Active axial stress in mouse aorta

The study verifies the development of active axial stress in the wall of mouse aorta over a range of physiological loads when the smooth muscle cells are stimulated to contract. The results obtained show that the active axial stress is virtually independent of the magnitude of pressure, but depends predominately on the longitudinal stretch ratio. The dependence is non-monotonic and is similar to the active stress-stretch dependence in the circumferential direction reported in the literature. The expression for the active axial stress fitted to the experimental data shows that the maximum active stress is developed at longitudinal stretch ratio 1.81, and 1.56 is the longitudinal stretch ratio below which the stimulation does not generate active stress. The study shows that the magnitude of active axial stress is smaller than the active circumferential stress. There is need for more experimental investigations on the active response of different types of arteries from different species and pathological conditions. The results of these studies can promote building of refined constrictive models in vascular rheology.     

Linked mechanical and biological aspects of remodeling in mouse pulmonary arteries with hypoxia-induced hypertension

Supravalvular aortic stenosis (SVAS) is associated with decreased elastin and altered arterial mechanics. Mice with a single deletion in the elastin gene (ELN(+/-)) are models for SVAS. Previous studies have shown that elastin haploinsufficiency in these mice causes hypertension, decreased arterial compliance, and changes in arterial wall structure. Despite these differences, ELN(+/-) mice have a normal life span, suggesting that the arteries remodel and adapt to the decreased amount of elastin. To test this hypothesis, we performed in vitro mechanical tests on abdominal aorta, ascending aorta, and left common carotid artery from ELN(+/-) and wild-type (C57BL/6J) mice. We compared the circumferential and longitudinal stress-stretch relationships and residual strains. The circumferential stress-stretch relationship is similar between genotypes and changes <3% with longitudinal stretch at lengths within 10% of the in vivo value. At mean arterial pressure, the circumferential stress in the ascending aorta is higher in ELN(+/-) than in wild type. Although arterial pressures are higher, the increased number of elastic lamellae in ELN(+/-) arteries results in similar tension/lamellae compared with wild type. The longitudinal stress-stretch relationship is similar between genotypes for most arteries. Compared with wild type, the in vivo longitudinal stretch is lower in ELN(+/-) abdominal and carotid arteries and the circumferential residual strain is higher in ELN(+/-) ascending aorta. The increased circumferential residual strain brings the transmural strain distribution in ELN(+/-) ascending aorta close to wild-type values. The mechanical behavior of ELN(+/-) arteries is likely due to the reduced elastin content combined with adaptive remodeling during vascular development.     

Site specific inelasticity of arterial tissue

Understanding the mechanical behaviour of arterial tissue is vital to the development and analysis of medical devices targeting diseased vessels. During angioplasty and stenting, stress softening and permanent deformation of the vessel wall occur during implantation of the device, however little data exists on the inelastic behaviour of cardiovascular tissue and how this varies through the arterial tree. The aim of this study was to characterise the magnitude of stress softening and inelastic deformations due to loading throughout the arterial tree and to investigate the anisotropic inelastic behaviour of the tissue. Cyclic compression tests were used to investigate the differences in inelastic behaviour for carotid, aorta, femoral and coronary arteries harvested from 3-4 month old female pigs, while the anisotropic behaviour of aortic and carotid tissue was determined using cyclic tensile tests in the longitudinal and circumferential directions. The differences in inelastic behaviour were correlated to the ratio of collagen to elastin content of the arteries. It was found that larger inelastic deformations occurred in muscular arteries (coronary), which had a higher collagen to elastin ratio than elastic arteries (aorta), where the smallest inelastic deformations were observed. Lower magnitude inelastic deformations were observed in the circumferential tensile direction than in the longitudinal tensile direction or due to radial compression. This may be as a result of non-collagenous components in the artery becoming more easily damaged than the collagen fibres during loading. Stress softening was also found to be dependent on artery type. In the future, computational models should consider such site dependant, anisotropic inelastic behaviour in order to better predict the outcomes of interventional procedures such as angioplasty and stenting.     

