Plausibility and parameter sensitivity of micro-finite element-based joint load prediction at the proximal femur

A micro-finite element-based method to estimate the bone loading history based on bone architecture was recently presented in the literature. However, a thorough investigation of the parameter sensitivity and plausibility of this method to predict joint loads is still missing. The goals of this study were (1) to analyse the parameter sensitivity of the joint load predictions at one proximal femur and (2) to assess the plausibility of the results by comparing load predictions of ten proximal femora to in vivo hip joint forces measured with instrumented prostheses (available from www.orthoload.com). Joint loads were predicted by optimally scaling the magnitude of four unit loads (inclined \(-20^{\circ }\) to \(100^{\circ }\) with respect to the vertical axis) applied to micro-finite element models created from high-resolution computed tomography scans (\(30.3~\upmu \)m voxel size). Parameter sensitivity analysis was performed by varying a total of nine parameters and showed that predictions of the peak load directions (range 10\(^{\circ }\)–\(30^{\circ }\)) are more robust than the predicted peak load magnitudes (range 2344.8–4689.5 N). Comparing the results of all ten femora with the in vivo loading data of ten subjects showed that peak loads are plausible both in terms of the load direction (in vivo: \(18.2\pm 2.0^{\circ }\), predicted: \(20.0^{\circ }\)) and magnitude (in vivo: \(2707.6\pm 443.3~\hbox {N}\), predicted: \(3372.2\pm 597.9~\hbox {N}\)). Overall, this study suggests that micro-finite element-based joint load predictions are both plausible and robust in terms of the predicted peak load direction, but predicted load magnitudes should be interpreted with caution.     

Improved identifiability of myocardial material parameters by an energy-based cost function

Myocardial stiffness is a valuable clinical biomarker for the monitoring and stratification of heart failure (HF). Cardiac finite element models provide a biomechanical framework for the assessment of stiffness through the determination of the myocardial constitutive model parameters. The reported parameter intercorrelations in popular constitutive relations, however, obstruct the unique estimation of material parameters and limit the reliable translation of this stiffness metric to clinical practice. Focusing on the role of the cost function (CF) in parameter identifiability, we investigate the performance of a set of geometric indices (based on displacements, strains, cavity volume, wall thickness and apicobasal dimension of the ventricle) and a novel CF derived from energy conservation. Our results, with a commonly used transversely isotropic material model (proposed by Guccione et al.), demonstrate that a single geometry-based CF is unable to uniquely constrain the parameter space. The energy-based CF, conversely, isolates one of the parameters and in conjunction with one of the geometric metrics provides a unique estimation of the parameter set. This gives rise to a new methodology for estimating myocardial material parameters based on the combination of deformation and energetics analysis. The accuracy of the pipeline is demonstrated in silico, and its robustness in vivo, in a total of 8 clinical data sets (7 HF and one control). The mean identified parameters of the Guccione material law were \(C_1=3000\pm 1700\,\hbox {Pa}\) and \(\alpha =45\pm 25\) (\(b_f=25\pm 14\), \(b_{ft}=11\pm 6\), \(b_{t}=9\pm 5\)) for the HF cases and \(C_1=1700\,\hbox {Pa}\) and \(\alpha =15\) (\(b_f=8\), \(b_{ft}=4\), \(b_{t}=3\)) for the healthy case.     

Ultrasound palpation for fast in-situ quantification of articular cartilage stiffness,thickness and relaxation capacity

Most current cartilage testing devices require the preparation of excised samples and therefore do not allow intra-operative application for diagnostic purposes. The gold standard during open or arthroscopic surgery is still the subjective perception of manual palpation. This work presents a new diagnostic method of ultrasound palpation (USP) to acquire applied stress and strain data during manual palpation of articular cartilage. With the proposed method, we obtain cartilage thickness and stiffness. Moreover, repeated palpations allow the quantification of relaxation effects. USP measurements on elastomer phantoms demonstrated very good repeatability for both, stage-guided (97.2%) and handheld (96.0%) applications. The USP measurements were compared with conventional indentation experiments and revealed very good agreement on elastomer phantoms (\(r = 0.98\)) and good agreement on porcine cartilage samples (\(r = 0.76\)). Artificially degenerated cartilage samples showed reduced stiffness, weak capacity to relax after palpation and an increase of stiffness of approximately 50% with each single palpation. Intact cartilage was measured by USP directly at the patella (in situ) and after excision and removal of the subchondral bone (ex situ), leading to stiffness values of \(12.1\pm 5.5\) and \(8.5\pm 5.9\,\hbox {MPa}\) (\(p<0.05\)), respectively. The results demonstrate the potential of the USP system for cartilage testing, its sensitivity to degenerative changes and as a method for quantifying relaxation processes by means of repeated palpations. Furthermore, the differences in the results of in-situ and ex-situ measurements are of general interest, since such comparison has not been reported previously. We point out the limited comparability of ex-situ cartilage with its in-situ biomechanical behavior.     

Backbone resonance assignments for the SET domain of the human methyltransferase NSD2

Aberrant NSD2 methyltransferase activity is implicated as the oncogenic driver in multiple myeloma, suggesting opportunities for novel therapeutic intervention. The methyltransferase activity of NSD2 resides in its catalytic SET domain, which is conserved among most lysine methyltransferases. Here we report the backbone \(\hbox {H}^{\mathrm{N}}\), N, C\(^{\prime }\), \(\hbox {C}^\alpha\) and side-chain \(\hbox {C}^\beta\) assignments of a 25 kDa NSD2 SET domain construct, spanning residues 991–1203. A chemical shift analysis of C\(^{\prime }\), \(\hbox {C}^\alpha\) and \(\hbox {C}^\beta\) resonances predicts a secondary structural pattern that is in agreement with homology models.     

Structural alteration of the endothelial glycocalyx: contribution of the actin cytoskeleton

The endothelial glycocalyx is a carbohydrate–protein layer that lines the luminal surface of the endothelium. It anchors to the cell membrane via its core proteins that share extended link to the actin cytoskeleton. It is widely accepted that those protein domains and the attached carbohydrates are susceptible to pathological changes. It is unclear, however, to what extent the actin cytoskeleton contributes to the glycocalyx stability. In this study, we investigate the role of the actin cytoskeleton in the maintenance of the glycocalyx under static and laminar flow conditions in vitro. Our results show that in the static culture medium neither rapid actin depolymerisation nor prolonged actin disturbance leads to glycocalyx disruption from the apical surface of human umbilical vein endothelial cells. However, when endothelial cells are exposed to laminar flow for 24 h, the glycocalyx is seen to shift to the downstream peripheral region of the cell surface. The mean fluorescence intensity decreases to \(91.9 \pm 2.5\%\) of the control. When actin depolymerisation is introduced, the intensity decreases significantly to \(54.7 \pm 1.3\%\), indicating a severe disruption of the glycocalyx. Similar changes are observed in human aortic endothelial cells, where the intensity of the glycocalyx is reduced to \(72.8 \pm 1.6\%\) of the control. Collectively, we demonstrate that the actin cytoskeleton contributes to structural stability of the glycocalyx under shear stress. Our results can be used to develop new strategies to prevent shedding of the glycocalyx in cardiovascular diseases.     

Understanding the effects of isolation on seed and pollen flow,spatial genetic structure and effective population size of the dioecious tropical tree species <Emphasis Type="Italic">Myracrodruon urundeuva</Emphasis>

This study examines the levels of gene flow, the distance and the patterns of pollen and seed dispersal, the intra-population spatial genetic structure (SGS) and the effective population size of a spatially isolated Myracrodruon urundeuva population using five microsatellite loci. The study was carried out in the Paulo de Faria Ecological Station, São Paulo State, Brazil and included the sampling and mapping of 467 adult-trees and 149 juveniles. Open-pollinated seeds (514) from 29 seed-trees were also sampled and genotyped. Significant SGS was detected in both adult (S _p  = 0.0269) and juveniles trees (S _p  = 0.0246), indicating short-distance seed dispersal. Using maternity analysis, all juveniles had the mother-tree assigned within the stand. A father-tree within the stand was also assigned for 97.3% of the juveniles and 98.4% of offspring. The average pollen dispersal distance measured in juveniles \( \left( {\hat{\delta } = 1 3 8\pm 1 6 9 {\text{ m}},{\text{ mean}} \pm {\text{SD}}} \right) \) and offspring \( \left( {\hat{\delta } = 2 5 2\pm 20 4 {\text{ m}}} \right) \) were higher than the average seed dispersal distance measured in juveniles \( \left( {\hat{\delta } = 1 2 4\pm 1 50{\text{ m}}} \right) \). About 70% of the pollen from juveniles and 51% from offspring traveled less than 200 m and, 72% of the seeds traveled less than 50 m. The effective population size of the studied sample indicates that the 467 adult-trees and 145 juveniles correspond respectively to 335 and 63 individuals that are neither inbred nor relatives. The results are discussed in relation to their impact on seed collection practices and genetic conservation.     

Oestrogen regulates the expression of cathepsin E-A-like gene through ER$$\upbeta $$ in liver of chicken ( Gallus gallus)

The cathepsin E-A-like, also known as ‘similar to nothepsin’, is a new member of the aspartic protease family, which may take part in processing of egg yolk macromolecules, due to it was identified in the chicken egg-yolk. Previously, studies have suggested that the expression of cathepsin E-A-like increased gradually during sexual maturation of pullets, but the exact regulation mechanism is poorly understood. In this study, to gain insight into the function and regulation mechanism of the gene in egg-laying hen, we cloned the cathepsin E-A-like gene and evaluated its evolutionary origin by using both phylogenetic and syntenic methods. The mode of the gene expression regulation was analysed through stimulating juvenile hens with \(17\upbeta \)-estradiol and chicken embryo hepatocytes with \(17\upbeta \)-estradiol combined with oestrogen receptor antagonists including MPP, ICI 182,780 and tamoxifen. Our results showed that cathepsin E-A-like was an orthologoues gene with nothepsin, which is present in birds but not in mammals. The expression of cathepsin E-A-like significantly increased in a dose-dependent manner after the juvenile hens were treated with \(17\upbeta \)-estradiol (\(P~<~0.05\)). Compared with the \(17\upbeta \)-estradiol treatment group, the expression of cathepsin E-A-like was not significantly changed when the hepatocytes were treated with \(17\upbeta \)-estradiol combined with MPP (\(P~<~0.05\)). In contrast, compared with the \(17\upbeta \)-estradiol combined with MPP treatment group, the expression of cathepsin E-A-like was significantly downregulated when the hepatocytes were treated with \(17\upbeta \)-estradiol combined with tamoxifen or ICI 182,780 (\(P~<~0.05\)). These results demonstrated that cathepsin E-A-like shared the same evolutionary origin with nothepsin. The expression of cathepsin E-A-like was regulated by oestrogen, and the regulative effect was predominantly mediated through ER-\(\upbeta \) in liver of chicken.     

