摘要:

SUMMARY1The review is mainly concerned with Carboniferous non‐marine Anthracosiidae and Myalinidae, of which only the shells are known, and with certain unspecialized non‐byssate suspension‐feeding bivalves which had smooth shells and burrowed shallowly.2Limited experimental evidence and observation of living bivalves suggest that in certain Recent siphonate species and in some members of the non‐siphonate Anthracosiidae the shape of the shell was functionally related to movement through the sediment in the same way. Predicted optimum shapes of shells for downward burrowing and for upward near‐vertical movement in sands and silts were apparently realized in the Anthracosiidae, which constituted a series of highly variable opportunistic assemblages. It is stressed, however, that the shape of the shell always appears to be a compromise between several functional requirements.3In both the early Anthracosiidae and in several analogous Recent marine genera, orientation of the long axis of the shell was the same for downward burrowing and for upward pushing, that is near the vertical, with posterior end upward.4Invasion of the Pennine late‐Namurian delta took place when marine bivalves pushed upward, thereby avoiding sedimentation from delta lobes moving seaward relatively swiftly. The evolution ofCarbonicolaoccurred at about this time (the Marsdenian Age) when the bivalves acquired a smooth elongate shell of ‘streamlined’ form, having a hinge plate with swellings and depressions on it (later to evolve into teeth). All these features tend to characterize the active shallow burrowers of today.5Entry into soft‐bottom eutrophic conditions of fresh water is characterized in several unionids by increase in height/length (H/Lorw/m) ratio of shell, in anterior end/length (A/L) and in obesity (T/L) (see Fig. 2, centre). These changes also took place in established faunas ofCarbonicolacharacteristic of richly carbonaceous shales, in faunas of supposedAnthraconaiain more carbonaceous sediments of mid‐and late‐Carboniferous times in the U.S.A. and inAnthraconautaof the British late Carboniferous (Westphalian C and D). The genusAnthracosphaeriumepitomizes the culmination of these trends in the Anthracosiidae, and species of the genus were probably epifaunal or shallowly infaunal active burrowers on soft bottoms in Westphalian upper A and B time.6Two contrasting patterns of growth characterize the shells of the widely variable unionidMargaritifera margaritifera.In the first, dorsal arching of the shell, with straightening and reflexion of the ventral margin, provides increased weight but decreased ligamental strength. In the second, in which the ‘hinge line’ tends to remain straight while the dorsal margin becomes more rounded and obesity increases, there is increased metabolic efficiency for active surface movement. The maintenance of these trends within the species, which may be regarded as secondarily opportunistic, affords a means of insurance for survival within the highly variable environments of fresh water. The same trends are recognizable in established faunas ofCarbonicola, where it is likely that they performed the same function, as well as in Mesozoic and Cenozoic Unionidae.7The functional explanation outlined in paragraph (6) may be extended to provide an ecological meaning for Ortmann's ‘Law of Stream Distribution’, which states that obesity of unionid shells increases downstream. This applies broadly, within a fairly wide range of variation, a fact which again suggests ‘insurance’ of faunas against the variable hazards of fresh‐water habitats.8In bivalves having considerable thickness of shell in relation to their size, and having strong umbonal development, specific gravity of the living bivalve is correlated withH/LandT/Lratios of the shell, as in the veneridVenerupis rhomboides.In this species, differences in the specific gravities of the bivalves, as well as their shape, appear to be functionally related to shallowly infaunal burrowing in different substrates.9The conclusions of paragraphs 6 and 8 provide a functional explanation, in terms of selection, for the palaeoecological ‘law’ of Eagar (1973), which is applicable to established faunas ofCarbonicolain mid‐Carboniferous time, and relates variational trends in two main groups of shells primarily to increases in the relative water velocities of the palaeoenvironments.10Where the growth of relatively unspecialized bivalve shells has been measured, allometric relationships have usually been found inH‐LandT‐Lscatters. Logarithmic lines have two inflexions and linear scatters a sigmoidal form. A similar pattern of allometric growth has been found in bothCarbonicola (H‐L) andAnthraconauta (m‐w).These patterns appear to be related to the optimum requirements of water‐borne larvae, the initial byssal phase of settlement, when ability to burrow quickly is essential, and the main period of growth and activity. It is herein suggested that the final second inflexion, which indicates a falling off of gain inH/LandT/Lratios, may be a genetically controlled modification of the growth pattern which counteracts the operation of the ‘cube‐square rule’ (of Thompson, 1942) and prolongs productive life.11Patterns of relative growth of the shell may be significantly modified by conditions of the habitat; bothT/LandH/Lratios may be increased, with general reduction in size, in the less ‘favourable’ habitats. Both these ratios have been similarly modified, the one in the ‘natural laboratory’ of a lake formed by the damming up of streams, and the other in transplant experiments with livingVenerupis.In both these latter cases, phenotypic changes took place in the same direction as those expected on the basis of natural selection. Direct response to environmental factors cannot therefore be ruled out as an agent in similar changes noted inCarbonicolaand supposedAnthraconaiain paragraphs (5) and (

