Mechanobiology E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 917

Veröffentlichungsjahr: 2017

Beschreibung

An emerging field at the interface of biology and engineering, mechanobiology explores the mechanisms by which cells sense and respond to mechanical signals--and holds great promise in one day unravelling the mysteries of cellular and extracellular matrix mechanics to cure a broad range of diseases. Mechanobiology: Exploitation for Medical Benefit presents a comprehensive overview of principles of mechanobiology, highlighting the extent to which biological tissues are exposed to the mechanical environment, demonstrating the importance of the mechanical environment in living systems, and critically reviewing the latest experimental procedures in this emerging field. Featuring contributions from several top experts in the field, chapters begin with an introduction to fundamental mechanobiological principles; and then proceed to explore the relationship of this extensive force in nature to tissues of musculoskeletal systems, heart and lung vasculature, the kidney glomerulus, and cutaneous tissues. Examples of some current experimental models are presented conveying relevant aspects of mechanobiology, highlighting emerging trends and promising avenues of research in the development of innovative therapies. Timely and important, Mechanobiology: Exploitation for Medical Benefit offers illuminating insights into an emerging field that has the potential to revolutionise our comprehension of appropriate cell biology and the future of biomedical research.

Leseprobe

Table of Contents

Cover

Title Page

List of Contributors

Preface

1 Extracellular Matrix Structure and Stem Cell Mechanosensing

1.1 Mechanobiology

1.2 Stem Cells

1.3 Substrate Stiffness in Cell Behavior

1.4 Stem Cells and Substrate Stiffness

1.5 Material Structure and Future Perspectives in Stem Cell Mechanobiology

1.6 Conclusion

References

2 Molecular Pathways of Mechanotransduction

2.1 Introduction: Mechanically Influenced Cellular Behavior

2.2 Mechanosensitive Molecular Mechanisms

2.3 Methods Enabling the Study of Mechanobiology

2.4 Conclusion

Acknowledgements

References

3 Sugar‐Coating the Cell

3.1 What is the Glycocalyx?

3.2 Composition of the Glycocalyx

3.3 Morphology of the Glycocalyx

3.4 Mechanical Properties of the Glycocalyx

3.5 Mechanobiology of the Endothelial Glycocalyx

3.6 Does the Glycocalyx Play a Mechanobiological Role in Bone?

3.7 Glycocalyx in Muscle

3.8 How Can the Glycocalyx be Exploited for Medical Benefit?

3.9 Conclusion

References

4 The Role of the Primary Cilium in Cellular Mechanotransduction

4.1 Introduction

4.2 The Primary Cilium

4.3 Cilia‐Targeted Therapeutic Strategies

4.4 Conclusion

Acknowledgements

References

5 Mechanosensory and Chemosensory Primary Cilia in Ciliopathy and Ciliotherapy

5.1 Introduction

5.2 Mechanobiology and Diseases

5.3 Primary Cilia as Biomechanics

5.4 Modulating Mechanobiology Pathways

5.5 Conclusion

References

6 Mechanobiology of Embryonic Skeletal Development

6.1 Introduction

6.2 An Overview of Embryonic Skeletal Development

6.3 Regulation of Joint Formation

6.4 Regulation of Endochondral Ossification

6.5 An Overview of Relevant Osteoarthritic Joint Changes

6.6 Lessons for Osteoarthritis from Joint Formation

6.7 Lessons for Osteoarthritis from Endochondral Ossification

6.8 Conclusion

Acknowledgements

References

7 Modulating Skeletal Responses to Mechanical Loading by Targeting Estrogen Receptor Signaling

7.1 Introduction

7.2 Biomechanical Activation of Estrogen Receptor Signaling:

In Vitro

Studies

7.3 Skeletal Consequences of Altered Estrogen Receptor Signaling:

In Vivo

Mouse Studies

7.4 Skeletal Consequences of Human Estrogen Receptor Polymorphisms: Human Genetic and Exercise‐Intervention Studies

7.5 Conclusion

References

8 Mechanical Responsiveness of Distinct Skeletal Elements

8.1 Introduction

8.2 Anatomy and Loading‐Related Stimuli

8.3 Preosteogenic Responses

In Vitro

8.4 Site‐Specific, Animal‐Strain Differences

8.5 Exploitation of Regional Information

8.6 Conclusion

References

9 Pulmonary Vascular Mechanics in Pulmonary Hypertension

9.1 Introduction

9.2 Pulmonary Vascular Mechanics

9.3 Measurements of Pulmonary Arterial Mechanics

9.4 Mechanobiology in Pulmonary Hypertension

9.5 Computational Modeling in Pulmonary Circulation

9.6 Impact of Pulmonary Arterial Biomechanics on the Right Heart

9.7 Conclusion

References

10 Mechanobiology and the Kidney Glomerulus

10.1 Introduction

10.2 Glomerular Filtration Barrier

10.3 Podocyte Adhesion

10.4 Glomerular Disease

10.5 Forces in the Glomerulus

10.6 Mechanosensitive Components and Prospects for Therapy

10.7 Conclusion

