Dissection properties of the human aortic media: an experimental study

Aortic dissection occurs frequently and is clinically challenging; the underlying mechanics remain unclear. The present study investigates the dissection properties of the media of 15 human abdominal aortas (AAs) by means of direct tension tests (n=8) and peeling tests (n=12). The direct tension test demonstrates the strength of the media in the radial direction, while the peeling test allows a steady-state investigation of the dissection propagation. To explore the development of irreversible microscopic changes during medial dissection, histological images (n=8) from four AAs at different peeling stages are prepared and analyzed. Direct tension tests of coin-shaped medial specimens result in a radial failure stress of 140.1+/-15.9 kPa (mean+/-SD, n=8). Peeling tests of rectangular-shaped medial strips along the circumferential and axial directions provide peeling force/width ratios of 22.9+/-2.9 mN/mm (n=5) and 34.8+/-15.5 mN/mm (n=7); the related dissection energies per reference area are 5.1+/-0.6 mJ/cm(2) and 7.6+/-2.7 mJ/cm(2), respectively. Although student's t-tests indicate that force/width values of both experimental tests are not significantly different (alpha=0.05, p=0.125), the strikingly higher resisting force/width obtained for the axial peeling tests is perhaps indicative of anisotropic dissection properties of the human aortic media. Peeling in the axial direction of the aorta generates a remarkably "rougher" dissection surface with respect to the surface generated by peeling in the circumferential direction. Histological analysis of the stressed specimens reveals that tissue damage spreads over approximately six to seven elastic laminae, which is about 15-18% of the thickness of the abdominal aortic media, which forms a pronounced cohesive zone at the dissection front.     

Strains inside xylem and inner bark of a stem submitted to a change in hydrostatic pressure

Tangential strains were measured with strain gauges at the surface of xylem and inner bark of saplings of Cryptomeria japonica D. Don. and Fagus silvatica L. during a pressurization test. The test consists in submitting the whole sapling to an artificially imposed hydrostatic pressure of increasing magnitude. The elastic response of the stems was found linear both at the surface of xylem and inner bark. A simple geometric model allows to compute radial strains in each tissue from tangential strain data. Inside inner bark, radial strains are much larger than tangential strains, because tangential strains are restrained by the core of wood. The material compliance of each tissue was computed as the ratio between the radial strain and the pressure that caused it. The material compliance of xylem is much lower than that of inner bark, but, as its thickness is much larger, its contribution to the apparent behavior of the stem is not negligible. Computation of material compliances by this pressurization test provides information about the specific behavior of each tissue in response to hydrostatic pressure. This can be used to estimate and interpret the calibration factor linking the water status of the plant to the apparent strain measured at its surface.T. Okuyama: deceased     

Smooth muscles in relation to the gill skeleton of Perca fluviatilis: organization and innervation

Summary Two laminae composed of smooth muscles, elastic tissue and collagen have been described in relation with the gill skeleton in Perca fluviatilis. A transverse smoothmuscle lamina joins the base of the cartilage rods of the two opposite hemibranchs. A longitudinal smooth-muscle lamina runs parallel to the afferent branchial artery and joins the cartilage rods from one filament to the other. In both laminae, the formaldehyde-induced fluorescence technique (Falck-Hillarp) reveals a network of nerve fibers displaying a green fluorescence characteristic of catecholamines. At the ultrastructural level, the presence of nerve endings containing clear and granular vesicles, and the degeneration of these endings after 6-hydroxydopamine treatment confirm the aminergic nature and the sympathetic origin of this innervation. Surgical denervation brings evidence that the innervation of both laminae is supplied by the metatrematic branches of the branchial nerves. The role of these smooth-muscle laminae remains speculative.     