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1978.tb01436.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

SUMMARY1After an outline description of pancreatic structure and function, and a more detailed account of acinar cell morphology, this review traces the pathway of amino acids as they are taken up by the acinar cell, incorporated into digestive hydrolases, transported through the cell and finally discharged from the cell, and considers the mechanisms by which these steps are controlled. At all stages comparisons are made with other secretory cells.2The use of radioautography and cell‐fractionation techniques in determining this pathway in the pancreas are described. The route and kinetics of the process in pancreas are compared with those in other cells.3Amino‐acid entry is by an active mechanism. However the intracellular pool of accumulated amino acids may not be used directly in protein synthesis. Selection of amino acids for incorporation into proteins may occur whilst they are associated with carrier systems within the plasma membrane. There is no convincing evidence that amino‐acid entry can be influenced by the pancreatic secretagogues, cholecystokin‐pancreazymin (CCK‐PZ) or acetylcholine.4Secretory proteins are synthesized on ribosomes bound to the endoplasmic reticulum (ER) and the nascent proteins vectorially transferred across the ER membrane into the ER cisternae. All messenger RNA molecules which are templates for secretory proteins appear to possess an initial sequence of codons whose translation produces a ‘signal’ sequence of amino acids. This signal sequence somehow triggers attachment of the ribosomes to the ER, thereby automatically determining that the final translation product is destined for the ER cisternae.5The effects of CCK‐PZ and acetylcholine on pancreatic protein synthesis are controversial. Whereas stimulation can be observed in vivo, this has not been convincingly demonstrated in vitro. I conclude that while CCK‐PZ and acetylcholine may accelerate protein synthesis, the physiological significance of this effect remains to be clarified. Long‐term stimulation can modify pancreatic enzyme synthesis and this, together with other factors, may be the means of dietary adaptation by the gland.6Newly synthesized proteins travel from the ER cisternae via the peripheral Golgi components to the Golgi cisternae. Transport from ER to Golgi cisternae may occur by a vesicle shuttle service or by direct tubular connexions. Although sustained stimulation with CCK‐PZ analogues can accelerate this intracellular transport step, pancreatic secretagogues have not yet been shown to accelerate transport under physiological conditions.7The Golgi complex has a number of functions including: glycosylation and, where appropriate, sulphation of glycoprotein and mucopolysaccharide components of the zymogen granules (ZG) and granule membranes; sequestration of divalent cations which bind to secretory proteins; the formation of condensing vacuoles (CV) from the inner Golgi cisternae.8Aggregation of proteins occurs passively within CV so as to form osmotically inert complexes, thereby reducing internal osmotic activity and causing water to diffuse out. This condensation imparts a gel‐like consistency to the mature ZG so formed.9Discharge of ZG occurs by a process of exocytosis involving fusion of the ZG membrane with the apical plasma membrane, release of the ZG contents, and retrieval of the ZG membrane from the plasma membrane by endocytotic mechanisms. The mechanisms responsible for migration of ZG towards the cell apex and for exocytosis remain unknown but may involve the participation of microtubules and/or microfilaments. Although there is a small, basal discharge of ZG at all times, stimulation with CCK‐PZ or acetylcholine greatly accelerates the process.10The basic tenet of the secretory mechanism summarized above is that, following synthesis, secretory proteins are confined within an intracellular organelle at all times. This ‘segregation’ hypothesis has been challenged by the ‘equilibrium’ hypothesis in which secretory proteins are suggested to move across cellular membranes and are therefore at equilibrium within the various compartments of the cell. While many of the observations on which the equilibrium hypothesis are based are tenuous, some others cannot readily be explained by the segregation model. Proponents of the equilibrium hypothesis therefore suggest that preferential release of individual hydrolases from ZG occurs, followed by their separate transport across the apical cell membrane. The claims of this alternative model are discussed.11In the final section are discussed the intracellular mechanisms by which CCK‐PZ and acetylcholine act on the acinar cell to cause discharge. The overall membrane perturbations brought about by CCK‐PZ and acetylcholine appear to be the same and include cell depolarization, and perhaps increased phospholipid turnover. Both events may be related to an altered membrane permeability to cations. CCK‐PZ, but not acetylcholine, will activate adenylate cyclase, but cyclic AMP does not appear to be involved in regulating enzyme discharge. Instead, Ca2+is the major intracellular second messenger. However, rather than increase Ca2+uptake into the cell, CCK‐PZ and acetylcholine appear to raise the intracellular Ca2+concentration by causing release of Ca2+from intracellular stores. The mechanism by which they do this, and the role of Ca

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1978.tb01437.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1978.tb01438.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY