Subcutaneous Tissue Mechanical Behavior is Linear and Viscoelastic Under Uniaxial Tension

Subcutaneous tissue is part of a bodywide network of “loose” connective tissue including interstitial connective tissues separating muscles and surrounding all nerves and blood vessels. Despite its ubiquitous presence in the body and its potential importance in a variety of therapies utilizing mechanical stretch, as well as normal movement and exercise, very little is known about loose connective tissue's biomechanical behavior. This study aimed to determine elastic and viscoelastic mechanical properties of ex-vivo rat subcutaneous tissue in uniaxial tension with incremental stress relaxation experiments. The elastic response of the tissue was linear, with instantaneous and equilibrium tensile moduli of 4.77 kPa and 2.75 kPa, respectively. Using a 5 parameter Maxwell solid model, material parameters μ₁ = 0.95 ± 0.24 Ns/m and μ₂ = 8.49 ± 2.42 Ns/m defined coefficients of viscosity related to time constants τ_1M = 3.83 ± 0.15 sec and τ_2M = 30.15 ± 3.16 sec, respectively. Using a continuous relaxation function, parameters C = 0.25 ± 0.12, τ_1C = 1.86 ± 0.34 sec, and τ_2C = 110.40 ± 25.59 sec defined the magnitude and frequency limits of the relaxation spectrum. This study provides baseline information for the stress-strain behaviors of subcutaneous connective tissue. Our results underscore the differences in mechanical behaviors between loose and high-load bearing connective tissues and suggest that loose connective tissues may function to transmit mechanical signals to and from the abundant fibroblasts, immune, vascular, and neural cells present within these tissues.     

大鼠不同部位疏松结缔组织撕片的制备和观察

目的　对比大鼠皮下取材和肠系膜取材制备疏松结缔组织撕片的差异;观察同一部位取材的疏松结缔组织HE染色和醛复红-亮绿-橘黄G染色差异;分析不同部位取材的疏松结缔组织两种染色方法的结果差异。 方法　Wistar大鼠腹腔注射10 g/L苔盼蓝生理盐水溶液2.5 ml,1次/d,连续3 d,分别在皮下、肠系膜取疏松结缔组织,铺片。两个部位的铺片分别采用HE染色、醛复红-亮绿-橘黄G染色。 结果　皮下取材疏松结缔组织经HE染色可见大量成纤维细胞,肥大细胞明显,巨噬细胞可见,弹性纤维和胶原纤维可见,但不明显;皮下取材疏松结缔组织经醛复红-亮绿-橘黄G染色弹性纤维呈紫红色、胶原纤维呈橙色,细胞不易着色;肠系膜取材疏松结缔组织经HE染色,可见成纤维细胞、肥大细胞、巨噬细胞明显,弹性纤维呈蓝紫色、胶原纤维呈淡红色;肠系膜取材疏松结缔组织经醛复红-亮绿-橘黄G染色,弹性纤维被染成紫红色、胶原纤维染成鲜艳的绿色,肥大细胞被染成紫红色,核呈圆或椭圆形、棕黄色,巨噬细胞清晰可见、形态不规则,胞质中可见粗大呈蓝紫色的苔盼蓝颗粒,细胞核呈圆形、棕黄色;成纤维细胞胞质无着色,核呈棕黄色。 结论　大鼠肠系膜取材制备的疏松结缔组织撕片经醛复红-亮绿-橘黄G染色能够更好的显示各种类型细胞和纤维,各结构间对比明显。     

A rate-insensitive linear viscoelastic model for soft tissues

It is well known that many biological soft tissues behave as viscoelastic materials with hysteresis curves being nearly independent of strain rate when loading frequency is varied over a large range. In this work, the rate-insensitive feature of biological materials is taken into account by a generalized Maxwell model. To minimize the number of model parameters, it is assumed that the characteristic frequencies of Maxwell elements form a geometric series. As a result, the model is characterized by five material constants: micro(0), tau, m, rho and beta, where micro(0) is the relaxed elastic modulus, tau the characteristic relaxation time, m the number of Maxwell elements, rho the gap between characteristic frequencies, and beta=micro(1)/micro(0) with micro(1) being the elastic modulus of the Maxwell body that has relaxation time tau. The physical basis of the model is motivated by the microstructural architecture of typical soft tissues. The novel model shows excellent fit of relaxation data on the canine aorta and captures the salient features of vascular viscoelasticity with significantly fewer model parameters.     