Stable Isotopes in Fish Eye Lenses as Potential Recorders of Trophic and Geographic History

We evaluated eye lenses as potential recorders of stable isotope histories in fish because they consist of metabolically inert optical proteins that are deposited in successive, concentric circles (laminae) much like otolith circuli and tree rings. We conducted four different tests on lenses from red snapper, red grouper, gag, and white grunt. The first test was a low-resolution screening of multiple individuals (4–5 radial groups of laminae per lens, all species except white grunt). Along the radial axis, all individuals exhibited substantial isotopic variability. Red snapper individuals separated into two groups based on δ ¹⁵N and gag separated into two groups based on δ ¹³C. Two gag with the greatest variation were chosen for high-resolution temporal analysis using individual laminae from their second eye lenses. The first-order patterns from the high-resolution analysis generally mimicked patterns from the low-resolution screening of grouped laminae, yet the high-resolution plots revealed early-life details that were not apparent in the low-resolution screenings. For the third test, left- versus right-eye variation was compared using high-resolution methods. White grunt left- and right-eye radial isotopic patterns were almost identical for both δ ¹³C and δ ¹⁵N, suggesting the variations observed among individual fish were not artifacts. The final test evaluated intra-laminar variation; multiple samples were analyzed from different parts of the same lamina. Seven laminae from three individuals of two species were analyzed in this manner; variations among laminae were found to be much higher than variations within laminae. However, nominal intra-laminar variations were comparable to nominal differences between left and right lenses, suggesting intra-laminar variation established measurement precision. Eye lens isotopes appear to be useful for reconstructing the isotopic histories of individual fish; these histories can be compared with spatially-derived isoscapes to reconstruct individual histories for site fidelity, movement and trophic position.     

Stiffness- and relaxation-based quantitation of radial left ventricular oscillations: elucidation of regional diastolic function mechanisms.

Traditionally, global and longitudinal (i.e., regional) left ventricular (LV) diastolic function (DF) assessment has utilized features of transmitral Doppler E and A waves or Doppler tissue imaging (DTI)-derived mitral annular E' and A' waves, respectively. Quantitation of regional DF has included M-mode echocardiography-based approaches and strain and strain rate imaging (in selected imaging planes), while analysis of mitral annular "oscillations" has recently provided a new window into longitudinal (long-axis) function. The remaining major spatial degree of kinematic freedom during diastole, radial (short-axis) motion, has not been fully characterized, nor has it been exploited for its potential to provide radial LV stiffness (k'(rad)) and relaxation/damping (c'(rad)) indexes. Prior characterization of regional (longitudinal) DF used only annular E'- and A'-wave peak velocities or, alternatively, myocardial strain and strain rate. By kinematically modeling short-axis tissue motion as damped radial oscillation, we present a novel method of estimating k'(rad) and c'(rad) during early filling. As required by the (near) constant-volume property of the heart and tissue/blood incompressibility, in subjects (n = 10) with normal DF, we show that oscillation duration-determined longitudinal (k'(long) and c'(long)) and radial (k'(long) and c'(rad)) parameters are highly correlated (R = 0.69 and 0.92, respectively). Selected examples of diabetic and LV hypertrophic subjects yield radial (k'(long) and c'(rad)) parameters that differ substantially from controls. Results underscore the utility of the incompressibility-based causal relation between DTI-determined mitral annular long-axis (longitudinal mode) and short-axis (radial mode) oscillations in healthy subjects. Selected pathological examples provide mechanistic insight and illustrate the value and potential role of regional (longitudinal and radial) DF indexes in fully characterizing normal vs. impaired DF states.     

A finite element model of the human left ventricular systole

Local wall stress is the pivotal determinant of the heart muscle's systolic function. Under in vivo conditions, however, such stresses cannot be measured systematically and quantitatively. In contrast, imaging techniques based on magnetic resonance (MR) allow the determination of the deformation pattern of the left ventricle (LV) in vivo with high accuracy. The question arises to what extent deformation measurements are significant and might provide a possibility for future diagnostic purposes.

