[Show abstract][Hide abstract] ABSTRACT: The bivalve mollusc Arctica islandica has received considerable
attention in recent years because of its potential as an archive of
marine palaeoclimate, based on its annually resolved incremental shell
growth, longevity, and synchronous growth within populations. The robust
interpretation of the archive depends on a detailed understanding of the
shell formation process, and this in turn requires a reliable
understanding of the shell microstructure. Research into this aspect,
however, has so far been relatively limited. This study uses secondary
ion mass spectrometry (NanoSIMS) to examine the compositions of the two
annually formed growth increments, i.e., a narrow band of relatively
slow growth referred to as growth increment I (GI I) and a usually wider
accretion called growth increment II (GI II). High resolution
composition maps are presented which clearly show lower concentrations
of the organic ions 12C14N- and
32S- in GI I relative to GI II. This is consistent
with the growth of larger crystallites in GI I, which is clearly
demonstrated using a novel analysis method involving focused ion beam
(FIB) milling. Electron backscatter diffraction (EBSD) analysis is also
presented, and demonstrates that the orientation of the aragonite c-axis
is the same in both GI I and GI II, and that the a- and b-axes assume
preferred orientations consistent with the known angle of twinning in
aragonite. By analyzing individual crystallites it is deduced that the
(001) plane is likely to be the mineralizing face in GI I, and that the
(011) and (102) planes are low energy interfaces in GI II.
[Show abstract][Hide abstract] ABSTRACT: Annually resolved growth increments in the shell of the bivalve mollusc Arctica islandica have previously been used in combination with geochemical measurements to successfully construct high-resolution proxy records of past marine environmental conditions. However, to ensure the accuracy of these paleoenvironmental reconstructions it is essential that the annual growth series of increments within the examined shells are reliably identified, and can be distinguished from spurious lines caused by nonannual perturbations such as those resulting from storm disturbance. The current methods used for identifying the growth increment series are sometimes compromised because of ambiguity that results from the employed preparation methods. Here it is shown that backscattered electron imaging of polished shell cross sections may be used to clearly discriminate between the two compositionally and structurally distinct increments that comprise 1 year of outer shell growth. This method, involving minimal specimen preparation, is likely to be primarily useful as a validation technique of particular value in cases where increment identification using existing methods is difficult or ambiguous.
Journal of Microscopy 01/2011; 241(1):29-36. DOI:10.1111/j.1365-2818.2010.03403.x · 2.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nucleus pulposus (NP) of the intervertebral disc develops from the notochord. Humans and other species in which notochordal cells (NCs) disappear to be replaced by chondrocyte-like mature NP cells (MNPCs) frequently develop disc degeneration, unlike other species that retain NCs. The reasons for NC disappearance are unknown. In humans, the change in cell phenotype (to MNPCs) coincides with changes that decrease nutrient supply to the avascular disc. We undertook this study to test the hypothesis that the consequent nutrient stress could be associated with NC disappearance.
We measured cell densities and metabolic rates in 3-dimensional cultures of porcine NCs and bovine MNPCs, and we determined survival rates under conditions of nutrient deprivation. We used scanning electron microscopy to examine end plate porosity of discs with NCs and those with MNPCs. Nutrient-metabolite profiles and cell viability were calculated as a function of cell density and disc size in a consumption/diffusion mathematical model.
NCs were more active metabolically and more susceptible to nutrient deprivation than were MNPCs. Hypoxia increased rates of glycolysis in NCs but not in MNPCs. Higher end plate porosity in discs with NCs suggested greater nutrient supply in keeping with higher nutritional demands. Mathematical simulations and experiments using an analog disc diffusion chamber indicated that a fall in nutrient concentrations resulting from increased diffusion distance during growth and/or a fall in blood supply through end plate changes could instigate NC disappearance.
NCs demand more energy and are less resistant to nutritional stress than MNPCs, which may shed light on the fate of NCs in humans. This provides important information about prospective NC tissue engineering approaches.