Elastic arteries in invertebrates: mechanics of the octopus aorta

Extracellular Matrix in Vascular Disease, Part 2/4: JACC Focus Seminar

Medium-sized and large arteries consist of 3 layers: the tunica intima, tunica media, and tunica adventitia. The tunica media accounts for the bulk of the vessel wall and is the chief determinant of mechanical compliance. It is primarily composed of circumferentially arranged layers of vascular smooth muscle cells that are separated by concentrically arranged elastic lamellae; a form of extracellular matrix (ECM). The tunica media is separated from the tunica intima and tunica adventitia, the innermost and outermost layers, respectively, by the internal and external elastic laminae. This second part of a 4-part JACC Focus Seminar discusses the contributions of the ECM to vascular homeostasis and pathology. Advances in genetics and proteomics approaches have fostered significant progress in our understanding of vascular ECM. This review highlights the important role of the ECM in vascular disease and the prospect of translating these discoveries into clinical disease biomarkers and potential future therapies.

Blood pressure in the Greenland shark as estimated from ventral aortic elasticity

We conducted in vitro inflations of freshly excised ventral aortas of the Greenland shark, Somniosus microcephalus, and used pressure-diameter data to estimate the point of transition from high to low compliance, which has been shown to occur at the mean blood pressure in other vertebrates including fishes. We also determined the pressure at which the modulus of elasticity of the aorta reached 0.4 MPa, as occurs at the compliance transition in other species. From these analyses, we predict the average ventral aortic blood pressure in S. microcephalus to be about 2.3-2.8 kPa, much lower than reported for other sharks. Our results support the idea that this species is slow moving and has a relatively low aerobic metabolism. Histological investigation of the ventral aorta shows that elastic fibres are present in relatively low abundance and loosely connected, consistent with this aorta having high compliance at a relatively low blood pressure.

The mechanical properties of the systemic and pulmonary arteries of Python regius correlate with blood pressures

Pythons are unique amongst snakes in having different pressures in the aortas and pulmonary arteries because of intraventricular pressure separation. In this study, we investigate whether this correlates with different blood vessel strength in the ball python Python regius. We excised segments from the left, right, and dorsal aortas, and from the two pulmonary arteries. These were subjected to tensile testing. We show that the aortic vessel wall is significantly stronger than the pulmonary artery wall in P. regius. Gross morphological characteristics (vessel wall thickness and correlated absolute amount of collagen content) are likely the most influential factors. Collagen fiber thickness and orientation are likely to have an effect, though the effect of collagen fiber type and cross-links between fibers will need further study.

Vascular extracellular matrix and arterial mechanics

An important factor in the transition from an open to a closed circulatory system was a change in vessel wall structure and composition that enabled the large arteries to store and release energy during the cardiac cycle. The component of the arterial wall in vertebrates that accounts for these properties is the elastic fiber network organized by medial smooth muscle. Beginning with the onset of pulsatile blood flow in the developing aorta, smooth muscle cells in the vessel wall produce a complex extracellular matrix (ECM) that will ultimately define the mechanical properties that are critical for proper function of the adult vascular system. This review discusses the structural ECM proteins in the vertebrate aortic wall and will explore how the choice of ECM components has changed through evolution as the cardiovascular system became more advanced and pulse pressure increased. By correlating vessel mechanics with physiological blood pressure across animal species and in mice with altered vessel compliance, we show that cardiac and vascular development are physiologically coupled, and we provide evidence for a universal elastic modulus that controls the parameters of ECM deposition in vessel wall development. We also discuss mechanical models that can be used to design better tissue-engineered vessels and to test the efficacy of clinical treatments.

Structure of the elastin gene

The isolation and characterization of cDNAs encompassing the full length of chicken, cow, rat and human elastin mRNA have led to the elucidation of the primary structure of the respective tropoelastins. Large segments of the sequence are conserved but there are also considerable variations which range in extent from relatively small alterations, such as conservative amino acid substitutions, to variation in the length of hydrophobic segments and largescale deletions and insertions. In general, smaller differences are found among mammalian tropoelastins and greater ones between chicken and mammalian tropoelastins. Although only a single elastin gene is found per haploid genome, the primary transcript is subject to considerable alternative splicing, resulting in multiple tropoelastin isoforms. Functionally distinct hydrophobic and cross-link domains of the protein are encoded in separate exons which alternate in the gene. The introns of the human gene are rich in Alu repetitive sequences, which may be the site of recombinational events, and there are also several dinucleotide repeats, which may exhibit polymorphism and, therefore, be effective genetic markers. The 5' flanking region is G+C rich and contains potential binding sites for numerous modulating factors, but no TATA box or functional CAAT box. The basic promoter is contained within a 136 bp segment and transcription is initiated at multiple sites. These findings suggest that the regulation of elastin gene expression is complex and takes place at several levels.