利用醛复红染色法制作家兔皮下疏松结缔组织铺片标本

目的 探讨适用于家兔皮下疏松结缔组织铺片标本的制片方法。方法 家兔3只,分别经腹腔注射10g/L锥虫蓝生理盐水溶液10～12ml。每天1次,连续注射3d后,于第4天取皮下疏松结缔组织进行铺片。待铺片晾干后,用福尔马林-酒精固定液固定约6h。然后,分别入醛复红、核固红和伊红染色液进行染色。每步之间用自来水冲洗。最后,常规脱水、透明、封片。 结果 巨噬细胞呈不规则形,分布在纤维中间,胞质中可见粗大的锥虫蓝颗粒。细胞核呈红色。醛复红染色30～40min时,弹性纤维呈紫色或蓝紫色,胶原纤维呈浅红色。结论 该法操作步骤简单,结果稳定可靠,是制作家兔皮下疏松结缔组织铺片标本适宜的方法。     

An Evaluation of the Quasi-Linear Viscoelastic Properties of the Healing Medial Collateral Ligament in a Goat Model

The viscoelastic properties of the healing medial collateral ligament (MCL) at 12 weeks after isolated injury were investigated in a goat model. The stress-strain relationships, static and cyclic stress-relaxation behaviors of the healing MCL up to 5% strain were determined experimentally using a femur-MCL-tibia complex. These experimental data were used in combination with the quasi-linear viscoelastic (QLV) theory of Fung (1972) to characterize the reduced relaxation function, G(t) (described by constants C, tau1, and tau2) and the elastic response, sigmae(epsilon) (described by constants A and B) of this tissue. It was found that the percentage of stress relaxation for the healing MCLs was significantly greater than those for sham-operated controls (49.0 +/- 12.1% vs. 26.5 +/- 8.1%, respectively; p < 0.05). The product of constants A x B, i.e. the initial slope of the stress-strain curves, was found to be significantly lower for healing MCLs compared to those for sham-operated controls (32.9 +/- 15.8 MPa vs. 118.8 +/- 48.3 MPa; p < 0.05). The dimensionless constant C, i.e. the magnitude of the viscous response, was nearly three times greater for healing MCLs, while constant tau1 was found to be similar between the two groups (0.80 +/- 0.43 s vs. 0.89 +/- 0.52 s, respectively). Constant tau2 for the healing MCL was significantly less than the controls (1269 +/- 38 s vs. 1845 +/- 431 s; p < 0.05) indicating that the stress relaxation reached a plateau earlier. These constants of the QLV theory used to describe the healing MCL were validated for the strain level utilized in this experiment (approximately equal to 4.5%) by predicting the peak stresses during a cyclic stress-relaxation experiment. The theoretically determined values closely matched the experimentally measured values. Thus, this study demonstrates that the QLV theory could be successfully used to describe the viscoelastic behavior of the MCL during the early phases of healing.     

Structure of the rat subcutaneous connective tissue in relation to its sliding mechanism

