analysis of energy metabolism shows lack of correlation between AMP‐deaminase activity and glycolysis in skeletal muscle during exercise.4. The purine nucleotide cycle appears to control the level of citric acid cycle intermediates in skeletal muscle. Pharmacological inhibition of adenylosuccinate lyase or adenylosuccinate synthetase leads to a reduced availability of four‐carbon ‘sparker’ molecules to the Krebs cycle with a concomitant impairment of aerobic energy production during muscular work.5. The cycle appears to be a major pathway for amino acid deamination in skeletal muscle and brain of vertebrates, but not in kidney or liver.
MATING FREQUENCY AND FECUNDITY IN INSECTS
Authors: RIDLEY M. 
Published: January 2008

Summary1. The paper summarizes the published evidence on the relation between mating frequency and fecundity in insects. There is experimental evidence of varying quality for 63 species and non‐experimental evidence for about 60.2. Repeated mating may be universally necessary for full fecundity and fertility in female insects (in species in which the females normally mate more than once).3. The evidence is remarkably poor. We need more properly designed experiments (and not just observations of natural variation), with sufficient sample sizes and statistics, and measurements of the fecundities and fertilities of singly and multiply mated females, when the multiple matings are separated by many days or weeks. Most of the existing experiments of this sort are defective in some way.4. In species with greater total fecundity and longevity, multiple mating may be more likely to enhance fertility than in species with small fecundity and short life span.5. Females in naturally monandrous species do not show increased fecundity or fertility with repeated mating, whereas females of polyandrous species do.6. There is no obvious connexion between paternal investment, in so far as we know about it, and the increase of fecundity by repeated mating.7. There is a small tendency for females to breed more quickly and be shorter lived if they mate repeatedly.