Zero-stress State Research Articles

A two-layered mechanical model of the rat esophagus. Experiment and theory

BackgroundThe function of esophagus is to move food by peristaltic motion which is the result of the interaction of the tissue forces in the esophageal wall and the hydrodynamic forces in the food bolus. The structure of the esophagus is layered. In this paper, the esophagus is treated as a two-layered structure consisting of an inner collagen-rich submucosa layer and an outer muscle layer. We developed a model and experimental setup for determination of elastic moduli in the two layers in circumferential direction and related the measured elastic modulus of the intact esophagus to the elastic modulus computed from the elastic moduli of the two layers.MethodsInflation experiments were done at in vivo length and pressure-diameters relations were recorded for the rat esophagus. Furthermore, the zero-stress state was taken into consideration.ResultsThe radius and the strain increased as function of pressure in the intact as well as in the individual layers of the esophagus. At pressures higher than 1.5 cmH2O the muscle layer had a larger radius and strain than the mucosa-submucosa layer. The strain for the intact esophagus and for the muscle layer was negative at low pressures indicating the presence of residual strains in the tissue. The stress-strain curve for the submucosa-mucosa layer was shifted to the left of the curves for the muscle layer and for the intact esophagus at strains higher than 0.3. The tangent modulus was highest in the submucosa-mucosa layer, indicating that the submucosa-mucosa has the highest stiffness. A good agreement was found between the measured elastic modulus of the intact esophagus and the elastic modulus computed from the elastic moduli of the two separated layers.

Combined compression and elongation experiments and non-linear modelling of liver tissue for surgical simulation

Uniaxial stress-strain data were obtained from in vitro experiments on 20 porcine livers for compressions, elongations and cycles of compression and then elongation. There were about 70 cylindrical samples, with diameter 7mm and varying height (4-11 mm). The combined compression and elongation test provide a unified framework for both compression and elongation for applications such as computer-aided surgical simulation. It enable the zero stress state of the experimental liver sample to be precisely determined. A new equation that combined both logarithmic and polynomial strain energy forms was proposed in modelling these experimental data. The assumption of incompressibility was justified from a preliminary Poisson's ratio for elongation and compression at 0.43+/-0.16 and 0.47+/-0.15, respectively. This equation provided a good fit for the observed mechanical properties of liver during compression-elongation cycles and for separate compressions or elongations. The root mean square errors were 91.92+/-17.43 Pa, 57.55+/-13.23 Pa and 29.78+/-17.67 Pa, respectively. In comparison with existing strain energy functions, this combined model was the better constitutive equation. Application of this theoretical model to small liver samples and other tissues demonstrated its suitability as the material model of choice for soft tissue.

Tissue remodeling of rat pulmonary arteries in recovery from hypoxic hypertension.

The reversibility of tissue remodeling is of general interest to medicine. Pulmonary arterial tissue remodeling during hypertension induced by hypoxic breathing is well known, but little has been said about the recovery of the arterial wall when the blood pressure is lowered again. We hypothesize that tissue recovery is a function of the oxygen concentration, blood pressure, location on the vascular tree, and time. We measured the changes of blood pressure, vessel lumen, vessel wall thicknesses, and opening angle of each segment of the blood vessel at its zero-stress state after step changes of the oxygen concentration in the breathing gas. The zero-stress state of each vessel is emphasized because it is important to the analysis of stress and strain and in morphometry. Experimental results are presented as histories of tissue parameters after step changes of the oxygen level. Tissue characteristics are examined under the hypothesis that they are linearly related to changes in the local blood pressure. Under this linearity hypothesis, each aspect of the tissue change can be expressed as a convolution integral of the blood pressure history with a kernel called the indicial response function. It is shown the indicial response function for rising blood pressure is different from that for falling blood pressure. This difference represents a major nonlinearity of the tissue remodeling process of the blood vessels.