Mammalian skin can extensively slide over most parts of the body. To study the mechanism of this mobility of the skin, the structure of the subcutaneous connective tissue was examined by light microscopy. The subcutaneous connective tissue was observed to be composed of multiple layers of thin collagen sheets containing elastic fibers. These piled-up collagen sheets were loosely interconnected with each other, while the outer and inner sheets were respectively anchored to the dermis and epimysium by elastic fibers. Collagen fibers in each sheet were variable in diameter and oriented in different directions to form a thin, loose meshwork under conditions without mechanical stretching. When a weak shear force was loaded between the skin and the underlying abdominal muscles, each collagen sheet slid considerably, resulting in a stretching of the elastic fibers which anchor these sheets. When a further shear force was loaded, collagen fibers in each sheet seemed to align in a more parallel manner to the direction of the tension. With the reduction or removal of the force, the arrangement of collagen fibers in each sheet was reversed and the collagen sheets returned to their original shapes and positions, probably with the stabilizing effect of elastic fibers. Blood vessels and nerves in the subcutaneous connective tissue ran in tortuous routes in planes parallel to the unloaded skin, which seemed very adaptable for the movement of collagen sheets. These findings indicate that the subcutaneous connective tissue is extensively mobile due to the presence of multilayered collagen sheets which are maintained by elastic fibers.     

Biomechanical and biochemical characteristics of a human fibroblast-produced and remodeled matrix

We report on a culture method for the rapid production of a strong and thick natural matrix by human cells for tissue engineering applications. Dermal fibroblasts were cultured for three weeks at high density on porous substrates in serum-containing or chemically defined media. The mechanical and biochemical properties of the resulting cell-derived matrix (CDM) were compared to those of standard fibroblast-populated collagen and fibrin gels and native human skin. We found that the ultimate tensile strength of CDM cultured in our chemically defined media (313+/-8.7 kPa) is significantly greater than for collagen gels (168+/-39.3 kPa), fibrin gels (133+/-8.0 kPa) and CDM cultured with serum (223+/-9.0 kPa), but less than native skin (713+/-55.2 kPa). In addition to the biomechanics, this *CDM is also biochemically more similar to native skin than the collagen and fibrin gels in terms of all parameters measured. As *CDM is produced by human cells in a chemically defined culture medium and is mechanically robust, it may be a viable living tissue equivalent for many connective tissue replacement applications requiring initial mechanical stability yet a high degree of biocompatibility.     

Biaxial failure properties of planar living tissue equivalents

Quantification of the mechanical properties of living tissue equivalents (LTEs) is essential for assessing their ultimate functionality as tissue substitutes, yet their delicate nature makes failure testing problematic. For this study, we evaluated the validity of using an inflation device for quantifying the biaxial tensile failure properties of extremely delicate fibroblast-populated collagen gels (CGs) and fibrin gels (FGs). Small samples were circularly clamped and then inflated until rupture. Each sample assumed an approximately spherical shape and burst at its center indicating effective clamping. After two weeks in culture, all LTEs tested were fragile, but the FGs were significantly stronger and more extensible than the CGs (ultimate tensile strength 6.0 kPa +/- 2.0 kPa vs. 2.8 kPa +/- 0.7 kPa; failure strain 3.5 +/- 0.9 vs. 0.26 +/- 0.05, n = 4). After an additional 11 days of culture, the strength of the FGs increased significantly (26.5 kPa +/- 12.7 kPa), and the extensibility decreased (1.9 +/- 0.8, n = 3). This study demonstrates that subtle differences in the properties of LTEs can be measured using inflation methods with minimal sample handling and without having to grow the tissues into anchors or cut the specimens.     

针灸经穴的数字解剖学研究

目的 探索人体经穴的解剖学构成和针灸疗法的机制。方法 在男、女图像数据集的基础上，通过图像分割和三维重建，构建了人体全身筋膜结缔组织的网状结构支架，与传统中医针灸疗法的刺激部位进行对比分析。结果 根据筋膜所处的解剖学部位可分为5类：1．真皮的网状层和乳头层致密结缔组织；2．皮下疏松结缔组织；3．肌间隔疏松结缔组织；4．神经血管束疏松结缔组织；5．内脏器官门及被膜疏松结缔组织。将构建的筋膜与中医针灸的刺激部位进行比较，数字人研究结果显示，中医针灸治疗的解剖学基础可能是分布于全身的筋膜结缔组织。结论 人体可能存在由结缔组织筋膜构成的维持机体内环境稳定的功能系统——自体监控系统。该系统可能是针灸疗法的解剖学基础。     

