Gravity plays a central role in vertebrate development and evolution. Gravitational forces acting on mammalian tissues cause the net muscle forces required for locomotion to be higher on earth than ...on a body subjected to a microgravitational field. As body mass increases during development, the musculoskeleton must be able to adapt by increasing the size of its functional units. Thus mechanical forces required to do the work (mechanical energy) of locomotion must be sensed by cells and converted into chemical energy (synthesis of new tissue). Extracellular matrices (ECMs) are multicomponent tissues that transduce internal and external mechanical signals into changes in tissue structure and function through a process termed mechanochemical transduction. Under the influence of an external gravitational field, both mineralized and unmineralized vertebrate tissues exhibit internal tensile forces that serve to preserve a synthetic phenotype in the resident cell population. Application of additional external forces alters the balance between the external gravitational force and internal forces acting on resident cells leading to changes in the expression of genes and production of protein that ultimately may alter the exact structure and function of the extracellular matrix. Changes in the equilibrium between internal and external forces acting on ECMs and changes in mechanochemical transduction processes at the cellular level appear to be important mechanisms by which mammals adjust their needs to store, transmit, and dissipate energy that is required during development and for bodily movements. Mechanosensing is postulated to involve many different cellular and extracellular components. Mechanical forces cause direct stretching of protein-cell surface integrin binding sites that occur on all eukaryotic cells. Stress-induced conformational changes in the extracellular matrix may alter integrin structure and lead to activation of several secondary messenger pathways within the cell. Activation of these pathways leads to altered regulation of genes that synthesize and catabolize extracellular matrix proteins as well as to alterations in cell division. Another aspect by which mechanal signals are transduced involves deformation of gap junctions containing calcium-sensitive stretch receptors. Once activated, these channels trigger secondary messenger activation through pathways similar to those involved in integrin-dependent activation and allow cell-to-cell communications between cells with similar and different phenotypes. Another process by which mechanochemical transduction occurs is through the activation of ion channels in the cell membrane. Mechanical forces have been shown to alter cell membrane ion channel permeability associated with Ca(+2) and other ion fluxes. In addition, the application of mechanical forces to cells leads to the activation of growth factor and hormone receptors even in the absence of ligand binding. These are some of the mechanisms that have evolved in vertebrates by which cells respond to changes in external forces that lead to changes in tissue strcture and function.
Mammals elastically store energy in leg and foot tendons during locomotion. In the turkey, much of the force generated by the gastrocnemius muscle is stored as elastic energy during tendon ...deformation and not within the muscle. During growth, avian tendons mineralize in the portions distal to the muscle and show increased tensile strength and modulus as a result. The purpose of this study was to evaluate the viscoelastic behavior of turkey tendons and self-assembled collagen fiber models to determine the molecular basis for tendon deformation. The stress-strain behavior of tendons and self-assembled collagen fibers was broken into elastic and viscous components. The elastic component was found to be to a first approximation independent of source of the collagen and to depend only on the extent of cross-linking. In the absence of cross-links the elastic component of the stress was found to be negligible for self-assembled type I collagen fibers. In the presence of cross-links the behavior approached that found for mineralized turkey tendons. The elastic constant for turkey tendon was shown to be between 5 and 7.75 GPa while it was about 6.43 GPa for self-assembled collagen fibers aged for 6 months at 22 degrees C. The viscous component for mineralized turkey tendons was about the same as that of self-assembled collagen fibers aged for 6 months, a result suggesting that addition of mineral does not alter the viscous properties of tendon. It is concluded that elastic energy storage in tendons involves direct stretching of the collagen triple-helix, nonhelical ends, and cross-links between the molecules and is unaffected by mineralization. Furthermore, it is hypothesized that mineralization of turkey tendons is an efficient means of preserving elastic energy storage while providing for increased load-bearing ability required for locomotion of adult birds.
Background
The modulus and resonant frequency of cancer cells and extracellular matrix are increased in both basal cell and squamous cell carcinomas, and in addition, the collagen stiffness is ...increased. The organization of the extracellular matrix surrounding cancer cells is clearly different than the extracellular matrix that is seen in normal skin.
Materials and Methods
We have used vibrational optical coherence tomography (VOCT) to measure the resonant frequency and stiffness of collagen, vascular, and reorganized fibrous extracellular matrix components. Measurements of vessels and fibrotic collagen content made on basal cell carcinomas (BCCs) are compared to similar measurements made on normal skin from different anatomical locations.