Influence of muscle groups’ activation on proximal femoral growth tendency

Muscle and joint contact force influence stresses at the proximal growth plate of the femur and thus bone growth, affecting the neck shaft angle (NSA) and femoral anteversion (FA). This study aims to illustrate how different muscle groups’ activation during gait affects NSA and FA development in able-bodied children. Subject-specific femur models were developed for three able-bodied children (ages 6, 7, and 11 years) using magnetic resonance images. Contributions of different muscle groups—hip flexors, hip extensors, hip adductors, hip abductors, and knee extensors—to overall hip contact force were computed. Specific growth rate for the growth plate was computed, and the growth was simulated in the principal stress direction at each element in the growth front. The predicted growth indicated decreased NSA and FA (of about \(0.1 {^{\circ }}\) over a four-month period) for able-bodied children. Hip abductors contributed the most, and hip adductors, the least, to growth rate. All muscles groups contributed to a decrease in predicted NSA (\(\sim \)0.01\({^{\circ }}\)–0.04\({^{\circ }})\) and FA (\(\sim \)0.004\({^{\circ }}\)–\(0.2{^{\circ }}\)), except hip extensors and hip adductors, which showed a tendency to increase the FA (\(\sim \)0.004\({^{\circ }}\)–\(0.02{^{\circ }}\)). Understanding influences of different muscle groups on long bone growth tendency can help in treatment planning for growing children with affected gait.     

Sensitivity of Chaos Measures in Detecting Stress in the Focusing Control Mechanism of the Short-Sighted Eye

When fixating on a stationary object, the power of the eye’s lens fluctuates. Studies have suggested that changes in these so-called microfluctuations in accommodation may be a factor in the onset and progression of short-sightedness. Like many physiological signals, the fluctuations in the power of the lens exhibit chaotic behaviour. A breakdown or reduction in chaos in physiological systems indicates stress to the system or pathology. The purpose of this study was to determine whether the chaos in fluctuations of the power of the lens changes with refractive error, i.e. how short-sighted a subject is, and/or accommodative demand, i.e. the effective distance of the object that is being viewed. Six emmetropes (EMMs, non-short-sighted), six early-onset myopes (EOMs, onset of short-sightedness before the age of 15), and six late-onset myopes (LOMs, onset of short-sightedness after the age of 15) took part in the study. Accommodative microfluctuations were measured at 22 Hz using an SRW-5000 autorefractor at accommodative demands of 1 D (dioptres), 2 D, and 3 D. Chaos theory analysis was used to determine the embedding lag, embedding dimension, limit of predictability, and Lyapunov exponent. Topological transitivity was also tested for. For comparison, the power spectrum and standard deviation were calculated for each time record. The EMMs had a statistically significant higher Lyapunov exponent than the LOMs (\(0.64\pm 0.33\) vs. \(0.39\pm 0.20~\hbox {D}/\hbox {s}\)) and a lower embedding dimension than the LOMs (\(3.28\pm 0.46\) vs. \(3.67\pm 0.49\)). There was insufficient evidence (non-significant p value) of a difference between EOMs and EMMs or EOMs and LOMs. The majority of time records were topologically transitive. There was insufficient evidence of accommodative demand having an effect. Power spectrum analysis and assessment of the standard deviation of the fluctuations failed to discern differences based on refractive error. Chaos differences in accommodation microfluctuations indicate that the control system for LOMs is under stress in comparison to EMMs. Chaos theory analysis is a more sensitive marker of changes in accommodation microfluctuations than traditional analysis methods.     

Stability of a Stochastic Model of an SIR Epidemic with Vaccination

We prove almost sure exponential stability for the disease-free equilibrium of a stochastic differential equations model of an SIR epidemic with vaccination. The model allows for vertical transmission. The stochastic perturbation is associated with the force of infection and is such that the total population size remains constant in time. We prove almost sure positivity of solutions. The main result concerns especially the smaller values of the diffusion parameter, and describes the stability in terms of an analogue \(\mathcal{R}_\sigma\) of the basic reproduction number \(\mathcal{R}_0\) of the underlying deterministic model, with \(\mathcal{R}_\sigma \le \mathcal{R}_0\). We prove that the disease-free equilibrium is almost sure exponentially stable if \(\mathcal{R}_\sigma <1\).     