The modulus of elasticity of fibrillin-containing elastic fibres in the mesoglea of the hydromedusa Polyorchis penicillatus

Hydromedusan jellyfish swim by rhythmic pulsation of their mesogleal bells. A single swimming muscle contracts to create thrust by ejecting water from the subumbrellar cavity. At the end of the contraction, energy stored in the deformation of the mesogleal bell powers the refilling stage, during which water is sucked back into the subumbrellar cavity. The mesoglea is a mucopolysaccharide gel reinforced with radially oriented fibres made primarily of a protein homologous to mammalian fibrillin. Most of the energy required to power the refill stroke is thought to be stored by stretching these fibres. The elastic modulus of similar fibrillin-rich fibres has been measured in other systems and found to be in the range of 0.2 to 1.1 MPa. In this paper, we measured the diameters of the fibres, their density throughout the bell, and the mechanical behaviour of the mesoglea, both in isolated samples and in an intact bell preparation. Using this information, we calculated the stiffness of the fibres of the hydromedusa Polyorchis penicillatus, which we found to be approximately 0.9 MPa, similar in magnitude to other species. This value is two orders of magnitude more compliant than the stiffness of the component fibrillin microfibrils previously reported. We show that the structure of the radial fibres can be modelled as a parallel fibre-reinforced composite and reconcile the stiffness difference by reinterpreting the previously reported data. We separate the contributions to the bell elasticity of the fibres and mesogleal matrix and calculate the energy storage capacity of the fibres using the calculated value of their stiffness and measured densities and diameters. We conclude that there is enough energy potential in the fibres alone to account for the energy required to refill the subumbrellar cavity.

Function-structure relationship of elastic arteries in evolution: from microfibrils to elastin and elastic fibres

Evolution of species has led to the appearance of circulatory systems including blood vessels and one or more pulsatile pumps, typically resulting in a low-pressurised open circulation in most invertebrates and a high-pressurised closed circulation in vertebrates. In both open and closed circulations, the large elastic arteries proximal to the heart damp out the pulsatile flow and blood pressure delivered by the heart, in order to limit distal shear stress and to allow regular irrigation of downstream organs. To achieve this goal, networks of resilient and stiff proteins adapted to each situation--i.e. low or high blood pressure--have been developed in the arterial wall to provide it with non-linear elasticity. In the low-pressurised circulation of some invertebrates, the mechanical properties of arteries can almost be entirely microfibril-based, whereas, in high-pressurised circulations, they are due to an interplay between a highly resilient protein, an elastomer in the octopus and elastin in most vertebrates, and the rather stiff protein collagen. In vertebrate development, elastin is incorporated in elastic fibres, on a earlier deposited scaffold of microfibrils. The elastic fibres are then arranged in functional concentric elastic lamellae and, with the smooth muscle cells, lamellar units. The microfibrils may also play a direct functional role in all mature arteries of high- and low-pressurised circulations. Finally, since blood pressure regularly increases with developmental stages, it appears possible that the early deposition of microfibrils, which are highly-conserved in evolution, corresponds, at least in part, to an early microfibril-driven elasticity in low-pressurised arteries, present across species. In vertebrates, when pressure developmentally rises above a threshold value, the vascular wall stress may turn on the expression of other resilient protein genes, including the elastin gene. Elastin would then be deposited on microfibrils and resulting in the elastic fibre network and elastic lamellae whose mechanical properties are adapted to allow for proper arterial work at higher pressures.

Microfibrils provide non-linear elastic behaviour in the abdominal artery of the lobster Homarus americanus

1. Microfibrils are becoming increasingly recognized as an important component of the extra-cellular matrix. However, almost nothing is known about their mechanical role in the diversity of tissues in which they are found. 2. Microfibrils form the principal structural component in the wall of the abdominal artery of the lobster Homarus americanus. We have used previous estimates of the mechanical properties of these microfibrils, estimates of the fraction of the aorta wall volume occupied by the microfibrils, and their angular distribution as a function of strain in a numerical model that predicts the macroscopic mechanical properties of the whole tissue. 3. Microfibrils alone, when their reorientation and deformation are accounted for, characterize the stress-strain behaviour of the vessel. Evidence of the evolutionary conservation of fibrillin between medusans, echinoderms and vertebrates implies that the mechanical properties of lobster microfibrils may apply to microfibrillar function in other taxa. This will have profound implications on the perceived roles of microfibrils in development, physiology and disease.