The contractile forces cause deformation of LV myocardial tissue in terms of wall thickening, longitudinal shortening, twisting rotation and radial constriction. The myocardium is thereby understood to act as a densely interlaced mesh. Yet, whole cycle image sequences display a distribution of wall strains as function of space and time heralding a significant amount of inhomogeneity even under healthy conditions. We made similar observations previously by direct measurement of local contractile activity. The major reasons for these inhomogeneities derive from regional deviations of the ventricular walls from an ideal spheroidal shape along with marked disparities in focal fibre orientation.

In response to a lack of diagnostic tools able to measure wall stress in clinical routine, this communication is aimed at an analysis and functional interpretation of the deformation pattern of an exemplary human heart at end-systole. To this end, the finite element (FE) method was used to simulate the three-dimensional deformations of the left ventricular myocardium due to contractile fibre forces at end-systole. The anisotropy associated with the fibre structure of the myocardial tissue was included in the form of a fibre orientation vector field which was reconstructed from the measured fibre trajectories in a post mortem human heart. Contraction was modelled by an additive second Piola–Kirchhoff active stress tensor.

As a first conclusion, it became evident that longitudinal fibre forces, cross-fibre forces and shear along with systolic fibre rearrangement have to be taken into account for a useful modelling of systolic deformation. Second, a realistic geometry and fibre architecture lead to typical and substantially inhomogeneous deformation patterns as they are recorded in real hearts. We therefore, expect that the measurement of systolic deformation might provide useful diagnostic information.     

Differences in Membrane Properties in Simulated Cases of Demyelinating Neuropathies: Internodal Focal Demyelinations without Conduction Block

The membrane properties (intracellular, extracellular, electrotonic potentials, strength-duration time constants, rheobasic currents and recovery cycles), which can now be measured in healthy subjects and patients with demyelinating neuropathies, are investigated in simulated cases of focal reduction (70%) of the myelin sheath in one, two and three successive internodal segments along the length of human motor fibres. The internodally focally demyelinated cases (termed as IFD1, IFD2 and IFD3, respectively) are simulated using our previous double cable model of the fibres. The results show that the intracellular potentials are with reduced amplitude and slowed conduction velocity in the vicinity of demyelinated segments, however the segmental conduction block is not achieved. The radial decline of the extracellular potential amplitudes slightly increases with the increase of the radial distance and demyelination. In contrast, the electrotonic potentials, strength-duration time constants and rheobases are normal. In the recovery cycles, the refractoriness, supernormality and less late subnormality are close to the normal, showing that the pathology is relatively minor. The obtained abnormalities in the potentials and excitability properties provide new information about the pathophysiology of the demyelinated human motor axons and can be observed in vivo in patients with acquired demyelinating neuropathies.     

Bicoid gradient formation mechanism and dynamics revealed by protein lifetime analysis

Embryogenesis relies on instructions provided by spatially organized signaling molecules known as morphogens. Understanding the principles behind morphogen distribution and how cells interpret locally this information remains a major challenge in developmental biology. Here, we introduce morphogen‐age measurements as a novel approach to test models of morphogen gradient formation. Using a tandem fluorescent timer as a protein age sensor, we find a gradient of increasing age of Bicoid along the anterior–posterior axis in the early Drosophila embryo. Quantitative analysis of the protein age distribution across the embryo reveals that the synthesis–diffusion–degradation model is the most likely model underlying Bicoid gradient formation, and rules out other hypotheses for gradient formation. Moreover, we show that the timer can detect transitions in the dynamics associated with syncytial cellularization. Our results provide new insight into Bicoid gradient formation and demonstrate how morphogen‐age information can complement knowledge about movement, abundance, and distribution, which should be widely applicable to other systems.     

A new hypothesis for the evolution of biological electron transport

A new hypothesis for the evolution of biological electron transport is presented. According to this hypothesis biological electron transport originated close to the potential of the hydrogen electrode and evolved in various advantageous directions including, when molecular oxygen became available on the Earth, that of the oxygen electrode. This implies stepwise evolution along and across the potential scale. The hypothesis is based mainly on existing information obtained from studies of primary and tertiary structural relationships of proteins. It is hoped to provide a framework for closer understanding of both evolution and mechanisms of cellular oxidation-reduction as well as energy coupling reactions.     