Towards the prediction of flow-induced shear stress distributions experienced by breast cancer cells in the lymphatics

Tumour metastasis in the lymphatics is a crucial step in the progression of breast cancer. The dynamics by which breast cancer cells (BCCs) travel in the lymphatics remains poorly understood. The goal of this work is to develop a model capable of predicting the shear stresses metastasising BCCs experience using numerical and experimental techniques. This paper models the fluidic transport of large particles (\(\eta =d_{\mathrm{p}}/W=0.1-0.4\) where \(d_{\mathrm{p}}\) is the particle diameter and W is the channel width) subjected to lymphatic flow conditions (\({ Re}=0.04\)), in a \(100\times 100\,\upmu \hbox {m}\) microchannel. The feasibility of using the dynamic fluid body interaction (DFBI) method to predict particle motion was assessed, and particle tracking experiments were performed. The experiments found that particle translational velocity decreased from the undisturbed fluid velocity with increasing particle size (5–14% velocity lag for \(\eta =0.1-0.3\)). DFBI simulations were found to better predict particle behaviour than theoretical predictions; however, mesh restrictions in the near-wall region (\(0.2\,\mathrm{W}>y>0.8\,\mathrm{W}\)) result in computationally expensive models. The simulations were in good agreement with the experiments (\(<12\%\) difference) across the channel (\(0.2\,\mathrm{W}\le y\le 0.8\,\mathrm{W}\)), with differences up to 25% in the near-wall region. Particles experience a range of shear stresses (0.002–0.12 Pa) and spatial shear gradients (\(0.004-0.137\,\hbox {Pa}/\upmu \hbox {m}\)) depending on their size and radial position. The predicted shear gradients are far in excess of values associated with BCC apoptosis (\(0.004-0.023\,\hbox {Pa}/\upmu \hbox {m}\)). Increasing our understanding of the shear stress magnitudes and gradients experienced by BCCs could be leveraged to elucidate whether a particular BCC size or location exists that encourages metastasis within the lymphatics.     

Physical parameter estimation from porcine ex vivo vocal fold dynamics in an inverse problem framework

This study presents a framework for a direct comparison of experimental vocal fold dynamics data to a numerical two-mass-model (2MM) by solving the corresponding inverse problem of which parameters lead to similar model behavior. The introduced 2MM features improvements such as a variable stiffness and a modified collision force. A set of physiologically sensible degrees of freedom is presented, and three optimization algorithms are compared on synthetic vocal fold trajectories. Finally, a total of 288 high-speed video recordings of six excised porcine larynges were optimized to validate the proposed framework. Particular focus lay on the subglottal pressure, as the experimental subglottal pressure is directly comparable to the model subglottal pressure. Fundamental frequency, amplitude and objective function values were also investigated. The employed 2MM is able to replicate the behavior of the porcine vocal folds very well. The model trajectories’ fundamental frequency matches the one of the experimental trajectories in \(98.6\%\) of the recordings. The relative error of the model trajectory amplitudes is on average \(9.5\%\). The experiments feature a mean subglottal pressure of 10.16 (SD \(= 2.31\)) \({\text {cmH}}_2{\text {O}}\); in the model, it was on average 7.61 (SD \(= 2.40\)) \({\text {cmH}}_2{\text {O}}\). A tendency of the model to underestimate the subglottal pressure is found, but the model is capable of inferring trends in the subglottal pressure. The average absolute error between the subglottal pressure in the model and the experiment is 2.90 (SD \(= 1.80\)) \({\text {cmH}}_2{\text {O}}\) or \(27.5\%\). A detailed analysis of the factors affecting the accuracy in matching the subglottal pressure is presented.     