Biomechanical and magnetic resonance characteristics of a cartilage-like equivalent generated in a suspension culture

OBJECTIVE: To generate a cartilage biomaterial using a suspension culture with biophysical properties similar to native articular cartilage. DESIGN: A novel cartilage tissue equivalent (CTE) using a no-scaffold, high-density suspension culture of neonatal porcine chondrocytes was formed on poly 2-hydroxyethyl methacrylate-treated plates for up to 16 weeks. Equilibrium aggregate modulus and hydraulic permeability were measured at 8 and 16 weeks using confined compression stress relaxation experiments. The CTE proteoglycan composition was characterized using sodium and T(1rho) magnetic resonance imaging methods after 8 weeks. RESULTS: The resultant CTE produces a biomaterial consistent with a hyaline cartilage phenotype in appearance and expression of type II collagen and aggrecan. The equilibrium aggregate modulus and permeability for the 8-week specimens were 41.6 (standard deviation (SD) 4.3) kPa and 2.85(-13) (SD 2.45(-13)) m(4)/Ns, respectively, and, for the 16-week specimens, 35.2 (SD 7.6) kPa and 2.67(-13) (SD 1.06(-13)) m(4)/Ns, respectively. Average sodium concentration of the 8-week CTE ranged from 260 to 278 mM and average T(1rho) relaxation times from 105 to 107 ms, indicating proteoglycan content similar to that of native articular cartilage. CONCLUSION: The high-density culture method produced a CTE with characteristics that approach those of native articular cartilage. The CTE mechanical properties are similar to those of the native cartilage. The CTE developed in this study represents a promising methodological advancement in cartilage tissue engineering and cartilage repair.     

Long-term intermittent compressive stimulation improves the composition and mechanical properties of tissue-engineered cartilage

Tissue engineering of articular cartilage is a promising alternative for cartilage repair. However, it has been difficult to develop tissue in vitro that mimicks native cartilage. Cartilaginous tissue formed in vitro does not accumulate enough extracellular matrix, is deficient in collagen, and possesses only a fraction of the mechanical properties of native cartilage. In this study, we investigated whether long-term intermittent compressive stimulation would improve the quality of the generated tissue. Chondrocyte cultures were established on the surface of porous calcium polyphosphate substrates and allowed to form cartilaginous tissue. In vitro-formed tissues were subjected to different stimulation protocols for 1 week. The optimal mechanical stimulation parameters identified in this short-term study were then applied to the cultures for up to 4 weeks. Mechanical stimulation applied at a 5% compressive amplitude at a frequency of 1 Hz for 400 cycles every second day resulted in the greatest increase in collagen synthesis (37 +/- 9% over control) while not significantly affecting proteoglycan synthesis (2 +/- 8% over control). This condition, applied to the chondrocyte cultures for 4 weeks, resulted in a significant increase in the amount of tissue that formed (stimulated, 2.4 +/- 0.2 mg dry wt; unstimulated, 1.61 +/- 0.08 mg dry wt). Stimulated tissues contained approximately 40% more collagen (stimulated, 590 +/- 58 microg; unstimulated, 420 +/- 42 microg), and 30% more proteoglycans (stimulated, 393 +/- 34 microg; unstimulated, 302 +/- 32 microg) as well as displaying a 2- to 3-fold increase in compressive mechanical properties (maximal equilibrium stress: stimulated, 10 +/- 1 kPa; unstimulated, 5 +/- 1 kPa; maximal equilibrium modulus: stimulated, 80 +/- 23 kPa; unstimulated, 24 +/- 6 kPa). The results of this study demonstrate that intermittent mechanical stimulation can increase collagen synthesis and, when applied over a 4-week period, can accelerate extracellular matrix accumulation as well as improve the material properties of the developed tissue. Interestingly, only short periods of mechanical stimulation (6 min every second day) were needed to affect the quality of cartilaginous tissue formed in vitro.     