Mechanical behavior of human aortas: Experiments,material constants and 3-D finite element modeling including residual stress

Segments of fresh human ascending, thoracic descending and abdominal aortas from eight male sexagenarians were pressurized under closed-end and free extension conditions. The median unpressurized inner radii for the ascending, thoracic and abdominal locations were 14.21, 9.67 and 7.16 mm, respectively. The median thickness was similar in the ascending and thoracic regions, at about 1.6 mm, while it was 1.2 mm in the abdominal region. The opening angle was not statistically different between regions, with a median of ?38°. Under 13.3 kPa pressure, the median circumferential stretch ratio was about 1.26 in all three aortic locations; the median longitudinal stretch ratio was similar in the ascending and thoracic regions, at about 1.13, while it was 1.05 in the abdominal region. Material constants for a three-dimensional hyperelastic anisotropic constitutive model were determined. Experimental, analytical and finite element results showed excellent agreement, validating the novel experimental approach and the numerical methods used. When residual stress was not taken into account, stresses were highest on the inside of the aorta, with a gradient across the wall of about 200 and 50 kPa in the circumferential and longitudinal directions, respectively. When residual stress was included as described by negative opening angles, stresses were highest on the outside of the aorta, with a gradient across the wall in excess of 400 kPa for the circumferential direction, and on the order of 150 kPa for the longitudinal direction. The mechanical consequences of negative opening angles had not been appreciated so far, and deserve further investigation.     

Tissue softening of guinea pig oesophagus tested by the tri-axial test machine

Tissue softening is commonly reported during mechanical testing of biological tissues in vitro. The loss of stiffness may be due to viscoelasticity-induced softening (the time-history of load-caused softening) and strain-induced stress softening (the maximum previous load-caused softening). However, the knowledge about tissue softening behaviour is presently poor. The aims of this study were to distinguish whether the loss of the stiffness during preconditioning was due to strain softening or viscoelasticity and to test the tissue softening in circumferential and longitudinal direction in the guinea pig oesophagus. Eight repeated pressure controlled ramp distensions and eight uniaxial tensile-release ramp stretches in three series were done on eight guinea pig oesophagi. The stress–strain curves were used to display the time-dependency (viscoelasticity) and the maximum previous load-caused softening (strain softening) in circumferential and longitudinal directions. For both the longitudinal and the circumferential softening, the peak stress and stiffness produced during the first loading were bigger than those produced in the remaining loadings. The stress loss due to strain softening was about three times more than that due to viscoelasticity in the longitudinal direction. The strain increased more than two times between the strain softening and viscoelastic softening in the circumferential direction. With a stress level of 20 kPa, the stiffness in the circumferential direction lost more than that in the longitudinal direction (P<0.05), indicating the anisotropic softening properties in the oesophagus. In conclusion, the stiffness loss during preconditioning is mainly attributed to strain softening, appears irreversible and is anisotropic.     

Influence of tensile strain on smooth muscle cell orientation in rat blood vessels.

Blood vessels are subject to tensile stress and associated strain which may influence the structure and organization of smooth muscle cells (SMCs) during physiological development and pathological remodeling. This study focused on the influence of the major tensile strain on the SMC orientation in the blood vessel wall. Several blood vessels, including the aorta, the mesenteric artery and vein, and the jugular vein of the rat were used to observe the normal distribution of tensile strains and SMC orientation; and a vein graft model was used to observe the influence of altered strain direction on the SMC orientation. The circumferential and longitudinal strains in these blood vessels were measured by using a biomechanical technique, and the SMC orientation was examined by fluorescent microscopy at times of 10, 20, and 30 days. Results showed that the SMCs were mainly oriented in the circumferential direction of straight blood vessels with an average angle of approximately 85 deg between the SMC axis and the vessel axis in all observed cases. The SMC orientation coincided with the principal direction of the circumferential strain, a major tensile strain, in the blood vessel wall. In vein grafts, the major tensile strain direction changed from the circumferential to the longitudinal direction at observation times of 10, 20, and 30 days after graft surgery. This change was associated with a decrease in the angle between the axis of newly proliferated SMCs and that of the vessel at all observation times (43 +/- 11 deg, 42 +/- 10 deg, and 41 +/- 10 deg for days 10, 20, and 30, respectively), indicating a shift of the SMC orientation from the circumferential toward the longitudinal direction. These results suggested that the major tensile strain might play a role in the regulation of SMC orientation during the development of normal blood vessels as well as during remodeling of vein grafts.     