References

11 Dynamic Remodeling of the Heart and Blood Vessels

11.1 Introduction

11.2 Causes of Remodeling

11.3 Mechanical Transduction in Cardiac Remodeling

11.4 The Remodeling Process

11.5 Conclusion

References

12 Aortic Valve Mechanobiology

12.1 Introduction

12.2 Mechanobiology at the Organ Level

12.3 Mechanobiology at the Cellular Level

12.4 Conclusion

Acknowledgments

References

13 Testing the Perimenopause Ageprint using Skin Visoelasticity under Progressive Suction

13.1 Introduction

13.2 Gender‐Linked Skin Aging

13.3 Dermal Aging, Thinning, and Wrinkling

13.4 Skin Viscoelasticity under Progressive Suction

13.5 Skin Tensile Strength during the Perimenopause

13.6 Conclusion

Acknowledgements

References

14 Mechanobiology and Mechanotherapy for Skin Disorders

14.1 Introduction

14.2 Skin Disorders Associated with Mechanobiological Dysfunction

14.3 Mechanotherapy

14.4 Conclusion

Acknowledgement

References

15 Mechanobiology and Mechanotherapy for Cutaneous Wound‐Healing

15.1 Introduction

15.2 The Mechanobiology of Cutaneous Wound‐Healing

15.3 Mechanotherapy to Improve Cutaneous Wound‐Healing

15.4 Future Considerations

References

16 Mechanobiology and Mechanotherapy for Cutaneous Scarring

16.1 Introduction

16.2 Cutaneous Wound‐Healing and Mechanobiology

16.3 Cutaneous Scarring and Mechanobiology

16.4 Cellular and Tissue Responses to Mechanical Forces

16.5 Keloids and Hypertrophic Scars and Mechanobiology

16.6 Relationship Between Scar Growth and Tension

16.7 A Hypertrophic Scar Animal Model Based on Mechanotransduction

16.8 Mechanotherapy for Scar Prevention and Treatment

16.9 Conclusion

References

17 Mechanobiology and Mechanotherapy for the Nail

17.1 Introduction

17.2 Nail Anatomy

17.3 Role of Mechanobiology in Nail Morphology

17.4 Nail Diseases and Mechanical Forces

17.5 Current Nail Treatment Strategies

17.6 Mechanotherapy for Nail Deformities

17.7 Conclusion

References

18 Bioreactors

18.1 The Mechanical Environment: Forces in the Body

18.2 Bioreactors: A Short History

18.3 Bioreactor Types

18.4 Commercial versus Homemade Bioreactors

18.5 Automated Cell‐Culture Systems

18.6 The Future of Bioreactors in Research and Translational Medicine

References

19 Cell Sensing of the Physical Properties of the Microenvironment at Multiple Scales

19.1 Introduction

19.2 Cells Sense their Mechanical Microenvironment at the Nanoscale Level

19.3 Cell Sensing of the Nanoscale Physicochemical Landscape of the Environment

19.4 Cell Sensing of the Microscale Geometry and Topography of the Environment

19.5 Conclusion

References

20 Predictive Modeling in Musculoskeletal Mechanobiology

20.1 What is Mechanobiology? Background and Concepts

20.2 Examples of Mechanobiological Experiments

20.3 Modeling Mechanobiological Tissue Regeneration

20.4 Mechanoregulation Theories for Bone Regeneration

20.5 Use of Computational Modeling Techniques to Corroborate Theories and Predict Experimental Outcomes

20.6 Horizons of Computational Mechanobiology

References

21 Porous Bone Graft Substitutes

21.1 Introduction

21.2 Bone: The Ultimate Smart Material

21.3 Bone‐Grafting Classifications

21.4 Synthetic Bone Graft Structures

21.5 Conclusion

References

22 Exploitation of Mechanobiology for Cardiovascular Therapy

22.1 Introduction

22.2 Arterial Wall Mechanics and Mechanobiology

22.3 Mechanical Signal and Mechanotransduction on the Arterial Wall

22.4 Physiological and Pathological Responses to Mechanical Signals

22.5 The Role of Vascular Mechanics in Modulating Mechanical Signals

22.6 Therapeutic Strategies Exploiting Mechanobiology

22.7 The Role of Hemodynamics in Mechanobiology

22.8 Conclusion

References

Index

End User License Agreement

List of Tables

Chapter 03

Table 3.1 Comparison of AFM indentation experiments measuring the mechanical properties of the glycocalyx.

Chapter 04

Table 4.1 Tissues in which primary cilia play a role in mechanotransduction.

Chapter 05

Table 5.1 Mechanical forces within a blood vessel.

Table 5.2 Mechanotransduction diseases.

Table 5.3 Ciliary proteins, by subcellular localization.

Chapter 10

Table 10.1 Many kidney diseases originate from mutations in genes encoding for adhesion proteins in the glomerular filtration barrier; these are some examples. Gene names in capitals indicate where human mutations have been described; those in lower case indicate where a phenotype has been observed in mice. FSGS, focal segmental glomerulosclerosis.

Chapter 13

Table 13.1 Progressive suction procedure. Median and range values of viscoelastic parameters in nonmenopausal (n = 65) and perimenopausal (n = 65) women.

Chapter 18

Table 18.1 Generalized considerations for determining bioreactor conditions.