Characterization of Changes to the Mechanical Properties of Arteries due to Cold Storage Using Nanoindentation Tests

Understanding the effect of cold storage on arterial tissues is essential in various clinical and experimental practices. Cold storage techniques could significantly affect the post-cryosurgical or post-cryopreservation mechanical behavior of arteries. Previously, arteries were considered homogenous and elastic and the changes in material properties due to cold storage were inconclusive. In this study, using a custom-made nanoindentation device, changes to the local viscoelastic properties of porcine thoracic aorta wall due to three common storage temperatures (+4, -20, and -80 °C) within 24 h, 48 h, 1 week, and 3 weeks were characterized. The changes to both elastic and relaxation behaviors were investigated considering the multilayer, heterogeneous nature of the aortic wall. The results showed that the average instantaneous Young's modulus (E) of +4 °C storage samples decreased while their permanent average relaxation amplitude (G (∞)) increased and after 48 h these changes became significant (10 and 13% for E and G (∞), respectively). Generally, in freezer storage, E increased and G (∞) showed no significant change. In prolonged preservation (>1 week), the results of -20 °C showed significant increase in E (20% after 3 weeks) while this increase for -80 °C was not significant, making it a better choice for tissue cold storage applications.     

Tight-skin, a new mutation of the mouse causing excessive growth of connective tissue and skeleton.

A new dominant mutation, tight-skin (Tsk), is located on Chromosome 2, two recombination units distal to pallid (pa). Heterozygotes (Tsk/+) have tight skins with marked hyperplasia of the subcutaneous loose connective tissues, increased growth of cartilage and bone, and small tendons with hyperplasia of the tendon sheaths. In the loose connective tissue there are large accumulations of microfibrils in the intercellular space. In spite of the increased skeletal size, body weight is not increased. Increase in size of the thoracic skeleton is especially pronounced and leads to pathologic distentsion of the hollow thoracic viscera. Concentration of growth hormone in the pituitary and plasma is normal. Homozygotes (Tsk/TSK) die in utero at 7 to 8 days of gestation. We propose the hypothesis that Tsk might act by causing defective cell receptors with high affinity for a somatomedin-like factor promoting growth of cartilage, bone, and connective tissue and low affinity for a multiplication-stimulating factor promoting embryonic growth.     

Optically based-indentation technique for acute rat brain tissue slices and thin biomaterials

Currently, micro-indentation testing of soft biological materials is limited in its capability to test over long time scales due to accumulated instrumental drift errors. As a result, there is a paucity of measures for mechanical properties such as the equilibrium modulus. In this study, indentation combined with optical coherence tomography (OCT) was used for mechanical testing of thin tissue slices. OCT was used to measure the surface deformation profiles after placing spherical beads onto submerged test samples. Agarose-based hydrogels at low-concentrations (w/v, 0.3-0.6%) and acute rat brain tissue slices were tested using this technique over a 30-min time window. To establish that tissue slices maintained cell viability, allowable testing times were determined by measuring neuronal death or degeneration as a function of incubation time with Fluor-Jade C (FJC) staining. Since large deformations at equilibrium were measured, displacements of surface beads were compared with finite element elastic contact simulations to predict the equilibrium modulus, μ(∞) . Values of μ(∞) for the low-concentration hydrogels ranged from 0.07 to 1.8 kPa, and μ(∞) for acute rat brain tissue slices was 0.13 ± 0.04 kPa for the cortex and 0.09 ± 0.015 kPa for the hippocampus (for Poisson ratio = 0.35). This indentation technique offers a localized, real-time, and high resolution method for long-time scale mechanical testing of very soft materials. This test method may also be adapted for viscoelasticity, for testing of different tissues and biomaterials, and for analyzing changes in internal structures with loading.     