Rheological properties and wall structures of large veins

The static and dynamic viscoelastic properties were studied of longitudinal and circumferential strips excised from various large veins of dogs. The mechanical behavior in longitudinal direction could be regarded as elastic, while that in circumferential direction was highly viscoelastic. No distinct regionality was observed in either of the longitudinal and the circumferential groups. Noradrenaline and papaverine did not alter the elastic behavior of the longitudinal strips. In circumferential strips, however, noradrenaline caused a considerable decrease in stress relaxation and some steepening in the slope of the upper limb of hysteresis loop. Papaverine did not affect the circumferential characteristics. These findings suggest the dominant contribution of smooth muscle tone to the circumferential characteristics of venous walls. Pretreatment with formic acid abolished the occurrence of stress relaxation in circumferential direction but produced no change in the longitudinal behavior. This indicates that elastin fibers may be a principal determinant of the elastic behavior in longitudinal direction and that a residual tension observed in stress-relaxation tests of circumferential strips may be due to stretched elastin fibers. The elastic moduli of elastase pretreated venous walls were in the order of 10(8) dynes/cm2, about 1000 times higher than those of the control. Accordingly, collagen fibers seemed not to play any appreciable role in the rheological behavior of venous walls under physiological conditions. This inference was supported by histological observations of venous walls under unstretched and stretched states. Models were proposed in regard to the architecture of the fibrous elements in the venous walls.     

Rheological properties of the thoracic aorta in normal and WHHL rabbits

The tension-strain, stress-strain and stress relaxation curves of longitudinal and circumferential strips of proximal thoracic aortas in normal and WHHL rabbits of different ages were determined using a tensile testing instrument. Wall distensibility of longitudinal and circumferential strips was the greatest in the normal aorta and decreased with advancing age in the atherosclerotic aorta. The wall thickness of the atherosclerotic aorta was positively related to age with a correlation coefficient of 0.66(p less than 0.01). The incremental elastic moduli calculated from the stress-strain curves increased with advancing age in the atherosclerotic aorta. Accordingly, the decreased distensibility of the atherosclerotic wall may be due to the increased wall thickness caused by the intimal thickening as well as to the increase in wall stiffness caused by the increased elastic modulus. The viscoelasticity of the atherosclerotic aorta was larger than that of the normal aorta. This reflects the mechanical effect of atherosclerotic changes that occurred in the thickened intima.     

Effect of hypertension on elasticity and geometry of aortic tissue from dogs

Inflation-extension experiments were carried out on segments of the descending thoracic aortas from 4 normotensive and 4 hypertensive dogs rendered hypertensive using either unilateral or bilateral renal artery constriction. Intravascular pressures up to 200 mm Hg and axial forces up to 200 g were used. The external diameter of the segment and the distance between two longitudinally spaced gage marks were recorded photographically at each pressure-force level combination. Dimensions in the underformed configuration were measured at the end of the inflation-extension experiment. Data were analyzed for changes in geometry and force-deformation response. Results indicate that: 1. Under sustained hypertension the wall thickness in the underformed configuration increases with a concurrent reduction in the in-situ longitudinal extension ratio. 2. This dual tissue response accomplishes substantial reductions in the circumferential and longitudinal stresses from the levels that would be reached at equivalent pressures in the absence of these geometric changes. 3. At comparable intravascular pressures the extensibility in the circumferential direction is slightly greater for the hypertensive aortas as compared to normals. However, the stress-extension ratio relationship in the circumferential direction is similar in the two groups. 4. The stress-extension ratio relationship in the longitudinal direction indicates that the hypertensive aorta is stiffer than its normotensive counterpart.     

Viscoelastic properties of the passive mechanical behavior of the porcine carotid artery: Influence of proximal and distal positions

The viscoelastic properties of porcine carotid tissue are investigated in this work. Experimental uniaxial stress relaxation tests along the longitudinal and circumferential directions of the vessel were performed for carotid strips extracted from 10 vessels. Directional and local differences - distal versus proximal position - in the tissue behavior were investigated. The experimental tests reveal a highly anisotropic, non-linear viscoelastic response and local dependence of the samples. The carotid artery shows anisotropic relaxation behavior for both proximal and distal samples. The highest stress relaxation was found in the circumferential tensile test for the highest applied strain at the distal position. For the circumferential direction, the relaxation stress was higher than in the longitudinal being at its highest in the distal position. These facts show that the stress relaxation is higher in the distal than in the proximal position. However, there are no differences between both positions in the longitudinal direction. In addition, a constitutive law that takes into account the fundamental features, including non-linear viscoelasticity, of the arterial tissue is proposed. The present results are correlated with the purely elastic response and the microstructural analysis of the tissue by means of histological quantification presented in a previous study.     