Elastic properties of human supraorbital and mandibular bone

Elastic constants, including the elastic modulus, the shear modulus, and Poisson's ratio, were measured on human craniofacial bone specimens obtained from the supraorbital region and the buccal surfaces of the mandibles of unembalmed cadavers. Constants were determined using an ultrasonic wave technique in three directions relative to the surface of each sample: 1) normal, 2) tangential, and 3) longitudinal. Statistical analysis of these elastic constants indicated that significant differences in the relative proportions of elastic properties existed between the regions. Bone from the mandible along its longitudinal axis was stiffer than bone from the supraorbital region. Directional differences in both locations demonstrated that cranial bone was not elastically isotropic. It is suggested that differences in elastic properties correspond to regional differences in function. © 1993 Wiley-Liss, Inc.     

Material characterization of the pig kidney in relation with the biomechanical analysis of renal trauma

The objective of this study was an investigation of the material properties of the fresh pig kidney and parametric characterization of its elastic and inelastic material behavior. The material investigation included density measurements, uniaxial as well as three-dimensional compression tests, tensile tests. and shear tests on the samples extracted from the fresh pig kidney. For comparison, density measurements on a number of soft synthetic materials were also performed. Compression tests on the radial and the tangential specimens from the cortex tissue were performed at various loading rates. Three-axial compression tests were performed on the cortex tissues placed in a compression chamber. Shear tests were performed by punching a cylinder into a slice of the cortex. Tensile tests were carried out on the outer capsule. For characterization of the material behavior, a non-linear theoretical simulation based on a two parameter Blatz model was used. For characterization of the time-dependent behavior of the pig kidney cortex, a four-parameter linear viscoelastic model was employed. From the present experimental and theoretical studies, a number of conclusions were derived: (1) The general behavior of the pig kidney cortex samples under compression showed the general non-linear features typical of the soft tissues; the stress strain diagram was composed of a very flat part at very low stress level to about 30% relative deformation which was followed by a steeply rising stiffening leading to the radial rupture of samples marked by a maximum nominal rupture strain of about 50%. (2) The uniaxial compression tests on the radial and the tangential samples from the cortex tissue showed an increase of the rupture stress with the increase in the loading rate, but a decrease in the related rupture strain. (3) The long-term uniaxial compression tests on the cortex specimens under sustained constant load showed an instantaneous deformation followed by a creep response which eventually approached an asymptote. (4) Simulation of the non-linear material behavior of the cortex tissue under uniaxial compression by the Blatz model gave two pairs of material parameters for the cortex in the radial and the tangential directions. Furthermore, fitting of the assumed four-parameter linear viscoelastic model with the experimental data resulted in the viscoelastic material parameters.     

Biomechanics of the columnar cactus Pachycereus pringlei

We report the longitudinal variations in stiffness and bulk density of tissue samples drawn from along the length of two Pachycereus pringlei plants measuring 3.69 and 5.9 m in height to determine how different tissues contribute to the mechanical stability of these massive vertical organs. Each of the two stems was cut into segments of uniform length and subsequently dissected to obtain and mechanically test portions of xylem strands, stem ribs, and a limited number of pith and cortex samples. In each case, morphometric measurements were taken to determine the geometric contribution each tissue likely made to the ability of whole stems to resist bending forces. The stiffness of each xylem strand increased basipetally toward the base of each plant where stiffness sharply decreased, reaching a magnitude comparable to that of strands 1 m beneath the stem apex. The xylem was anisotropic in behavior, i.e., its stiffness measured in the radial and in the tangential directions differed significantly. Despite the abrupt decrease in xylem strand stiffness at the stem base, the contribution made by this tissue to resist bending forces increased exponentially from the tip to the base of each plant due to the accumulation of wood. A basipetal increase in the stiffness of the pith (and, to limited extent, that of the cortex) was also observed. In contrast, the stiffness of stem rib tissues varied little as a function of stem length. These tissues were stiffer than the xylem in the corresponding portions of the stem along the upper two-fifths of the length of either plant. Tissue stiffness and bulk density were not significantly correlated within or across tissue types. However, a weak inverse relationship was observed for these properties in the case of the xylem and stem rib tissues. We present a simple formula that predicts when stem ribs rather than the xylem strands serve as the principal stiffening agents in stems. This formula successfully predicted the observed aspect ratio of the stem ribs (the average quotient of the radial and tangential dimensions of rib transections), and thus provided circumstantial evidence that the ribs are important for mechanical stability for the distal and younger regions of the stems examined.     