Small intestinal morphometric and biomechanical changes during physiological growth in rats

Changes in small intestinal geometry, residual strain and stress–strain properties during physiological growth were studied in rats ranging from 1 to 32 weeks of age. Small intestinal mass and dimensions increased many-fold with age, e.g. the weight per unit length increased five -fold with age and the wall cross-sectional area increased four-fold. The opening angle of duodenum obtained at zero-stress state was approximately 220° and 290° during the first and second week after birth and decreased to 170° at other ages (p<0.005). The opening angle of ileum ranged between 120° and 150°. The residual strain of duodenum at the mucosal surface did not vary with age ( p>0.05) whereas the residual strain of ileum at the mucosal surface decreased with age ( p<0.001). The circumferential and longitudinal stress–strain curves fitted well to a mono-exponential function. At a given circumferential stress, the corresponding strain values increased during the first 8 weeks of age ( p<0.05) where after no further change was observed. Hence, the small intestine became more compliant during early life. At a given longitudinal stress, the corresponding strains of ileum and duodenum became larger during the first 2–4 weeks of age ( p<0.05) where after no further change was observed. The small intestine was stiffer in longitudinal direction compared to the circumferential direction. In conclusion, pronounced morphometric and biomechanical changes were observed in the rat small intestine during physiological growth. Such data may prove useful in the understanding of the functional changes of the digestive tract during early life.

A constitutive formulation of arterial mechanics including vascular smooth muscle tone.

A pseudo-strain energy function (pseudo-SEF) describing the biomechanical properties of large conduit arteries under the influence of vascular smooth muscle (VSM) tone is proposed. In contrast to previous models that include the effects of smooth muscle contraction through generation of an active stress, in this study we consider the vascular muscle as a structural element whose contribution to load bearing is modulated by the contraction. This novel pseudo-SEF models not only arterial mechanics at maximal VSM contraction but also the myogenic contraction of the VSM in response to local increases in stretch. The proposed pseudo-SEF was verified with experimentally obtained pressure-radius curves and zero-stress state configurations from rat carotid arteries displaying distinct differences in VSM tone: arteries from normotensive rats displaying minimal VSM tone and arteries from hypertensive rats exhibiting significant VSM tone. The pressure-radius curves were measured in three different VSM states: fully relaxed, maximally contracted, and normal VSM tone. The model fitted the experimental data very well (r2 > 0.99) in both the normo- and hypertensive groups for all three states of VSM activation. The pseudo-SEF was used to illustrate the localized reduction of circumferential stress in the arterial wall due to normal VSM tone, suggesting that the proposed pseudo-SEF can be of general utility for describing stress distribution not only under passive VSM conditions, as most SEFs proposed so far, but also under physiological and pathological conditions with varying levels of VSM tone.

A new method to denude the endothelium without damage to media: structural, functional, and biomechanical validation.

The intimial thickening that occurs in human and animal atherogenesis can be induced by mechanical injury to the endothelium. The objective of the present study was to develop a new method to induce arterial endothelial injury without damage to the media for future investigations of mechanisms of intimal thickening and atherogenesis. A specifically designed catheter was inserted into the common femoral artery of Wistar rats (n = 9) through an arteriotomic mouth. After application of Tyrode solution containing 0.14 M KCl on the surface of the vessel, the vessel contracted onto the catheter. The catheter was then moved back and forth to scrape away the endothelium. The left common femoral artery of the same rat was subjected to the standard balloon injury model. The two models were evaluated structurally, functionally, and biomechanically. Structurally, we verified that both techniques remove the endothelium, but the balloon method damages the media. Functionally, we examined the contractile response of the artery to [K+] and norepinephrine 2 days after the denudation. We found that the right femoral artery underwent contraction in response to [K+], whereas the left artery did not. Furthermore, neither artery responded to norepinephrine. Biomechanically, we measured the pressure-diameter relationship and the zero-stress state of the vessel and computed the stress-strain relation. The circumferential stretch ratios at 120 mmHg were 1.38 +/- 0.08 for the control, 1.41 +/- 0.08 (P > 0.05) for the new method, and 1.56 +/- 0.09 for the balloon injury (P < 0.05). The opening angles at the zero-stress state were 113 +/- 21 degrees for the control, 102 +/- 18 degrees for the new method (P > 0.05), and 8 +/- 13 degrees for the balloon injury (P < 0.001). In conclusion, the new method removes the endothelium while maintaining the structure, contractile function, and biomechanical properties of the vessel.