Table 18.2 Experimental bioreactors designed to research the effects of mechanical forces on cells and tissues

in vitro

.

List of Illustrations

Chapter 01

Figure 1.1 Bone growth and development are affected by mechanical stress. (a) The response of tissues to mechanical stimulation can clearly be seen in the arms of a professional tennis player. The bone thickness and density are greater in the dominant right arm. (b) On hitting the ball with the racket, the skeletal muscle pulls against the bones, causing them to rebuild and become denser.

Figure 1.2 Stem cells and their niches. (a) Artur Pappenheim’s hypothesis of hematopoiesis from 1905. The center cell, designated a “stem cell,” represents the common progenitor of the entire blood system. (b) Stem cells exist in “niches” throughout the body, one of the best characterized being the bulge of the hair follicle. They become active during the anagen phase of the hair follicle cycle, replenishing many of the cell types that contribute to the follicle. Mechanical microenvironments such as topography may provide specific extracellular signals vital for keeping the stem cell in normal homeostasis. (c) Skin stem cells have been postulated to inhabit the rete ridge regions of the basal layer of the epidermis, formed by the epithelial morphology.

Figure 1.3 ESCs: differences in origin. (a) Murine ESCs form domelike, rounded colonies several cell layers thick, whereas (b) human ESCs form flattened, epithelial colonies. This may reflect differences in their origins. (c) Murine ESCs are thought to be analogous to cells of the inner cell mass of the embryo, which has no obvious polarity. (d) On the other hand, human ESCs (and murine EpiSCs) are likely to be more closely related to cells of the epiblast of the blastocyst. This structure is a polarized epithelium covering a basement membrane on the surface of the primitive endoderm (hypoblast).

Figure 1.4 Cellular mechanosensing of substrate thickness. (a) In order to contract a gel from A to B for a distance Δx, a cell needs to form focal adhesions on a solid support, as shown in the cartoon schematic (the cones represent the integrin connections), and then exert a force (right arrow). During this process, a tensile force (skewed lines represent the actin–myosin dependent contraction) is generated in its cytoskeleton. The material has to be able to resist and accommodate to the force that the cell applies, in this case a shear force. (b) and (c) illustrate in a simplified way the difference in force that a cell must exert in order to contract a thick versus a thin gel of equal shear modulus. (b) The shear strain is measured as the ratio between the transverse displacement of the gel (Δx) and its initial length (l). An attached cell exerts a shear force on the gel (top left) and deforms it to a distance Δx. An equal deformation Δx in the direction A → B requires a greater shear strain (ϒ

A→B

) on thin gel compared with thick. Even if the shear modulus of the material is the same, the shear stress required to deform the thin gel is greater than that required for the thick gel. As a consequence, the tension generated in the cytoskeleton may reach a critical threshold on thin gels, causing the cell to spread more; on thick gels, the cell may be unable to generate the same tension, and thus remains rounded. (c) For colonies of cells, the transverse displacement may be greater than that for a single cell. This may be a collective behavior mediated by tight intracellular interactions. Note that this figure is for explanatory purposes only and ignores many variables.

Figure 1.5 Mechanosensing of the stiffness and heterogeneity of the wound granulation tissue is important in wound‐healing. A new layer of epithelium has to migrate from the intact skin over the granulation tissue. Granulation tissue is formed in the wound bed after skin injury. It is a heterogeneous material, manly formed of fibrin and type III collagen. During the healing process, its stiffness varies, and this might influence skin repair.

Chapter 02

Figure 2.1 Overview of mechanotransduction pathways. Cells in tissue are subjected to stresses in the forms of compression, stretch, and shear; these perturbations may reach a cell through contact with matrix and extracellular fluids, or through cell–cell interfaces. Forces are passed through FAs at the cell surface, via a network of continuous mechanical linkages in the cytoskeleton, and into the nucleus. Structures within the matrix, cellular membrane, cytoskeleton, and nucleus are continuously remodeled in response to mechanical perturbations, and changes are transduced into molecular signaling pathways, such as through activation of ion channels or transcription factors (TFs). These signals are ultimately interpreted to affect cellular behavior.

Figure 2.2 Cell culture models for examining mechanobiological processes. Cells can be cultured within “static” model systems, though they remain mobile and may deform their surroundings, or in “dynamic” systems that are subjected to externally imposed loading cycles to better reflect the stresses and strains in living tissue. The simplest 2D models allow for easy cell manipulation, imaging, and scale‐up, but more sophisticated models with greater dimensionality offer a closer representation of tissue. The decellularization and repopulation of matrix extracted from tissue offers perhaps the closest model of the native chemical and mechanical environment short of carrying out experiments

in vivo

.

Chapter 03

Figure 3.1 Reconstructed confocal image from the Z‐stacks of hyaluronic acid (HA) coats (biotinylated HA‐binding protein) and primary cilia (antibody to acetylated alpha tubulin) on MLO‐A5 cells (murine preosteocytes). The co‐stain on the cilium indicates the presence of the glycocalyx on this fluid‐detecting organelle.

Figure 3.2 (a) RBC exclusion zone around a chondrocyte, indicated by the black arrow. This area corresponds to the PCM of the chondrocyte. Bar: 10 µm.