Eco-evolutionary consequences of habitat warming and fragmentation in communities

Eco-evolutionary dynamics can mediate species and community responses to habitat warming and fragmentation, two of the largest threats to biodiversity and ecosystems. The eco-evolutionary consequences of warming and fragmentation are typically studied independently, hindering our understanding of their simultaneous impacts. Here, we provide a new perspective rooted in trade-offs among traits for understanding their eco-evolutionary consequences. On the one hand, temperature influences traits related to metabolism, such as resource acquisition and activity levels. Such traits are also likely to have trade-offs with other energetically costly traits, like antipredator defences or dispersal. On the other hand, fragmentation can influence a variety of traits (e.g. dispersal) through its effects on the spatial environment experienced by individuals, as well as properties of populations, such as genetic structure. The combined effects of warming and fragmentation on communities should thus reflect their collective impact on traits of individuals and populations, as well as trade-offs at multiple trophic levels, leading to unexpected dynamics when effects are not additive and when evolutionary responses modulate them. Here, we provide a road map to navigate this complexity. First, we review single-species responses to warming and fragmentation. Second, we focus on consumer–resource interactions, considering how eco-evolutionary dynamics can arise in response to warming, fragmentation, and their interaction. Third, we illustrate our perspective with several example scenarios in which trait trade-offs could result in significant eco-evolutionary dynamics. Specifically, we consider the possible eco-evolutionary consequences of (i) evolution in thermal performance of a species involved in a consumer–resource interaction, (ii) ecological or evolutionary changes to encounter and attack rates of consumers, and (iii) changes to top consumer body size in tri-trophic food chains. In these scenarios, we present a number of novel, sometimes counter-intuitive, potential outcomes. Some of these expectations contrast with those solely based on ecological dynamics, for example, evolutionary responses in unexpected directions for resource species or unanticipated population declines in top consumers. Finally, we identify several unanswered questions about the conditions most likely to yield strong eco-evolutionary dynamics, how better to incorporate the role of trade-offs among traits, and the role of eco-evolutionary dynamics in governing responses to warming in fragmented communities.     

A framework for integrating thermal biology into fragmentation research

Habitat fragmentation changes thermal conditions in remnant patches, and thermal conditions strongly influence organism morphology, distribution, and activity patterns. However, few studies explore temperature as a mechanism driving ecological responses to fragmentation. Here we offer a conceptual framework that integrates thermal biology into fragmentation research to better understand individual, species, community, and ecosystem‐level responses to fragmentation. Specifically, the framework addresses how fragmentation changes temperature and how the effects of those temperature changes spread through the ecosystem, from organism response via thermal sensitivity, to changes in species distribution and activity patterns, to shifts in community structure following species' responses, and ultimately to changes in ecosystem functions. We place a strong emphasis on future research directions by outlining “Critical gaps” for each step of the framework. Empirical efforts to apply and test this framework promise new understanding of fragmentation's ecological consequences and new strategies for conservation in an increasingly fragmented and warmer world.     