Biomechanical and morphometric properties of the arterial wall referenced to the zero-stress state in experimental diabetes

Morphometric and passive biomechanical properties were studied in isolated segments of the thoracic and abdominal aorta, left common carotid artery, left femoral artery and the left pulmonary artery in 20 non-diabetic and 28 streptozotocin (STZ)-induced diabetic rats. The diabetic and non-diabetic rats were divided into groups living 1, 4, 8, and 12 weeks after the induction of diabetes (n = 7 for each diabetic group) or sham injection (n = 5 for each group). The mechanical test was performed as a distension experiment where the proximal end of the arterial segment was connected via a tube to the container used for applying pressures to the segment and the distal end was left free. The vessel diameter and length were obtained from digitized images of the arterial segments at pre-selected pressures and at no-load and zero-stress states. Circumferential and longitudinal stresses (force per area) and strains (deformation) were computed from the length, diameter and pressure data and from the zero-stress state data. The zero-stress state was obtained by cutting vessel rings radially causing the rings to open up into a sector. Diabetes was associated with pronounced morphometric changes, e.g., wall thickness. With respect to the biomechanical data, the opening angle increased and reached a plateau in 4 weeks after which it decreased again (p < 0.05). The opening angle was smallest in the thoracic aorta and largest in the pulmonary artery. Furthermore, it was found that the circumferential stiffness of the arteries studied increased with the duration of diabetes. In the longitudinal direction significant differences were found 8 weeks after injection of STZ in all arteries except the pulmonary artery. In the 12 weeks group, the femoral artery was stiffest in the circumferential direction whereas the thoracic aorta was stiffest in the longitudinal direction. The accumulated serum glucose level correlated with the arterial wall thickness and elastic modulus (correlation coefficient between 0.56 and 0.81).     

Uniform stress state in bone structure with residual stress.

Residual stress and strain in living tissues have been investigated from the viewpoint of mechanical optimality maintained by adaptive remodeling. In this study, the residual stresses in the cortical-cancellous bone complex of bovine coccygeal vertebrae were examined. Biaxial strain gages were bonded onto the cortical surface, so that the gage axes were aligned in the cephalocaudal and circumferential directions. Strains induced by removal of the end-plate and the cancellous bone were recorded sequentially. The results revealed the existence of compressive residual stress in the cortical bone and tensile residual stress in the cancellous bone in both the cephalocaudal and the circumferential direction. The observed strains were examined on the basis of the uniform stress hypothesis using a three-bar model for the cephalocaudal direction and a three-layered cylinder model for the circumferential direction. In this model study, the magnitude of effective stresses, which is defined as the macroscopic stress divided by the area fraction of bone material, was found not to differ significantly between cephalocaudal and circumferential directions, although they were evaluated using independent models. These results demonstrate that the uniform stress state of the cortical-cancellous bone structure is consistent with results obtained in the cutting experiment when the existence of residual stress is taken into account.     

Mechanical properties of the tracheal mucosal membrane in the rabbit. II. Morphometric analysis.

Folding of the airway mucosal membrane provides a mechanical load that impedes airway smooth muscle contraction. Mechanical testing of rabbit tracheal mucosal membrane showed that the membrane is stiffer in the longitudinal than in the circumferential direction of the airway. To explain this difference in the mechanical properties, we studied the morphological structure of the rabbit tracheal mucosal membrane in both longitudinal and circumferential directions. The collagen fibers were found to form a random meshwork, which would not account for differences in stiffness in the longitudinal and circumferential directions. The volume fraction of the elastic fibers was measured using a point-counting technique. The orientation of the elastic fibers in the tissue samples was measured using a new method based on simple geometry and probability. The results showed that the volume fraction of the elastic fibers in the rabbit tracheal mucosal membrane was approximately 5% and that the elastic fibers were mainly oriented in the longitudinal direction. Age had no statistically significant effect on either the volume fraction or the orientation of the elastic fibers. Linear correlations were found between the steady-state stiffness and the quantity of the elastic fibers oriented in the direction of testing.     