The nonlinear elastic and viscoelastic passive properties of left ventricular papillary muscle of a guinea pig heart

The mechanical behavior of the heart muscle tissues is the central problem in finite element simulation of the heart contraction, excitation propagation and development of an artificial heart. Nonlinear elastic and viscoelastic passive material properties of the left ventricular papillary muscle of a guinea pig heart were determined based on in-vitro precise uniaxial and relaxation tests. The nonlinear elastic behavior was modeled by a hypoelastic model and different hyperelastic strain energy functions such as Ogden and Mooney-Rivlin. Nonlinear least square fitting and constrained optimization were conducted under MATLAB and MSC.MARC in order to obtain the model material parameters. The experimental tensile data was used to get the nonlinear elastic mechanical behavior of the heart muscle. However, stress relaxation data was used to determine the relaxation behavior as well as viscosity of the tissues. Viscohyperelastic behavior was constructed by a multiplicative decomposition of a standard Ogden strain energy function, W, for instantaneous deformation and a relaxation function, R(t), in a Prony series form. The study reveals that hypoelastic and hyperelastic (Ogden) models fit the tissue mechanical behaviors well and can be safely used for heart mechanics simulation. Since the characteristic relaxation time (900 s) of heart muscle tissues is very large compared with the actual time of heart beating cycle (800 ms), the effect of viscosity can be reasonably ignored. The amount and type of experimental data has a strong effect on the Ogden parameters. The in vitro passive mechanical properties are good initial values to start running the biosimulation codes for heart mechanics. However, an optimization algorithm is developed, based on clinical intact heart measurements, to estimate and re-correct the material parameters in order to get the in vivo mechanical properties, needed for very accurate bio-simulation and for the development of new materials for the artificial heart.     

针刺过程中的针体受力分布

背景：针刺过程针体受力分布,不仅取决于不同穴位处组织结构,更受行针手法的影响。 目的：就一般穴位所共有的表皮、真皮和皮下结缔组织的组织结构和力学性质,分提插和捻转法进行针体受力研究。 方法：以提插法和捻转法为前提,以表皮组织、真皮组织和皮下结缔组织为区域对象,在前期研究的基础上建立类固体和软物质两种生物力学模型,对行针过程中的针体阻力分布进行分析,以得出定性化的结论。 结果与结论：表皮无血管无神经,致密而坚硬,可以看作类固体;真皮及皮下结缔组织中血管神经等分布广泛,疏松而柔软,是典型的软物质。提插行针过程中非“得气”时阻力主要来自表皮部分,在“得气”状况下阻力应该来自表皮和结缔组织中的纤维细胞共同作用,对针体而言,进入皮肤的所有针体部分都是受力点,只是靠近针尖部分的针体受力稍大一些,但是随着提插操作,进入皮肤的针体长度不断在发生变化。捻转法行针的主要阻力来自胶原纤维和弹性纤维的缠绕,对针体而言受力点主要集中在针尖及附近部位。     

The effects of heating on the mechanical properties of arterial elastin

Autoclaving is a standard way of purifying arterial elastin for mechanical testing, but recent evidence suggests that heating native elastin might affect its mechanical behavior. We therefore examined the quasi-static tensile properties of pig arterial tissue to see if the mechanical properties of native elastin are altered by autoclaving. From an analysis of the shapes of the stress-extension ratio curves of tissues before and after 8 h of autoclaving, we determined that the mechanical characteristics of elastin dominated the behavior of unautoclaved arterial tissue at wall stresses around 25+/-5 kPa. Autoclaving did not change the tangential modulus of the tissue at this wall stress (+/-4% 95% CI), indicating that elastin can be heated during purification without affecting its mechanical behavior. Autoclaved tissue was tested daily to determine the effects of prolonged heating of autoclaved elastin. Between tests the elastin was incubated at either 80 degrees C (experimental group) or 37 degrees C (control group). After 6 days the average modulus of the control group was unchanged from the initial value, while the average modulus of the experimental group was 7%+/-2% (95% CI) lower. At shorter times the modulus of the experimental group was not significantly reduced. The slight decrease in modulus suggests a slow chemical degradation may occur with prolonged heating, but its time course and magnitude are such that it would not affect standard mechanical tests.     

Scanning electron microscopic studies of tissue elastin components exposed by a KOH-collagenase or simple KOH digestion method.