Negative regulation of monocyte adhesion to arterial elastic laminae by signal regulatory protein alpha and Src homology 2 domain-containing protein-tyrosine phosphatase-1

Elastic laminae are extracellular matrix constituents that not only contribute to the stability and elasticity of arteries but also play a role in regulating arterial morphogenesis and pathogenesis. We demonstrate here that an important function of arterial elastic laminae is to prevent monocyte adhesion, which is mediated by the inhibitory receptor signal regulatory protein (SIRP) alpha and Src homology 2 domain-containing protein-tyrosine phosphatase (SHP)-1. In a matrix-based arterial reconstruction model in vivo, elastic laminae were resistant to leukocyte adhesion and transmigration compared with the collagen-dominant arterial adventitia. The density of leukocytes within the elastic lamina-dominant media was about 58-70-fold lower than that within the adventitia from 1 to 30 days. An in vitro assay confirmed the inhibitory effect of elastic laminae on monocyte adhesion. The exposure of monocytes to elastic laminae induced activation of SIRP alpha, which in turn activated SHP-1. Elastic lamina degradation peptides extracted from arterial specimens could also activate SIRP alpha and SHP-1. The knockdown of SIRP alpha and SHP-1 by specific small interfering RNA diminished the inhibitory effect of elastic laminae, resulting in a significant increase in monocyte adhesion. These observations suggest that SIRP alpha and SHP-1 potentially mediate the inhibitory effect of elastic laminae on monocyte adhesion.     

Winching up heavy loads with a compliant arm: a new local joint controller

A closed kinematic chain, like an arm that operates a crank, has a constrained movement space. A meaningful movement of the chain’s endpoint is only possible along the free movement directions which are given implicitly by the contour of the object that confines the movement of the chain. Many technical solutions for such a movement task, in particular those used in robotics, use central controllers and force–torque sensors in the arm’s wrist or a leg’s ankle to construct a coordinate system (task frame formalism) at the local point of contact the axes of which coincide with the free and constrained movement directions. Motivated by examples from biology, we introduce a new control system that solves a constrained movement task. The control system is inspired by the control architecture that is found in stick insects like Carausius morosus. It consists of decentral joint controllers that work on elastic joints (compliant manipulator). The decentral controllers are based on local positive velocity feedback (LPVF). It has been shown earlier that LPVF enables contour following of a limb in a compliant motion task without a central controller. In this paper we extend LPVF in such a way that it is even able to follow a contour if a considerable counter force drags the limb away along the contour in a direction opposite to the desired. The controller extension is based on the measurement of the local mechanical power generated in the elastic joint and is called power-controlled relaxation LPVF. The new control approach has the following advantages. First, it still uses local joint controllers without knowledge of the kinematics. Second, it does not need a force or torque measurement at the end of the limb. In this paper we test power-controlled relaxation LPVF on a crank turning task in which a weight has to be winched up by a two-joint compliant manipulator.     

Does pigeon homing depend on stimuli perceived during displacement?

Summary In order to test whether stimuli perceived during passive displacement are important for the subsequent homing, pigeons were transported in an apparatus designed to prevent them from receiving relevant information: The experimental birds were continuously rotated quite rapidly (1.5 cps, radial acceleration about 4 g); in addition, they were exposed to an artificial magnetic field and supplied with bottled air. Control birds were transported in open-air cages on top of the van with free view to all sides.Five pairs of releases from equal distances in opposite directions were conducted. Experienced birds were released at distances of about 15, 90, and 300 km from the loft, inexperienced birds at distances of about 180km. In each pair of releases both groups of pigeons were significantly homeward oriented. Neither in initial orientation nor in homing performance nor in the distribution of recoveries were the experimental birds inferior to the controls or in any perceptable way different from them.It is concluded that homing of passively displaced pigeons is not primarily based on information gathered during the outward journey.Abbreviations EP experimental pigeon(s) - CP control pigeon(s) The possibility to maintain our pigeon loft in a building that belongs to the Zoological Institute (Prof. M. Lindauer) of the University of Würzburg is gratefully acknowledged.     

Electron microscopic observations of elastic fibres in the lung and aorta of tight-skin and beta-aminopropionitrile-fed mice.