Optimal Therapy Scheduling Based on a Pair of Collaterally Sensitive Drugs

Despite major strides in the treatment of cancer, the development of drug resistance remains a major hurdle. One strategy which has been proposed to address this is the sequential application of drug therapies where resistance to one drug induces sensitivity to another drug, a concept called collateral sensitivity. The optimal timing of drug switching in these situations, however, remains unknown. To study this, we developed a dynamical model of sequential therapy on heterogeneous tumors comprised of resistant and sensitive cells. A pair of drugs (DrugA, DrugB) are utilized and are periodically switched during therapy. Assuming resistant cells to one drug are collaterally sensitive to the opposing drug, we classified cancer cells into two groups, \(A_\mathrm{R}\) and \(B_\mathrm{R}\), each of which is a subpopulation of cells resistant to the indicated drug and concurrently sensitive to the other, and we subsequently explored the resulting population dynamics. Specifically, based on a system of ordinary differential equations for \(A_\mathrm{R}\) and \(B_\mathrm{R}\), we determined that the optimal treatment strategy consists of two stages: an initial stage in which a chosen effective drug is utilized until a specific time point, T, and a second stage in which drugs are switched repeatedly, during which each drug is used for a relative duration (i.e., \(f \Delta t\)-long for DrugA and \((1-f) \Delta t\)-long for DrugB with \(0 \le f \le 1\) and \(\Delta t \ge 0\)). We prove that the optimal duration of the initial stage, in which the first drug is administered, T, is shorter than the period in which it remains effective in decreasing the total population, contrary to current clinical intuition. We further analyzed the relationship between population makeup, \(\mathcal {A/B} = A_\mathrm{R}/B_\mathrm{R}\), and the effect of each drug. We determine a critical ratio, which we term \(\mathcal {(A/B)}^{*}\), at which the two drugs are equally effective. As the first stage of the optimal strategy is applied, \(\mathcal {A/B}\) changes monotonically to \(\mathcal {(A/B)}^{*}\) and then, during the second stage, remains at \(\mathcal {(A/B)}^{*}\) thereafter. Beyond our analytic results, we explored an individual-based stochastic model and presented the distribution of extinction times for the classes of solutions found. Taken together, our results suggest opportunities to improve therapy scheduling in clinical oncology.     

Glutathione <Emphasis Type="BoldItalic">S</Emphasis>-transferase P1 gene polymorphisms and susceptibility to coronary artery disease in a subgroup of north Indian population

The present study aimed to investigate the association of \(\hbox {g}.313\hbox {A}{>}\hbox {G}\) and \(\hbox {g}.341\hbox {C}{>}\hbox {T}\) polymorphisms of GSTP1 with coronary artery disease (CAD) in a subgroup of north Indian population. In the present case–control study, CAD patients (\(n = 200\)) and age-matched, sex-matched and ethnicity-matched healthy controls (\(n = 200\)) were genotyped for polymorphisms in GSTP1 using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Genotype distribution of \(\hbox {g}.313\hbox {A}{>}\hbox {G}\) and \(\hbox {g}.341\hbox {C}{>}\hbox {T}\) polymorphisms of GSTP1 gene was significantly different between cases and controls (\(P = 0.005\) and 0.024, respectively). Binary logistic regression analysis showed significant association of A/G (odds ratio (OR): 1.6, 95% CI: 1.08–2.49, \(P = 0.020\)) and G/G (OR: 3.1, 95% CI: 1.41–6.71, P \(=\) 0.005) genotypes of GSTP1 \(\hbox {g}.313\hbox {A}{\!>\!}\hbox {G}\), and C/T (OR: 5.8, 95% CI: 1.26–26.34, \(P = 0.024\)) genotype of GSTP1 \(\hbox {g}.341\hbox {C}{>}\hbox {T}\) with CAD. The A/G and G/G genotypes of \(\hbox {g}.313\hbox {A}{>}\hbox {G}\) and C/T genotype of \(\hbox {g}.341\hbox {C}{>}\hbox {T}\) conferred 6.5-fold increased risk for CAD (OR: 6.5, 95% CI: 1.37–31.27, \(P = 0.018\)). Moreover, the recessive model of GSTP1 \(\hbox {g}.313\hbox {A}{>}\hbox {G}\) is the best fit inheritance model to predict the susceptible gene effect (OR: 2.3, 95% CI: 1.11–4.92, \(P = 0.020\)). In conclusion, statistically significant associations of GSTP1 \(\hbox {g}.313\hbox {A}{>}\hbox {G}\) (A/G, G/G) and \(\hbox {g}.341\hbox {C}{>}\hbox {T}\) (C/T) genotypes with CAD were observed.     