Figure 3.3 Schematic of the bone glycocalyx. (a) Osteocyte lying in the lacunocanalicular network, highlighting the disposition of the bone glycocalyx.

Chapter 04

Figure 4.1 Basic structure and components of the primary cilium.

Figure 4.2 Schematic of the primary cilium, illustrating (a) stretch‐activated ion channels under static conditions and (b) the opening of these channels in the presence of ciliary bending.

Chapter 05

Figure 5.1 Primary cilium as a sensory organelle. (a) Side view of the primary cilium, which acts as a cellular organelle. A primary cilium is projected at the apical membrane of many cell types. The cilium is extended from a mother centriole, also known as a basal body. (b) Based on the central pair of microtubules in the axoneme seen from the cross‐section, a primary cilium is generally categorized into a “9 + 0” structure. It was once thought that a cilium with “9 + 0” axoneme was always immotile, but some are now known to be motile, making classification more complex.

Figure 5.2 Mechanosensory primary cilia are dependent on functional sensory proteins. Polycystin‐1, polycystin‐2, and fibrocystin form a mechanosensory complex protein in the cilium to sense fluid shear stress. Polycystin‐1 and polycystin‐2 interact with each other at their COOH termini, forming a polycystin complex. It is predicted that fibrocystin interacts with this complex through polycystin‐2, with kif acting as an adaptor protein.

Figure 5.3 Intracellular signaling pathways are involved in transducing the mechanosensory function of primary cilia. Mechanistic divergence pathways initiated from primary cilia are responsible for blood pressure maintenance and aneurysm formation. Abnormal primary cilia induce high blood pressure earlier than aneurysm formation. However, abnormal survivin function is sufficient to form an aneurysm without altering blood pressure.

Figure 5.4 Ciliary dopamine receptor can regulate cilia length and function through a complex cellular pathway. Both calcium‐ and cAMP‐dependent protein kinases (PKC and PKA) are involved in regulating cilia length through MAP kinase (MAPK) and protein phosphatase 1 (PP‐1). PP‐1 plays an important role in actin rearrangement, which is a requirement for cilia length regulation. As cilia length optimally increases, the cilia function will become more sensitive in response to fluid shear stress.

Chapter 06

Figure 6.1 Joint cavitation is dependent upon embryo movement. Knee joints of embryonic chickens at 11 days into development. The distal femur and proximal tibia are visible in the sagittal plane. (a) A fully formed joint cavity in a normal embryo. (b) Failure of joint cavitation in response to pharmacological immobilization.

Figure 6.2 Growth plate cartilage from embryonic chickens at 18 days’ incubation. The “proliferative zone” where cells express proliferative markers such as PCNA, which is expressed in the S‐phase of the cell cycle, is indicated by dotted lines. (a) Growth plate of a normal embryo. (b) Growth plate of a pharmacologically immobilized embryo, demonstrating an expanded proliferative zone, resulting from the failure of cells to complete the cell cycle and progress through the growth plate.

Chapter 07

Figure 7.1 Schematic illustration of the mechanostat. (1) Bone remodels toward a habituated steady state in which customary levels of loading engender an acceptable mechanical strain stimulus. (2) When loading increases, such as during exercise, increased strain‐related stimuli activate osteoblasts, which are responsible for bone formation, leading to increased bone mass and improved architecture. (3) Consequently, the improvements in bone structure return strains to an acceptable level at the new level of loading. (4) Conversely, when loading is reduced, as occurs during bed rest, strains decrease, such that the resorptive activity of osteoclasts predominates. (5) This reduces bone mass and increases strain levels toward those experienced in the habituated state.

Figure 7.2 Simplified representation of the estrogen receptor signaling cascade. (1) Genomic ligand‐dependent estrogen receptor signaling is initiated by estrogens such as 17β‐estradiol (E2) diffusing across cell membranes to bind the estrogen receptors, primarily ERα and ERβ. These estrogen receptors then homo‐ or heterodimerize and translocate to the nucleus, where they interact with various cofactors to alter gene expression. (2) Nongenomic ligand‐dependent estrogen receptor signaling is initiated when E2 binds the estrogen receptors typically at the cell membrane to trigger activation of protein kinases, including ERK. (3) Ligand‐independent estrogen receptor signaling can be initiated by various growth factors binding cell‐surface receptors that activate protein kinases, again including ERK, which are able to phosphorylate (P) the estrogen receptors and thus activate them in the absence of E2.

Figure 7.3 Schematic representation of the structure of ERα and ERβ. Both estrogen receptors have an AF‐1 domain (which mediates interactions with other proteins), a short linker region, a DNA‐binding domain (DBD), a hinge region, and a ligand‐binding AF‐2 domain. Per cent homologies between the two receptors are based on those previously reported by Dey et al. (2013). Structural differences between the receptors are exploited in the pharmacological development of SERMs.