Study on cartilaginous and muscular strains of rat trachea

This paper introduces a new method, termed Twice Cutting, for obtaining the zero-stress states of cartilage and muscle of trachea. The method applied cuts at the two junctions of tracheal cartilage and muscle perpendicular to the tangent lines of cartilage at its tips. The cartilaginous and muscular opening angles are defined for the first time in Twice Cutting methods. Based on the analysis of cartilaginous and muscular geometric information in no-load and zero-stress states, it is found that there are compressive and tensile residual strains in the inner and outer walls of the cartilage respectively. Residual strains at the muscular inner wall of tracheal rings near bifurcation are negative, whereas those of other rings are positive, and residual strains at outer wall of all rings are positive. This phenomenon of tracheal muscle residual strains is different from those of vessel etc. The results also show that the absolute values of cartilaginous strains are considerably smaller than that of muscular ones, with the ratio being around 0.05. The values of all the tracheal parameters, including residual strains and opening angles, are reducing with the increasing value of tracheal rings' position. So the consequences obtained in this paper not only indicate that the trachea is a non-uniform tissue along the circumferential and axial directions, but also reveal the differences between the trachea and other living tissues, such as vessel, esophagus. This is a basic research for further work, such as determining stress in trachea, to which the cartilaginous and muscular zero-stress states should be referred.

Viscoelastic behavior of small intestine in streptozotocin-induced diabetic rats.

Diabetes is associated with remodeling of the morphology and elastic properties of the small intestine. This study aims to study remodeling of the viscoelastic (time-dependent) properties of the small intestine during experimental diabetes. Stress relaxation tests were performed on the duodenum, jejunum, and ileum in 10 nondiabetic and 28 streptozotocin (STZ) -induced diabetic rats. The rats were made diabetic by a single intraperitoneal injection of 50 mg/kg STZ. The diabetic rats were allocated into groups living four days or one, two, or four weeks after the induction of diabetes (N = 7 in each group). The mechanical test was performed using a machine that rapidly stretched the intestinal tube to 40% more than the resting length. The intestinal diameter and wall area were obtained from digitized images of the intestinal segments at no-load and zero-stress states. The stress (force per area) was computed and a reduced-stress relaxation function was applied. The log decay parameter C and the slow and fast time constants tau1 and tau2 were computed. STZ-induced diabetes was associated with a progressive increase in the wall thickness and wall cross-sectional area (P < 0.05). tau1 and tau2 increased and C decreased. The slope alpha became consistently less negative (P < 0.05), and the stress after 600-sec relaxation gradually increased during diabetes (P < 0.01). In conclusion, the viscoelastic behavior of the intestinal wall changed during the development of diabetes.

Biomechanical and morphometric intestinal remodelling during experimental diabetes in rats.

Morphometric and passive biomechanical properties were studied in the duodenum, jejunum and ileum in 10 non-diabetic and 40 streptozotocin-induced diabetic rats. The diabetic rats were divided into groups living 4 days, 1, 2, and 4 weeks after diabetes was induced ( n=10 for each groups). The mechanical test was done as a ramp distension experiment. The intestinal diameter and length were obtained from digitised images of the intestinal 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 geometry. The blood glucose concentration increased four- to fivefold in the diabetic rats. Streptozotocin-induced diabetes generated pronounced increase in the weight per centimetre length, wall thickness and wall cross-sectional area in all intestinal segments during diabetes ( p<0.05). Histological analysis showed that the thickness of the intestinal layers was increased in all segments during diabetes ( p<0.05). In the duodenum the opening angle did not change in the first 2 weeks and decreased after 4 weeks ( p<0.05). In the jejunum and ileum the opening angle increased after 1 week in the diabetic group. The residual strain showed the same pattern as the opening angle. Furthermore, it was found that the circumferential and longitudinal stiffness of the intestinal wall increased with the duration of diabetes ( p<0.05 and p<0.01). Morphological and biomechanical remodelling of the small intestine occurred during the development of diabetes.