Contribution of left ventricular residual stress by myocytes and collagen: existence of inter-constituent mechanical interaction

We quantify the contribution of myocytes, collagen fibers and their interactions to the residual stress field found in the left ventricle (LV) using both experimental and theoretical methods. Ring tissue samples extracted from normal rat, male and female, LV were treated with collagenase and decellularization to isolate myocytes and collagen fibers, respectively. Opening angle tests were then performed on these samples as well as intact tissue samples containing both constituents that served as control. Our results show that the collagen fibers are the main contributor to the residual stress fields found in the LV. Specifically, opening angle measured in collagen-only samples (106.45\(^\circ \) ± 23.02\(^\circ \)) and myocytes-only samples (21.00\(^\circ \) ± 4.37\(^\circ \)) was significantly higher and lower than that of the control (57.88\(^\circ \) ± 12.29\(^\circ \)), respectively. A constrained mixture (CM) modeling framework was then used to infer these experimental results. We show that the framework cannot reproduce the opening angle found in the intact tissue with measurements made on the collagen-only and myocytes-only samples. Given that the CM framework assumes that each constituent contributes to the overall mechanics simply by their mere presence, this result suggests the existence of some myocyte–collagen mechanical interaction that cannot be ignored in the LV. We then propose an extended CM formulation that takes into account of the inter-constituent mechanical interaction in which constituents are deformed additionally when they are physically combined into a mixture. We show that the intact tissue opening angle can be recovered in this framework.     

A Mathematical Model Supports a Key Role for Ae4 (Slc4a9) in Salivary Gland Secretion

We develop a mathematical model of a salivary gland acinar cell with the objective of investigating the role of two \(\mathrm{Cl}^-/\mathrm{HCO}_3^-\) exchangers from the solute carrier family 4 (Slc4), Ae2 (Slc4a2) and Ae4 (Slc4a9), in fluid secretion. Water transport in this type of cell is predominantly driven by \(\mathrm{Cl}^-\) movement. Here, a basolateral \(\mathrm{Na}^+/ \mathrm{K}^+\) adenosine triphosphatase pump (NaK-ATPase) and a \(\mathrm{Na}^+\)–\(\mathrm{K}^+\)–\(2 \mathrm{Cl}^-\) cotransporter (Nkcc1) are primarily responsible for concentrating the intracellular space with \(\mathrm{Cl}^-\) well above its equilibrium potential. Gustatory and olfactory stimuli induce the release of \(\mathrm{Ca}^{2+}\) ions from the internal stores of acinar cells, which triggers saliva secretion. \(\mathrm{Ca}^{2+}\)-dependent \(\mathrm{Cl}^-\) and \(\mathrm{K}^+\) channels promote ion secretion into the luminal space, thus creating an osmotic gradient that promotes water movement in the secretory direction. The current model for saliva secretion proposes that \(\mathrm{Cl}^-/ \mathrm{HCO}_3^-\) anion exchangers (Ae), coupled with a basolateral \(\mathrm{Na}^+/\hbox {proton}\) (\(\hbox {H}^+\)) (Nhe1) antiporter, regulate intracellular pH and act as a secondary \(\mathrm{Cl}^-\) uptake mechanism (Nauntofte in Am J Physiol Gastrointest Liver Physiol 263(6):G823–G837, 1992; Melvin et al. in Annu Rev Physiol 67:445–469, 2005.  https://doi.org/10.1146/annurev.physiol.67.041703.084745). Recent studies demonstrated that Ae4 deficient mice exhibit an approximate \(30\%\) decrease in gland salivation (Peña-Münzenmayer et al. in J Biol Chem 290(17):10677–10688, 2015). Surprisingly, the same study revealed that absence of Ae2 does not impair salivation, as previously suggested. These results seem to indicate that the Ae4 may be responsible for the majority of the secondary \(\mathrm{Cl}^-\) uptake and thus a key mechanism for saliva secretion. Here, by using ‘in-silico’ Ae2 and Ae4 knockout simulations, we produced mathematical support for such controversial findings. Our results suggest that the exchanger’s cotransport of monovalent cations is likely to be important in establishing the osmotic gradient necessary for optimal transepithelial fluid movement.     