Figure 7.4 Schematic representation of the actions of estrogen receptor in different stages of the osteoblast lineage. Functions of the estrogen receptors illustrated here are inferred from mechanistic studies of osteoblastic cell types used to model osteoblastic cells in different stages of differentiation. Early osteoblasts can proliferate or differentiate, and though ERα promotes their proliferation and suppresses differentiation, there is evidence that ERβ promotes differentiation while inhibiting proliferation. In mature osteoblasts, ERα promotes proliferation and ERβ reduces proliferation, but both reduce apoptosis. ERβ and ERα both contribute to these cells’ bone‐forming functions. In response to mechanical strain, ERα facilitates osteogenic signaling pathways, including IGF and Wnt/β‐catenin. Both receptors regulate

Sost

expression: ERβ mediates its acute downregulation via strain and estradiol, and ERα maintains its basal expression. Both estrogen receptors also reduce osteoclast recruitment by osteoblastic cells. β‐cat, β‐catenin.

Figure 7.5 Illustration of the mouse axial tibial loading model used to investigate influences on the mechanostat. (a) Schematic diagram of the noninvasive mouse axial tibial loading model. The flexed knee is placed in a cup attached to the actuator arm of an electromagnetic materials testing machine, while the flexed ankle is placed in a cup attached to a load cell, which measures forces applied. (b) Micro‐computed tomography (CT) images demonstrating the dramatic increase in both trabecular and cortical bone following 40 cycles of loading three times per week for 2 weeks.

Figure 7.6 Deletion of ERα impairs the osteogenic response to loading in the cortical bone of female mice. Sections taken from the distal ulnae of mice to demonstrate cortical bone formation in (a) wild type and (b) ERα global knockout mice following 2 weeks of mechanical loading. Bones are labeled with fluorescent fluorochromes administered on the first and last days of loading. The distance between labels indicates new bone formation (arrows). (c) Quantification of new bone formation, demonstrating a blunted response to mechanical loading in ERα global knockout mice.

Chapter 09

Figure 9.1 (a) Stress–strain curves of a large conduit PA, with SMCs at dilated state, basal tone, and constricted state. Note that the transition region is marked. (b) Stress–strain loops of a large conduit PA under dynamic loading in healthy and disease conditions at SMC dilated state. (c) Stress–strain curves of a large conduit PA in healthy and disease conditions at SMC dilated state.

Figure 9.2 Representative pulmonary vascular impedance (magnitude PVZ and phase θ) spectra obtained from a healthy mouse.

Figure 9.3 Eight‐chain model of crosslinked collagen in an artery wall. A single chain represents the tropocollagen molecules between neighboring crosslinks. This single chain is made up of multiple subunits with fixed length, which represent the repeating amino acid motif of a tropocollagen molecule. The length of each chain in the eight‐chain element depends on the number of subunits, which in turn depends on the density of crosslinks. The element has normalized dimensions Cr × Cθ × Cz along the material axes r, θ, are z, respectively.

Chapter 10

Figure 10.1 Glomerular filtration barrier. (a) The kidney glomerulus consists of a bundle of capillaries enclosed by Bowman’s capsule. Blood from the systemic circulation enters capillaries via afferent arterioles. Filtration occurs across specialized capillary walls, and the primary filtrate flows into the proximal tubule. Filtered blood returns to the circulation via efferent arterioles. (b) With electron microscopy, it is possible to visualize the ultrastructure of the filtration barrier, which comprises fenestrated endothelial cells (GEnCs), glomerular basement membrane (GBM), and specialized epithelial cells known as podocytes. (c) Podocytes cover the outer surface of glomerular capillaries with interdigitating foot processes extending from the cell’s body. The capillary wall experiences stretch associated with shear force within the lumen, in addition to the force associated with filtration across the wall.

Figure 10.2 Podocyte adhesion. Podocyte foot processes connect via a unique cell–cell junction known as the slit diaphragm. This protein complex includes nephrin, its homologue neph‐1, podocin, CD2AP, Nck, and Fyn. Adhesion to the GBM is via focal adhesion (FA) complexes that link to the actin cytoskeleton.

Chapter 12

Figure 12.1 Schematic of mechanical forces experienced by the aortic valve during (a) systole and (b) diastole. Insets show the effect of these forces on the valve endothelial and interstitial cells.

Figure 12.2 Schematic depicting the complex cross‐sectional structure of the aortic valve cusp and the various mechanical forces acting at this scale. The fibrosa layer serves for the load‐bearing function and is made up of type I and III collagen. The spongiosa layer located in between the fibrosa and ventricularis provides lubrication to the supporting layers during cardiac cycle and is made up of glycosaminoglycans. The layer closest to the LV, the ventricularis, serves to reduce the radial strain of the valve during a cardiac cycle and is made up of elastin.

Figure 12.3 Examples of bioreactor systems currently in use. (a) Aortic valve anchoring module. (b) Flow‐loop schematic of a pulsatile left heart simulator. (c) Synchronous multivalve aortic valve culture system. (d) Uniaxial cyclic biostretcher, used to stretch cell monolayers grown on an elastic silicone rubber substrate.

Figure 12.4 (a) Schematic depicting a valve cell co‐culture model, with VECs seeded atop VIC‐encapsulated hydrogel. (b) Co‐culture scaffold demonstrating zonally organized cell populations after 7 days in culture. CD31‐expressing VECs form a confluent monolayer on top of the gel, while encapsulated VICs express low levels of α‐SMA (scale bars = 50 µm).