Stress distribution in the layered wall of the rat oesophagus

The topic of this study is to obtain the constitutive equations for two layers in the oesophagus from inflation experiment of each layer in the in vivo state and to calculate the corresponding stress distribution referenced to a new stress-free state for multi-layered tissue. The oesophagus is treated as a two-layered structure consisting of an inner submucosa layer and an outer muscle layer. An anisotropic exponential pseudo-strain energy density function is used as the constitutive equation to fit the experimental loading curve and for the calculation of the stress distribution in each layer. Significant differences in the constitutive parameters and zero-stress parameter were found between the submucosa layer and the muscle layer. The stress in the submucosa layer was higher than that in the muscle layer and intact (non-separated) wall under the same loading conditions. The submucosa layer was stiffer than the muscle layer and the intact wall. The zero-stress state of the intact oesophagus and the zero-stress state after separation were used as a reference to compare the stress–strain distributions. The residual strain was discontinuous when using the zero-stress state after separation as a reference. Furthermore, the circumferential stress when using the zero-stress state of the intact wall as a reference was about 100 times higher than that referenced to the zero-stress state after separation. Hence, it is important to use the zero-stress state obtained after the layer separation as the stress-free state in the study of multi-layered tissue.

Biomechanical properties of esophagus during systemic treatment with epidermal growth factor in rats.

The epidermal growth factor (EGF) is a growth factor with effects on many cell types and tissue. Morphometric and passive biomechanical properties were studied in isolated segments of the esophagus in 22 EGF-treated rats and 12 control rats. The rats were divided into groups with EGF treatment for 2, 4, 7, and 14 days (n=6 for each group except n=4 for the 14 days EGF-treatment group) or saline treatment (n=3 for each group). The mechanical test was performed as a distension experiment in vitro where the whole esophagus was stretched to its in situ length and distended with pressures up to 10 cm H2O using a ramp distension protocol. The pressure and outer diameter were recorded. Circumferential stress (force per area) and strain (deformation) were computed from the diameter and pressure data using the zero-stress state as reference. The zero-stress state was obtained by cutting esophageal rings radially. This caused the rings to open up into a sector. EGF induced pronounced morphometric changes, e.g., the wall thickness, wall cross-sectional area, and inner and outer circumferential lengths significantly increased during the EGF treatment. Histological analysis showed mucosa and submucosa growth during EGF treatment. The opening angle and residual strains increased with the highest value in the 14 days EGF-treated group (P<0.05). The change in opening angle depended largely on the change in mucosa thickness. Furthermore, the circumferential stiffness of the esophagus reached a maximum after 7 days EGF treatment (P<0.01).

Some Unusual Considerations about Vessel Walls and Wall Stresses

The “zero-stress state” of blood vessels is usually defined with respect to the atmospheric pressure p a (≈750 mmHg). As a consequence, circumferential and axial wall stresses due to a positive transmural pressure can only be positive and thus, by definition, only tensile. If the zero-stress state were defined with respect to vacuum pressure (0 mmHg), the compressive stress −p a generated by p a everywhere in the wall would, however, be included so that negative (=compressive) wall stresses would formally become possible. In order to examine the consequences this alternative definition would have for arteries, we have compared radial, circumferential, and axial stresses calculated “conventionally” to the values they take when the zero-stress state is defined “correctly” by reference to the vacuum pressure. It turns out that, under normal physiologic conditions, axial stress and perhaps also circumferential stress might well be compressive in many elastic and conductance arteries, contrary to the intuitive conviction of many people. Since the type of stresses a vessel wall is submitted to may be highly relevant for its structure and mechanical properties, this unconventional way of considering wall stresses may reveal unsuspected relationships between wall stresses on one side, and wall structure, vessel growth, adaptation and repair processes, atherosclerosis, angioplasty or stenting on the other side. Similar considerations might also prove useful with regard to cardiac hypertrophy.