A KOH-collagenase or simple KOH digestion method was employed for scanning electron microscope (SEM) studies of elastin components in the rat thoracic aorta, mouse urinary bladder, and human ductus deferens. Immersion of the fixed tissues in 30% KOH solution for 8-10 min at 60 degrees C, with or without subsequent collagenase treatment, successfully removed collagen fibrils and basal laminae while leaving cellular and elastin elements unchanged at their original shapes and locations. The internal elastic lamina of the rat aorta appeared as a solid sheet formed by elastin fibrils 0.1-0.2 microns thick, while the medial elastic laminae were more fibrous because of the presence of numerous fine elastin fibers on their surface. Adventitial elastin fibers were of a cord-like shape complicatedly entangled among the adventitial fibroblasts. These fibers were seen as bundles of fibrils 0.1-0.2 microns thick. In the mouse urinary bladder, elastin formed a thin lace-like sheet just beneath the serosal covering of the peritoneum. This sheet was composed of small bundles of fine (0.1-0.2 microns thick) fibrils. The external connective tissue of the human ductus deferens was made up of a three-dimensional loose network of elastin fibers 0.1-1.5 microns thick. These fibers also appeared as bundles of the fine fibrils. These findings indicate that the present method is useful for SEM studies of elastin as well as cellular components in various tissues and organs. This study also maintains that elastin fibers and laminae are basically composed of unit fibrils of 0.1-0.2 microns thickness. As elastin components are arranged specific to individual organs and tissues, it is reasonable that these components are concerned in the characteristic mechanical properties of these tissues and organs.     

Nonclostridial anaerobic soft tissue infections

Fourteen cases of nonclostridial anaerobic infections of soft tissues caused by peptococci, peptostreptococci, bacteroids or their combinations with E. coli, Proteus, and Staphylococcus aureus were studied. They are characterized by a rapid spread of serous-suppurative inflammation and the causative agents in loose fibrous connective tissue of subcutaneous adipose tissue (cellutitis), surface fascia (fasciitis), or skeletal muscles (myositis). Dissolution of intercellular structures of the connective tissue, extensive necroses, mild leukocyte reaction, and a wide peripheral zone of toxic edema are observed in the focus of inflammation. This is due to a high toxicity and enzymatic activity of the causative agents. Gas formation is observed when anaerobes are combined with E. coli.     

Effects of auricular chondrocyte expansion on neocartilage formation in photocrosslinked hyaluronic acid networks

The overall objective of this study was to examine the effects of in vitro expansion on neocartilage formation by auricular chondrocytes photoencapsulated in a hyaluronic acid (HA) hydrogel as a next step toward the clinical application of tissue engineering therapies for treatment of damaged cartilage. Swine auricular chondrocytes were encapsulated either directly after isolation (p = 0), or after further in vitro expansion ( p = 1 and p = 2) in a 2 wt%, 50-kDa HA hydrogel and implanted subcutaneously in the dorsum of nude mice. After 12 weeks, constructs were explanted for mechanical testing and biochemical and immunohistochemical analysis and compared to controls of HA gels alone and native cartilage. The compressive equilibrium moduli of the p = 0 and p = 1 constructs (51.2 +/- 8.0 and 72.5 +/- 35.2 kPa, respectively) were greater than the p = 2 constructs (26.8 +/- 14.9 kPa) and the control HA gel alone (12.3 +/- 1.3 kPa) and comparable to auricular cartilage (35.1 +/- 12.2 kPa). Biochemical analysis showed a general decrease in glycosaminoglycan (GAG), collagen, and elastin content with chondrocyte passage, though no significant differences were found between the p = 0 and p = 1 constructs for any of the analyses. Histological staining showed intense and uniform staining for aggrecan, as well as greater type II collagen versus type I collagen staining in all constructs. Overall, this study illustrates that constructs with the p = 0 and p = 1 auricular chondrocytes produced neocartilage tissue that resembled native auricular cartilage after 12 weeks in vivo. However, these results indicate that further expansion of the chondrocytes (p = 2) can lead to compromised tissue properties.