The lung of the tight-skin (TSK) mouse was characterized by enlargement of the air spaces. Elastin in the alveolar walls of the TSK mouse exhibited fragmentation. The aorta of the TSK mouse was characterized by marked hyperplasia of loose connective tissue in the adventitia. Collagen fibres and ruthenium red-positive materials were markedly increased. Microfibrils surrounding elastin in the adventitia of the aorta were not clear in the TSK mouse. In the lung of the beta-aminopropionitrile (BAPN)-fed mouse, enlargement of the alveolar air spaces was not prominent compared with the TSK mouse. Elastic fibres in the alveolar walls did not show the fragmentation observed in the TSK mouse, and microfibrils surrounding elastin were clearly observed. However, elastic laminae in the media of the BAPN-fed mouse aorta were swollen and fragmented. Elastic fibres in the adventitia exhibited a normal appearance and microfibrils surrounding elastin in the adventitia were clearly observed. The results suggest that the mechanism of the connective tissue abnormality in the TSK mouse is different from that of BAPN, which inhibits the activity of lysyl oxidase. The abnormality of elastin and microfibrils surrounding elastin in the TSK mouse probably plays a role in the deformity or degradation of elastic fibres and the structural changes of the lung.     

Poro-elastic modeling of Syringomyelia – a systematic study of the effects of pia mater,central canal,median fissure,white and gray matter on pressure wave propagation and fluid movement within the cervical spinal cord

Syringomyelia, fluid-filled cavities within the spinal cord, occurs frequently in association with a Chiari I malformation and produces some of its most severe neurological symptoms. The exact mechanism causing syringomyelia remains unknown. Since syringomyelia occurs frequently in association with obstructed cerebrospinal fluid (CSF) flow, it has been hypothesized that syrinx formation is mechanically driven. In this study we model the spinal cord tissue either as a poro-elastic medium or as a solid linear elastic medium, and simulate the propagation of pressure waves through an anatomically plausible 3D geometry, with boundary conditions based on in vivo CSF pressure measurements. Then various anatomic and tissue properties are modified, resulting in a total of 11 variations of the model that are compared. The results show that an open segment of the central canal and a stiff pia (relative to the cord) both increase the radial pressure gradients and enhance interstitial fluid flow in the central canal. The anterior median fissure, anisotropic permeability of the white matter, and Poisson ratio play minor roles.     

Symbiotic vesicle ultrastructure in high pressure-frozen,freeze-substituted actinorhizae

Summary Using tissue stained en bloc with chromic acid or tissue prepared by high pressure-freezing and freeze-substitution, it was possible to analyze quantitatively the ultrastructure of symbiotic vesicle envelopes (SVE) inAlnus serrulata, Ceanothus americanus, Elaeagnus umbellata, andMyrica cerifera. The lamina measured about 4.7 nm in thickness in thin section. Despite diverse symbiotic vesicle morphology, the SVE thickness was similar in all of these symbioses: 36–71 nm, which corresponded to 6–15 laminae based on counts of chromic acid-stained SVEs. This similarity in structure suggests that a similar environmental signal regulates envelope thickness in the different root nodules. Based on previous studies, this is likely to be pO₂. Three types of envelope morphologies were distinguished: (1) theAlnus-type (as inAlnus andElaeagnus), which had localized thickenings around the vesicle and had thickest dimensions over the stalk; (2) theCeanothus-type. characterized as a relatively uniform envelope over both vesicle and attached hypha, and (3) theMyrica-type, which had no stalk region and a basal SVE thickness of about six laminae throughout except where localized thickening occurred. Localized thickening of the SVE resulted from extra numbers of laminae being deposited, generally over regions where septa contacted the edge of the vesicle. Freeze-substituted symbiotic vesicles had a variety of novel structures that are poorly preserved in chemically-fixed tissue. A paracrystalline body inAlnus symbiotic vesicles may be composed of particles that also exist free in the symbiotic vesicle cytoplasm. In addition, a previously unknown complex at the base of theAlnus-type symbiotic vesicle and within its stalk was evident in freeze-substituted tissues.Abbreviations HPF/FS high pressure-frozen/freeze-substituted - SV symbiotic vesicle - SVE symbiotic vesicle envelope Dedicated to the memory of Professor John G. Torrey