Biomechanical properties of ileum after systemic treatment with epithelial growth factor.

Systemic treatment with epidermal growth factor (EGF) leads to growth of all parts of the small intestine in normal functioning rats. In this study, we investigated the effect of this growth process on morphometric and biomechanical parameters of ileum. Rats were treated with EGF (150 microg.kg(-1)day(-1))or placebo via osmotic minipumps for 2, 4, 7, and 14 days. A segment of ileum was removed. The morphology at no-load state and zero-stress state was measured and passive biomechanical properties were assessed using a biaxial test machine (combined inflation and axial stretching). The ileum weight increased after EGF administration. After 4 days' EGF treatment, the wall thickness was increased. Significantly smaller inner perimeters were seen in 4 day and 7 day EGF treatment groups. The opening angle and residual strain began to increase after 7 days' EGF treatment. Wall stiffness, evaluated from the stress-strain curves, showed a continuous decrease in circumferential direction during the first 7 days' EGF treatment. The longitudinal stiffness increased during the first 7 days. The stress-strain curves for both circumferential and longitudinal direction tended to shift back to normal 14 days after starting EGF administration. EGF can cause significant changes both in the morphology and in the passive mechanical properties of the rat ileum.

Remodelling of the left anterior descending artery in a porcine model of supravalvular aortic stenosis.

Knowledge of the adaptive mechanisms of structure and function of coronary arteries in response to physical stress is important in human health and disease. To gain a better understanding of the adaptive mechanisms of morphology in the zero-stress state of the coronary arteries in a porcine model of hypertension and flow overload. The effects of simultaneous increases in pressure and flow were examined by studying the left anterior decending (LAD) artery in supravalvular aortic stenosis (SVAS). In this model, the pressure is uniformly increased along the length of the LAD artery trunk, whereas the increase in flow is significantly greater in the proximal than in the distal artery. The longitudinal variation of vessel dimension, medial and adventitial area, opening angle and residual strains were examined in the LAD arteries of aortic-banded (n = 5) and control (n = 5) pigs. Our results show that the wall shear stress was normalized, whereas the circumferential stress was increased, in the proximal portion of the LAD artery after 5 weeks of SVAS. In the distal artery, both shear and circumferential stresses were normalized. The vessel wall area was also increased in the remodelled vessels as the result of an increase in the medial and adventitial area. The major conclusion of this study is that, in the SVAS model, the remodelling process of the coronary artery is consistent with normalization of shear stress despite an increase in the circumferential stress. Furthermore, the remodelling of the zero-stress state is also dominated by flow overload.

Remodelling of zero-stress state of small intestine in streptozotocin-induced diabetic rats. Effect of gliclazide