Growth plate cartilage shows different strain patterns in response to static versus dynamic mechanical modulation

Longitudinal growth of long bones and vertebrae occurs in growth plate cartilage. This process is partly regulated by mechanical forces, which are one of the underlying reasons for progression of growth deformities such as idiopathic adolescent scoliosis and early-onset scoliosis. This concept of mechanical modulation of bone growth is also exploited in the development of fusionless treatments of these deformities. However, the optimal loading condition for the mechanical modulation of growth plate remains to be identified. The objective of this study was to evaluate the effects of in vitro static versus dynamic modulation and of dynamic loading parameters, such as frequency and amplitude, on the mechanical responses and histomorphology of growth plate explants. Growth plate explants from distal ulnae of 4-week-old swines were extracted and randomly distributed among six experimental groups: baseline (\(n=10\)), control (\(n=10\)), static (\(n=10\)) and dynamic (\(n=3\times 10\)). For static and dynamic groups, mechanical modulation was performed in vitro using an Indexed CartiGen bioreactor. A stress relaxation test combined with confocal microscopy and digital image correlation was used to characterize the mechanical responses of each explant in terms of peak stress, equilibrium stress, equilibrium modulus of elasticity and strain pattern. Histomorphometrical measurements were performed on toluidine blue tissue sections using a semi-automatic custom-developed MATLAB toolbox. Results suggest that compared to dynamic modulation, static modulation changes the strain pattern of the tissue and thus is more detrimental for tissue biomechanics, while the histomorphological parameters are not affected by mechanical modulation. Also, under dynamic modulation, changing the frequency or amplitude does not affect the biomechanical response of the tissue. Results of this study will be useful in finding optimal and non-damaging parameters for the mechanical modulation of growth plate in fusionless treatments.     

Simulation of extracellular matrix remodeling by fibroblast cells in soft three-dimensional bioresorbable scaffolds

To culture functional soft tissues and organs in three-dimensional scaffolds, it is essential to elucidate the optimal scaffold mechanical properties. However, mechanoregulated soft tissue remodeling is not well understood. In this study, we hypothesized that individual cells are capable of remodeling extracellular matrix within a short proximity of themselves in order to match the stiffness of the broader surrounding matrix. This theory was implemented in a three-dimensional finite element model to simulate soft tissue remodeling of human fibroblast cells in two collagen–chitosan scaffolds with different mechanical properties. Simulation results closely matched with previously reported experimental data, showing that soft tissue growth in compliant (Scaf-A, 4.30 kPa) and stiff (Scaf-B, 17.03 kPa) scaffolds led to an almost eightfold difference in the resulting stiffnesses after 10 days (8.40 kPa for Scaf-A, 59.25 kPa for Scaf-B). Furthermore, varying the simulated rate for tissue remodeling by \(\pm \)50 % caused unequal changes in the resulting stiffness (+3.6 and \(-\)23 % for Scaf-A, +5 and \(-\)17 % for Scaf-B), and \(\pm \)50 % changes in the assumed upper limit on tissue stiffness only had larger effects on the stiff scaffold (+42 and \(-\)44 % for Scaf-B). These results reinforce the notion that soft tissue remodeling is governed by the stiffness of the surrounding matrix, until meeting an upper limit on tissue stiffness.