Flow-induced shear strain in intima of porcine coronary arteries.

The in vivo circumferential strain has a small variation throughout the vascular system (aorta to arterioles). The axial strain has also been shown to be nearly the same as the circumferential strain under physiological loading. Since the endothelium is mechanically much softer than the media-adventitia in healthy arteries, the porcine intima was considered as a mechanically distinct layer from the media-adventitia in a two-layer computational model. Based on the simulation result, we hypothesize that the flow-induced shear strain in intima can be of similar value as the pressure-induced circumferential strain in healthy coronary arteries, even though the shear stress is orders of magnitude smaller than the circumferential stress. The nearly isotropic deformation (circumferential, axial, and shear strains) may have important implications for mechanical homeostasis of endothelial cells, mechanotransduction, growth, and remodeling of blood vessels.     

Variation of mechanical properties along the length of the aorta in C57bl/6 mice

The objective of the present study is to obtain a systematic set of data on the mechanical properties along the entire length of the mouse aorta. The ascending aorta of seven mice was cannulated near the aortic valve, and the aorta was preconditioned with several cyclic changes in pressure. The perfusion pressure was then increased in 30-mmHg increments from 0 to 150 mmHg. Cab-O-Sil, colloidal silica, was mixed into the perfusate to prevent flow through the microvessels and hence attain zero-flow distensions. Our results show that the residual circumferential strain leads to a uniformity of transmural strain of the aorta in the loaded state along the entire length of the aorta. This uniformity is attained in the range of 60-120 mmHg. At pressures <60 mmHg, the outer strain is greater than the inner strain, whereas at pressures >120 mmHg, the converse is true. Furthermore, we found that the circumferential and longitudinal stress-strain relationships are linear in the pressure range of 30-120 mmHg. Finally, the circumferential modulus is greatest (most rigid) near the diaphragm, and the majority of volume compliance (85%) is in the thoracic compared with the abdominal aorta. These findings are important for an understanding of the hemodynamics of the cardiovascular system of the normal mouse and will serve as a reference state for the study of various diseases in knock-in and knock-out models of this species.     

Is location a significant parameter in the layer dependent dissection properties of the aorta?

Proper characterisation of biological tissue is key to understanding the effect of the biomechanical environment in the physiology and pathology of the cardiovascular system. Aortic dissection in particular is a prevalent and sometimes fatal disease that still lacks a complete comprehension of its progression. Its development and outcome, however, depend on the location in the vessel. Dissection properties of arteries are frequently studied via delamination tests, such as the T-peel test and the mixed-mode peel test. So far, a study that performs both tests throughout different locations of the aorta, as well as dissecting several interfaces, is missing. This makes it difficult to extract conclusions in terms of vessel heterogeneity, as a standardised experimental procedure cannot be assured for different studies in literature. Therefore, both dissection tests have been here performed on healthy porcine aortas, dissecting three interfaces of the vessels, i.e., the intima-media, the media-adventitia and the media within itself, considering different locations of the aorta, the ascending thoracic aorta (ATA), the descending thoracic aorta and the infrarenal abdominal aorta (IAA). Significant differences were found for both, layers and location. In particular, dissection forces in the ATA were the highest and the separation of the intima-media interface required significantly the lowest force. Moreover, dissection in the longitudinal direction of the vessel generally required more force than in the circumferential one. These results emphasise the need to characterise aortic tissue considering the specific location and dissected layer of the vessel.

     

Shear modulus of porcine coronary artery: contributions of media and adventitia

The epicardial coronary arteries experience significant torsion in the axial direction due to changes in the shape of the heart during the cardiac cycle. The objective of this study was to determine the torsional mechanical properties of the coronary arteries under various circumferential and longitudinal loadings. The coronary artery was treated as a two-layer composite vessel consisting of intima-medial and adventitial layers, and the shear modulus of each layer was determined. Eight porcine hearts were obtained at a local abattoir, and their right coronary and left anterior descending arteries were isolated and tested in vitro with a triaxial torsion machine (inflation, longitudinal stretch, and circumferential twist). After the intact vessel was tested, the adventitia was dissected away, leaving an intact media that was then tested under identical triaxial loading conditions. We proposed a biomechanical analysis to compute the shear modulus of the adventitia from the measured shear moduli of the intact vessel and the media. To validate our predictions, we used four additional hearts in which the shear modulus of the adventitia was measured after dissection of media. Our results show that the shear modulus does not depend on the shear stress or strain but varies linearly with circumferential and longitudinal stresses and in a nonlinear way with the corresponding strains. Furthermore, we found that the shear modulus of the adventitia is larger than that of the intact vessel, which is larger than the vessel media. These results may have important implications for baroreceptor sensitivity, circulation of the vasa vasorum, and coronary dissection.     