Background. Biomechanical properties in terms of residual strains in diabetic small intestine have not been studied. Furthermore, no data have been reported on effect of gliclazide on gastrointestinal complications of diabetes. Aims. To determine remodelling of zero-stress state of small intestine in streptozotocin-induced diabetic rats and effect of gliclazide treatment. Materials. Morphological properties and residual strains were studied in duodenum, jejunum and ileum obtained from diabetic rats, gliclazide-treated diabetic rats and normal rats (n=8 each group). Methods. Diabetes was induced by single intraperitoneal injection of 65 mglkg streptozotocin. Gliclazide (10 mg kg −1 day −1) was injected directly into stomach lumen by intragastric gavage twice daily. Experimental period was 35 days. To approach no-load state; intestinal segments were surgically excised and cut transversely into short ring-shaped segments. Each ring was cut radially to obtain geometry of zero-stress state. Circumferential length, the wall thickness and opening angle were measured from digital images of each specimen and residual strains were computed. Results. Blood glucose level of diabetic group (∼20 mmol/1) was consistently higher than that in normal group (∼4 mmol/l) after induction of diabetes (p<0.001). Gliclazide lowered average blood glucose level to between 10 and 15 mmol/l (p<0. 001). Plasma insulin levels of both diabetic groups (average between 10 and 15 pmol/l) were significantly lower than those in normal group (average ∼18 pmol/l, p<0.05). Wet weight per unit length and wall thickness of duodenum, jejunum and ileum were significantly higher in Diabetes group than those in Normal group (p<0.05). Opening angle and absolute value of residual strain were significantly smaller in duodenum and larger in jejunum and ileum in Diabetes group than in Normal group (p<0.01). Gliclazide treatment partly restored these changes (p<0.05). Conclusions. Diabetes induced morphometric and biomechanical remodelling in intestine. Gliclazide partly restored these changes.

Matching gene activity with physiological functions.

Matching the activity of the genes with biomechanics and physiology is an effective way to use cDNA microarray technology. Required are data on the change of activities of genes associated with specific physiological functions with respect to a continuous variable such as time. For each pair of data (gene and physiological function) as functions of time, we can compute a coefficient of correlation, R. The correlation is perfect if R is +1 or -1; it is nonexistent if R = 0. By evaluating R for every gene in a microarray, we can arrange the genes in the order of the number R, thus learning which genes are best correlated with the mechanical or physiological function. We illustrate this procedure by studying the blood vessels in the lung in response to pulmonary hypoxic hypertension, including the remodeling of vascular morphometry, the elastic moduli, and the zero-stress state of the vessel wall. For each physiological function, we identify the top genes that correlate the best. We found that different genes correlate best with a given function in large and small arteries, and that the genes in pulmonary veins which respond to arterial functions are different from those in pulmonary arteries. We found one set of genes matching the remodeling of arterial wall thickness, but another set of genes whose integral of activity over time best fit the wall thickness change. Our method can be used to study other thought-provoking problems.

Zero-stress state of intra- and extraparenchymal airways from human, pig, rabbit, and sheep lung.

Alterations in airway wall anatomic properties and the consequential effects on airway narrowing have been assessed by use of computational models. In these models, it is generally assumed that at zero transmural pressure the airway wall exists in a zero-stress state. Many studies have shown that this is often not the case, as evidenced by a nonzero opening angle. In this study, we measured the opening angle of airway rings at zero transmural pressure to test this assumption. The airway tree was dissected from human, pig, sheep, and rabbit lungs. Airways were excised from the tree, and the opening angle was measured. There were obvious species and regional differences in opening angle. Rabbit airways from both extraparenchymal and intraparenchymal sites exhibited marked opening angles (7-82 degrees). Extraparenchymal airways from sheep had large opening angles (up to 50 degrees), but ovine intraparenchymal airways had small opening angles. Measurable opening angles were rarely observed in human and porcine airways of any size. The assumption of a stable zero-stress state at zero transmural pressure is therefore valid for human and porcine, but not rabbit and sheep, airways.

Morphological properties and residual strain along the small intestine in rats.