Results
Lesions with basal cell carcinomas exhibit characteristic resonant frequencies and moduli of reorganized extracellular matrix characteristic of fibrotic tissue. Cancerous lesions are characterized by dermal collagen (100 Hz), vascular (150 Hz), and fibrotic peaks (200‐230 Hz). The fibrotic peak is not found in areas of normal skin.
Conclusions
Based on this pilot study, it is hypothesized inflammation may promote precancerous lesion formation in the surrounding extracellular matrix. Measurement of the increases in vaculature and fibrotic content may be useful in early detection of BCCs.
Background
Increased tissue stiffness (also termed modulus) has been shown to be a characteristic of potential tumor metastasis. Measured values of the stiffness of tumors and cancer cells are ...reported in the literature to increase compared to neighboring normal tissues. Yet the relationship between the mechanical properties of cells and the extracellular matrix has yet to be correlated with the histopathology of cancerous lesions.
Materials and Methods
We have developed a technique to do virtual biopsies of skin lesions by combining images made using optical coherence tomography with stiffness measurements made simultaneously using vibrational analysis. The technique is termed vibrational optical coherence tomography (VOCT).
Results
In this paper, we report that precancerous and cancerous lesions are characterized by changes in both the morphology and stiffness of the cellular components of the skin. The ratio of the peak heights that correspond to the epidermal (40‐60Hz) and dermal (140‐160 Hz) resonant frequencies appear to be different for benign and cancerous or precancerous lesions compared with normal skin and scar.
Conclusions
Cell‐to‐cell and epidermal‐to‐dermal interactions may be very important in evaluating the potential of skin lesions to become malignant. These interactions can be evaluated using VOCT, a new technique for performing “virtual biopsies” of skin lesions.
1 Department of Pathology and Laboratory Medicine, UMDNJ-Robert Wood Johnson Medical School, Piscataway, New Jersey 08854; 2 Xium, LLC, Chelmsford, Massachusetts 01824; and 3 Department of ...Biochemistry and Molecular Pathology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272
Mechanical forces play a role in the development and evolution of extracellular matrices (ECMs) found in connective tissue. Gravitational forces acting on mammalian tissues increase the net muscle forces required for movement of vertebrates. As body mass increases during development, musculoskeletal tissues and other ECMs are able to adapt their size to meet the increased mechanical requirements. However, the control mechanisms that allow for rapid growth in tissue size during development are altered during maturation and aging. The purpose of this mini-review is to examine the relationship between mechanical loading and cellular events that are associated with downregulation of mechanochemical transduction, which appears to contribute to aging of connective tissue. These changes result from decreases in growth factor and hormone levels, as well as decreased activation of the phosphorelay system that controls cell division, gene expression, and protein synthesis. Studies pertaining to the interactions among mechanical forces, growth factors, hormones, and their receptors will better define the relationship between mechanochemical transduction processes and cellular behavior in aging tissues.
phosphorelay system; extracellular matrix; connective tissue; collagen; mechanical forces
Address for reprint requests and other correspondence: F. H. Silver, Dept. of Pathology and Laboratory Medicine, UMDNJ-Robert Wood Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854 (E-mail: silverfr{at}umdnj.edu ).
The possible means by which type I collagen may mediate mineralization in normal vertebrate bone, tendon, dentin and cementum as well as in pathological mineral formation are not fully understood. ...One consideration in this regard is that the structure of the protein is somehow important in binding calcium and phosphate ions in a stereochemical configuration conducive to nucleation of apatite crystals. In the present study, type I collagen, packed in a quarter-staggered arrangement in two dimensions and a quasi-hexagonal model of microfibrillar assembly in three dimensions, has been examined in terms of several of its charged amino acid residues. These included glutamic and aspartic acid, lysine, arginine, hydroxylysine and histidine, whose positions along the three alpha-chain axes of the collagen molecule were determined with respect to each other. It was found that the locations of these residues specified sites uniquely suited as potential apatite nucleation centers following binding of calcium and phosphate ions. From this analysis, it would appear that type I collagen provides a template of charged amino acid residues that dictates ion binding critical to subsequent nucleation events for mineral formation in vertebrate tissues.
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