Axial prestretch and circumferential distensibility in biomechanics of abdominal aorta

Elastic arteries are significantly prestretched in an axial direction. This property minimises axial deformations during pressure cycle. Ageing-induced changes in arterial biomechanics, among others, are manifested via a marked decrease in the prestretch. Although this fact is well known, little attention has been paid to the effect of decreased prestretch on mechanical response. Our study presents the results of an analytical simulation of the inflation–extension behaviour of the human abdominal aorta treated as nonlinear, anisotropic, prestrained thin-walled as well as thick-walled tube with closed ends. The constitutive parameters and geometries for 17 aortas adopted from the literature were supplemented with initial axial prestretches obtained from the statistics of 365 autopsy measurements. For each aorta, the inflation–extension response was calculated three times, with the expected value of the initial prestretch and with the upper and lower confidence limit of the initial prestretch derived from the statistics. This approach enabled age-related trends to be evaluated bearing in mind the uncertainty in the prestretch. Despite significantly decreased longitudinal prestretch with age, the biomechanical response of human abdominal aorta changes substantially depending on the initial axial stretch was used. In particular, substituting the upper limit of initial prestretch gave mechanical responses which can be characterised by (1) low variation in axial stretch and (2) high circumferential distensibility during pressurisation, in contrast to the responses obtained for their weakly prestretched counterparts. The simulation also suggested the significant effect of the axial prestretch on the variation of axial stress in the pressure cycle. Finally, the obtained results are in accordance with the hypothesis that circumferential-to-axial stiffness ratio is the quantity relatively constant within this cycle.     

Contribution of elastin and collagen to the inflation response of the pig thoracic aorta: assessing elastin's role in mechanical homeostasis

This study was undertaken to understand elastin's role in the mechanical homeostasis of the arterial wall. The mechanical properties of elastin vary along the aorta, and we hypothesized this maintained a uniform mechanical environment for the elastin, despite regional variation in loading. Elastin's physiological loading was determined by comparing the inflation response of intact and autoclave purified elastin aortas from the proximal and distal thoracic aorta. Elastin's stretch and stress depend on collagen recruitment. Collagen recruitment started in the proximal aorta at systolic pressures (13.3 to 14.6 kPa) and in the distal at sub-diastolic pressures (9.3 to 10.6 kPa). In the proximal aorta collagen did not contribute significantly to the stress or stiffness, indicating that elastin determined the vessel properties. In the distal aorta, the circumferential incremental modulus was 70% higher than in the proximal aorta, half of which (37%) was due to a stiffening of the elastin. Compared to the elastin tissue in the proximal aorta, the distal elastin suffered higher physiological circumferential stretch (29%, P=0.03), circumferential stress (39%, P=0.02), and circumferential stiffness (37%, P=0.006). Elastin's physiological axial stresses were also higher (67%, P=0.003). These findings do not support the hypothesis that the loading on elastin is constant along the aorta as we expected from homeostasis.     

Morphometry and strain distribution of the C57BL/6 mouse aorta

The goal of the present study was to obtain a systematic set of data along the length of the mouse aorta to study variations of morphometry (diameter, wall thickness, and curvature), strain, and stress of the mouse aorta. Five mice were imaged with a 13-MHz ultrasound probe to determine the in vivo diameter along the aorta. A cast was made of these aortas to validate the ultrasonic diameter measurements. The root mean squared and systematic errors for these measurements were 12.6% and 6.4% of the mean ultrasound diameter, respectively. The longitudinal variations of geometry, stress, and strain from the aortic valve to the common iliac bifurcation were documented. Our results show that the residual circumferential strain leads to a uniformity of transmural strain of the aorta in the loaded state along the entire length of the aorta. Furthermore, we validated the incompressibility condition along the length of the aorta. These data of normal mice will serve as a reference state for the study of disease in future knockout models.