Residual stress and strain are important for gastrointestinal function and relate to the geometric configuration, the loading conditions and the zero-stress state of the gastrointestinal tract. The purpose of this project is to provide morphometric data and residual strains for the rat small intestine ( n =11). To approach the no-load state, the intestine was surgically excised, transferred to an organ bath and cut transversely into short ring-shaped segments. Each ring was cut radially for obtaining the zero-stress state. The residual stress can be characterised by an opening angle. The strain difference between the zero-stress state and the no-load state is called residual strain. Large morphometric variations were found along the small intestine. The wall thickness was highest in the proximal duodenum and decreased in distal direction along the axis of the small intestine (P<0.001). The circumferential length of the inner and outer surfaces decreased rapidly along the length of duodenum by 30-50% (P<0.001). The wall area and lumen area showed a similar pattern (P<0.001). In zero-stress state the rings always opened up after making the cut. The experiments resulted in larger inner circumferential length and smaller outer circumferential length when compared to the no-load state. The wall thickness and wall area did not differ between the no-load and zero-stress state. The opening angle and tangent rotation angle increased along the length of the duodenum and had its highest value 30% down the intestine. Further down the intestine it decreased again (P<0.001). The serosal residual strain was tensile with the highest value close to the ligament of Treitz (P<0.001). The mucosal residual strain was compressive in all segments of the small intestine with average values between -0.25 and -0.4 and with the lowest values close to the ligament of Treitz (P<0.001). Axial variation in morphometric properties and residual strains were found in the small intestine. Existence of large residual strains indicates that the zero-stress state must be considered in future biomechanical studies in the gastrointestinal tract.

Opening angles and residual strains in normal rat trachea

The no-load state and zero-stress state of the normal rat trachea were analyzed. It was found that there exist compressive residual strains in the inner wall region of the rat trachea and tensile residual strains in the outer wall region. The fact that the opening angle of the rat trachea cut at the cartilaginous region is significantly larger than that cut at the muscular portion shows that residual strains exist mainly in the muscular region in the rat trachea. It was also indicated that the opening angles and residual strains expressed by cutting at the muscular portion are basically identical along longitudinal location and those expressed by cutting in the cartilaginous region tend to increase in the longitudinal direction in the normal rat, and that there exists quantitatively positive correlation between the opening angles and residual strains in rat trachea. The results will help to further understand the opening angles and residual strains in the trachea and study tracheal remodeling in response to mechanical environment.

Oesophageal morphometry and residual strain in a mouse model of osteogenesis imperfecta.

Recently, it was demonstrated in the oesophagus that the zero-stress state is not a closed cylinder but an open circular cylindrical sector. The closed cylinder with no external loads applied is called the no-load state and residual strain is the difference in strain between the no-load state and the zero-stress state. To understand the physiology and pathology of the oesophagus, it is necessary to know the zero-stress state and the stress-strain relationships of the tissues in the oesophagus, and the changes of these states and relationships due to biological remodelling of the tissues under stress. The aim of this study was to investigate the morphological and biomechanical remodelling at the no-load and zero-stress states in mutant osteogenesis imperfecta murine (oim) mice with collagen deficiency. The oesophagi of seven oim and seven normal wild-type mice were excised, cleaned, and sectioned into rings in an organ bath containing calcium-free Krebs solution with dextran and EGTA. The rings were photographed in the no-load state and cut radially to obtain the zero-stress state. Equilibrium was awaited for 30 min and the specimens were photographed again. Circumferences, submucosa and muscle layer thicknesses and areas, and the opening angle were measured from the digitized images. The oesophagi in oim mice had smaller layer thicknesses and areas compared to the wild types. The largest reduction in layer thickness in oim mice was found in the submucosa (approximately 36%). Oim mice had significantly larger opening angles (120.2 +/- 4.5 degrees ) than wild-type mice (93.0 +/- 11.2 degrees ). The residual strain was compressive at the mucosal surface and tensile at the serosal surface in both oim and wild types. In the oim mice, the residual strains at the serosal and mucosal surfaces and the mucosa-submucosal-muscle layer interface were higher than in the wild types (P < 0.05). The gradient of residual strain per unit thickness was higher in oim mice than in wild-type mice, and was highest in submucosa (P < 0.05). The only morphometric measure that was similar in oim and wild-type mice was the inner circumference in the no-load state. In conclusion, our data show significant differences in the residual strain distribution and morphometry between oim mice and wild-type mice. The data suggest that the residual stress in oesophagus is caused by the tension in the muscle layer rather than the stiffness of the submucosa in compression and that the remodelling process in the oim oesophagus is due mainly to morphometric and biomechanical alterations in the submucosa.