Chapter 13

Figure 13.1 Stress–strain curve obtained under progressive suction procedure, showing progressive linear increase in suction (S, mbar) of 25 mbar/s for 20 seconds followed by a relaxation recovery at the same rate. The maximum deformation (MD) and the residual deformation (RD) of the skin extensibility (E, mm) are recorded. Hysteresis (HY) is the area delimited by the suction–relaxation curves.

Figure 13.2 Examples of stress–strain relationships showing a variable value of energy dissipation (ed) and energy input (ei): (a) nonmenopausal woman; (b) perimenopausal woman with marked skin slackness; (c) perimenopausal woman with discrete skin atrophy.

Chapter 14

Figure 14.1 Anatomy of skin structure.

Figure 14.2 (a) Itchy and painful nodules on the anterior chest of a young male with keloids. (b) Keloid fibroblasts encounter both traction force and substratum stiffness.

Figure 14.3 Painful swelling fingers in a patient with scleroderma. Periungual telangiectasia is one of the clinical features.

Figure 14.4 Typical appearance of pincer nail. Overcurvature of the nail plate is observed.

Figure 14.5 Tense blisters, erosions, and crusts with itchy urticarial plaques or patches on (a) the trunk and (b) the forearm of a patient with bullous pemphigoid.

Chapter 16

Figure 16.1 Keloids, hypertrophic scars, and gray area. Typical hypertrophic scars generally grow within the boundaries of wounds, and typical keloids grow beyond the confines of their original wounds. However, even senior clinicians sometimes have difficulty in differentiating between the two conditions, particularly with atypical cases.

Figure 16.2 Severity of scars is modified by many factors. It is likely that the inflammatory status in scars is modified by many other risk factors, including genetic, systemic, and local factors, such as hypertension (high blood pressure).

Figure 16.3 Typical keloids. Keloids tend to occur at specific sites, including the anterior chest, shoulder, scapular, and lower abdomen–suprapubic regions.

Figure 16.4 Finite‐element analysis of the mechanical force distribution around keloids, showing that there is high skin tension at the edges. This observation strongly supports the notion that skin tension is closely associated with the pattern and degree of keloid growth.

Figure 16.5 Z‐plasty for shoulder‐joint keloids. Z‐plasty is effective in tension reduction and in decoupling long keloids and hypertrophic scars, especially those located on a joint.

Chapter 17

Figure 17.1 Nail anatomy.

Chapter 18

Figure 18.1 Cell culture has evolved significantly since its origins in the 19th century, but is still generally performed in two‐dimensional (2D) monolayers cultured under static conditions, which do not reflect the rich structural and mechanical environment of the native tissue. Certain cell types, such as osteoblasts, change their morphology

in vitro

as they differentiate from (a) mesenchymal stem cells (MSCs) into terminally differentiated osteocytes, which become encased in mineralizing matrix and form characteristic (b) three‐dimensional (3D) “nodule” structures. Mechanically stimulating these cells (in this case, by using magnetic nanoparticles directly against the mechanically gated ion channel, TREK1) shows that (c) mechanical cues can have a significant enhancement on tissue formation. Reproducible biomaterial scaffolds have now elevated cell culture into the third dimension, mimicking many of the structural and chemical aspects of the tissue; however, without mechanical cues the environment is still incomplete. The purpose of bioreactors, therefore, is to build a mechanical apparatus that integrates with existing cell culture, thereby facilitating a straightforward transition from (d) static culture in plates and flasks to dynamic microenvironmental culture, such as in (e) a hydrostatic bioreactor, which provides compressive forces to cells cultured in standard plates, augmenting the standard incubator with a mechanical component and bridging the gap between traditional and advanced cell culture.

Figure 18.2 Bioreactors can deliver shear forces in many different ways. (a) Stirred spinner flask. (b) Rotating‐wall bioreactor. These were initially developed for the industrial expansion of microorganisms, but adherent mammalian cells must first be seeded on to microcarriers or biomaterial scaffolds or cultured as tissue explants. (c) A simple method for providing shear is to perfuse the cell‐seeded scaffold with fluid, which can be pressurized; the nonuniform effects on flow rate and the subsequent cell responses are a common theme of mathematical modeling. (d) Laminar microfluidic flow system, which utilizes compressed air to drive media flow across an adherent cell monolayer.

Figure 18.3 Hydrostatic bioreactor designed and created in partnership between Keele University (Professor Alica El‐Haj) and TissueGrowthTechnologies (now a part of Instron). The key features of this bioreactor have been optimized to enhance high laboratory throughput and maintain maximum sterility, with the majority of the bioreactor and most of the moving hardware being located outside of the incubator environment, including the compressor and the computer control system. The bioreactor chamber itself exists as a “mini‐incubator”: a sealed, autoclavable chamber accommodating a standard multiwelled plate, allowing for standard experiments to be run under conventional, static incubator cultures and under dynamic hydrostatic loading. Separation from the outside space is provided by a replaceable and autoclavable filter, and is maintained by wide flanges to the chamber and a gasket, which helps reduce microbial infiltration when the chamber is opened in order to replace culture plates. The addition of inspection windows allows the cultures to be viewed, but reflections from the glass and the height above the culture restrict opportunities to derive measurable data or high‐resolution images – this will be a point of optimization of future models in this range, which has now been commercialized as “CartiGen HP” (hydrostatic pressure). (a) Bioreactor chamber, containing cells in a standard culture plate. (b) Chamber and valve‐control box – the only parts of the bioreactor placed within the incubator. (c) Schematic of the bioreactor.