Mechanisms of circulatory homeostasis and response in Aplysia

This review concerns the organization and function of arterial vasculature in Aplysia californica, especially the vasomotor reflexes that support circulatory homeostasis, and fixed patterns of response that may reroute blood flow during changes in behavioral state. The observations presented here raise three hypotheses for further study: 1) Arterial vasculature is functionally organized with precisely structured, independently regulated subdivisions; these are most evident for arterial systems serving digestive and reproductive processes; 2) arterial musculature is inherently responsive to local pressure changes, having both static and dynamic reflexes that promote efficient, evenly-distributed flow of blood; and 3) complex, long-lasting behaviors like egg laying have, as part of their makeup, equally prolonged and stereotypical changes in the pattern of circulation. Taken together, these observations support the view that maintenance and adjustment of blood flow in gastropod molluscs is an unexpectedly complex and highly integrated component of behavior.

Microfibrils, elastic anchoring components of the extracellular matrix, are associated with fibronectin in the zonule of Zinn and aorta

Microfibrils are striated tubules that play a role in the formation of elastin fibers by providing a scaffold upon which newly synthesized elastin is deposited. Ultrastructural and staining studies also demonstrate microfibrils that terminate where elastin is sparse or absent in basal laminae, plasma membranes, and the collagenous matrix. The most striking accumulation of microfibrils is found in the zonule of Zinn, the transparent and elastic suspensory ligament of the lens, which contains no elastin. Application of immunocytochemical staining with a peroxidase-antiperoxidase (PAP) procedure demonstrates that fibronectin is associated with the microfibrils of the zonule and aorta. Aggregates of microfibrils are identical to oxytalan ('acid enduring') fibers that have been described in peridontal membranes and other sites subject to mechanical stress and they can be found in sites as disparate as the rabbit zonule, rat hepatic stroma and human cardiac papillary muscle, indicating that microfibrils are a widely distributed connective tissue element with a function that extends beyond elastogenesis; their association with fibronectin and localization suggests that they serve as an elastic anchoring component of the extracellular matrix.

The infrastructure of aortic elastic fibers

Elastic fibers are composed of a central core of elastin that is amorphous and electron-lucent in conventional transmission electron micrographs and peripheral microfibrils. A complex infrastructure within the amorphous elastin of mature rat aorta is made visible by fixation and staining with a glutaraldehyde-ruthenium red mixture in phosphate buffer or osmium-ruthenium red in cacodylate buffer. The infrastructure is composed of at least two interlacing but distinct elastic structural components; a framework of circumferentially orientated microfibrils and a three-dimensional meshwork of filaments that permeate the fiber. The latter resembles a reticulum that has previously been observed in freeze-fractured and negatively stained elastin and attributed to the supramolecular organization of elastin. Microfibrils also extend from the core of the elastic fiber into the surrounding matrix where they appear to function as anchoring fibers. These observations indicate that the elastic properties of the arterial wall are an integrated function of both elastin and microfibrils.

The ultrastructure of the byssal apparatus of a mussel. V. Localization of collagenic and elastic components in the threads

Ultrastructural and cytochemical studies have been carried out on the proximal part of byssus threads (TPP) in an attempt to localize collagenic and elastic components. The results show that TPP autoclaving followed by hot alkali treatment causes the extraction of about two-thirds of hydroxyproline and the parallel removal of most of the matrix, leaving filaments unaffected. Moreover the results of the staining reactions signaletic for elastic tissues indicate that TPP filaments contain glycoproteins with a reactivity similar to that of many invertebrate elastic tissues. On the basis of these morphological findings, it seems reasonable to suggest that collagen may be located in TPP matrix, while filaments could be responsible for the elastic properties.

Structure-function relationship in the evolution of elastin

The evolution of the structure of the rubber-like protein elastin, found in connective tissues which are subjected to periodic physiological stress, was studied with respect to its phylogenetic distribution, fiber morphology and arrangement, response to deformation, and amino acid composition. Aortae and other tissues from several vertebrates and invertebrates were examined for the presence of elastin, which was defined on the basis of a characteristic amino acid composition, the presence of the unique crosslinks desmosine and isodesmosine, and by histologic criteria. The protein was present in all vertebrates except the primitive jawless fishes and was absent from all invertebrates which were examined. In addition, the morphology of aortic elastin fibers differed markedly among the vertebrate families. Biochemical analysis revealed increases in both the degree of crosslinking and hydrophobicity in elastins from higher vertebrates (mammals, birds) as compared to those from bony fish. Mammalian elastin displayed an increased tendency toward coacervation (polymerization into aggregated structures) at 37 degrees C and behaved differently from a conventional elastomer when stretched in a microcalorimeter. Selection for an increasingly hydrophobic elastin appears to have paralleled the development of a highly-pressurized, closed circulatory system in homeothermic animals. The data do not support a common genetic origin for elastin and other connective tissue proteins. Significant variations in amino acid composition among aortic elastins from different species, however, indicate that genetically distinct elastin types could have arisen by divergence from a common ancestral gene.