Figure 18.4 Schematic of a typical tension bioreactor, in which cells in a biomaterial scaffold such as collagen are cultured between two grips, which can be pulled apart to generate strain within the cell‐seeded construct. (a) A useful adaptation of the second‐generation Bose Electroforce series is the addition of space‐filling solid baffles (α and β), which require less medium in the bioreactor chamber, and thus provide substantial savings on the expensive growth factors used in culture. (b) A particular engineering challenge is (i) the grip–biomaterial interface, where a great deal of mechanical failure occurs. Researchers have thus developed various ways of reinforcing this region. Options include (ii) the use of composite materials with an integrated solid scaffold or sacrificial zone, which is mechanically more resilient than the region under tensile load. Alternatively, the cell‐seeded scaffold can be fabricated as a circular band and (iii) connected to the tension actuator via a loop or (iv) pinned on to the grips. Often, researchers will use a combination of these approaches, depending on the application (see Table 18.1). (c) The Electroforce series (now owned by TA Instruments) has been extensively used by researchers, as it conveniently attaches to existing mechanical testing systems, which include adaptable commercial software.

Figure 18.5 Commercial bioreactors are often designed principally to improve bioproduction methods – increasing cell yields and reducing costs through the extensive use of automation, in order to achieve consistent, reliable cell growth

in vitro

. Most of these systems use an element of perfusion to provide nutrients to cells, though the effects of this as a mechanical stimulus are generally a secondary consideration and are often limited to ameliorating the negative effects of excessive shear forces on cell viability. Nevertheless, this is likely to be the direction in which cells become commercialized as manufactured therapeutic products. Cell factories may be relatively easy to re‐engineer with more appropriate mechanical characteristics which support and enhance cell growth, ultimately leading to more effective cell products. (a) Robotic T‐flask handling (CompacT SelecT TAP Biosystems). (b) GE Healthcare’s WAVE Bioreactor 2050.

Chapter 19

Figure 19.1 Nanoscale architecture of focal adhesions (FAs). Left: average z‐position of different FA‐associated proteins (Liu et al. 2015). Right: super‐resolution image (top: top view; bottom: side view) of the actin network, with color‐coded z‐spatial information, at cell protrusions and FAs (scale bars top: 2 µm; bottom: 250 nm).

Figure 19.2 MSCs spreading on polydimethylsiloxane (PDMS) microposts with controlled flexural moduli.

Figure 19.3 Proposed model to account for the distinct cell response to the bulk moduli of ECM protein‐functionalized PAAm gels and PDMS substrates. Key parameters influencing the bulk modulus and cell response are summarized.

Figure 19.4 The differentiation of MSCs is regulated by matrix degradation in 3D. Top: osteogenic differentiation of MSCs in degradable hyaluronic acid gels. Bottom: adipogenic differentiation in nondegradable gels.

Figure 19.5 Impact of substrate topography on FA formation and cell spreading.

Figure 19.6 Impact of local and global ligand density on cell adhesion. Top: examples of nanopatterned substrates. Bottom: impact of ligand density on cell adhesion and spreading.

Figure 19.7 The dynamics of recruitment of vinculin is controlled by adhesive patch size.

Figure 19.8 ECM guides the orientation of cell division.

Figure 19.9 Cell shape controls the differentiation of keratinocytes.

Figure 19.10 Arrays of microepidermis with controlled partitioning of differentiated keratinocytes.

Chapter 20

Figure 20.1 Forearms of a professional tennis player (right‐hander). The image illustrates both widening and lengthening of the right arm as an adaptive response to mechanical loading through exercise.

Figure 20.2 Discoveries of Dr. Shinya Yamanaka that were awarded the Nobel Prize. He studied genes that regulate stem cell function and (a) transferred such genes into fibroblasts from a mouse. These fibroblasts (b) dedifferentiated into stem cells, which could (c) redifferentiate into, for example, musculoskeletal cells.

Figure 20.3 (a) Negative feedback loop, where the formation of new tissue matrix and cell phenotypes reduces the pericellular mechanical strains detected by cells. (b) Positive feedback loop, where sensitized cells respond more efficiently to the mechanical environment. New tissue matrices and cell phenotypes are formed to reduce the stimuli sensed by cells, but as the cells are more mechanosensitive, their response to loading is stronger than that of nonsensitized cells. Thus, tissue formation can become more effective.

Figure 20.4 The phases of fracture healing. Inflammatory phase: a hematoma, consisting of platelets, inflammatory cells, and signaling molecules, is formed. MSCs migrate from the periosteum or the bone‐lining surface to the gap, which organizes into connective granulation tissue. Reparative phase: bone formation begins from the periosteal surface and grows to form the callus. Stem cells start to differentiate, which results in soft‐tissue production (e.g., fibrous tissue and cartilage) in the gap. The cartilage is replaced by bone through endochondral ossification. Remodeling phase: after maturation of the bone, the tissue remodels and becomes more organized. The fracture callus gradually resorbs and restores the bone’s original shape.

Figure 20.5 Examples of experimental techniques for the investigation of cell mechanobiology. (a) Nanoindentation: a technique whereby the cell is carefully indented. The indenter records force and deformation data to investigate cell material properties. (b) Micropipette aspiration: a technique whereby the cell is sucked into a pipette with a known diameter, which deforms it. (c) Substrate stretch: a monolayer of cells is stretched by stretching the substrate on which it lies. Based on the stiffness of the substrate and the magnitude of stretch, the cells are exposed to mechanical strain. (d) Flow chambers: confluent layer of cells in a bioreactor are exposed to fluid flow. This technique is often used to investigate the effect of fluid shear on cells.

Figure 20.6 Selected illustrations of some mechanoregulation theories. (a) Theory of Pauwels (1960), who proposed that hydrostatic pressure and strain regulate stem cell differentiation into cartilage and fibrous tissue. He proposed that bone is formed only following the stabilization and maturation of the soft tissues. (b) Mechanoregulation theory of Carter et al. (1988), who propose hydrostatic stress and principal strain. (c) Theory of Prendergast et al. (1997), based on the idea that tissues are biphasic materials, in which the interstitial fluid flow and octahedral shear strains together are the main modulators of stem cell differentiation. (d) Mechanoregulatory tissue differentiation theory of Claes and Heigele (1999), which includes quantitative limits on when tissues form. This formulates a theory for the different differentiation pathways to which cells will commit.

Chapter 21

Figure 21.1 Schematic diagram of the different material and porosity volume fractions within a porous hydroxyapatite scaffold. V

S

, total strut volume fraction; V

M

, total macropore volume fraction; V

Cμ

, total closed micropore volume fraction; V

HA

, total hydroxyapatite volume fraction; V

Oμ

, total open micropore volume fraction. V

S

 = V

HA

 + V

Cμ

 + V

Oμ

. Total porosity (P

T

) = 1 − [V

HA

/(V

S

 + V

M

)]. Strut porosity (P

S

) = (V

Cμ

 + V

Oμ

)/V

S

. If ρ

HA

 = theoretical density of hydroxyapatite, then apparent density = ρ

HA

[V

HA

/(V

S

 + V

M

)] and real density = ρ

HA

[V

HA

/(V

Cμ

 + V

HA

)].

Figure 21.2 Dependence of extrinsic scaffold properties on the level of apparent density (i.e., total porosity) and strut porosity of hydroxyapatite BGS materials.

Chapter 22

Figure 22.1 Impact of vessel mechanics and geometry on blood flow dynamics through the interaction of flow with vessel wall. This promotes mechanosensing and mechanotransduction signaling cascades within vascular cells, which determines tissue homeostasis or remodeling and thus vascular healthy or diseased states.

Figure 22.2 Typical CFD process in a biomedical application. (a) Typical peripheral bypass grafting. (b) Solid computer‐aided model (CAD) for the computational domain. (c) Hybrid mesh consisting of quadrilateral cells near the wall boundaries and tetrahedral cells further from the wall. (d) Post‐processing of the CFD simulation results using streamlines and contours of the hemodynamic parameters.

Figure 22.3 Spatial distribution and localized sites of IT/IH in a typical ABG.

Figure 22.4 Industrial examples of the induction of helical flow. (a) Schematics of “transverse‐ribbed,” “multi‐start ribbed,” and “longitudinally finned” nuclear fuel pins used in the UK’s advanced gas‐cooled reactors (AGRs). (b) Schematic of helical ridges on the outer surface of “multi‐start” fuel pins. (c) Profile of the rib/ridge in the “multi‐start” design.

Figure 22.5 Schematics and dimensions of four geometric models.

Figure 22.6 Distributions of different hemodynamic parameters viewed from the host arterial wall, opened ventrally and shown

en face

: (a) time‐averaged wall shear stress (TAWSS); (b) TAWSS gradient; (c) oscillatory shear index (OSI); and (d) relative residence time (RRT). Note that the color scale of the TAWSS map is inverted for ease of comparison.

Guide

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Mechanobiology

Exploitation for Medical Benefit

Edited by Simon C. F. Rawlinson

Queen Mary University of London, Institute of Dentistry, England

Copyright © 2017 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750‐8400, fax (978) 750‐4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748‐6011, fax (201) 748‐6008, or online at http://www.wiley.com/go/permissions.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

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Library of Congress Cataloging‐in‐Publication data applied for

ISBN: 9781118966143

Cover credits: Background: Mimi Haddon/Gettyimages

List of Contributors

Kartik BalachandranDepartment of Biomedical EngineeringUniversity of Arkansas, FayettevilleAR, USA

Christoph BallestremWellcome Trust Centre for Cell‐MatrixResearch, Faculty of Life SciencesUniversity of ManchesterManchester, UK

Charlie CampionInstitute of Bioengineering and School ofEngineering and Materials ScienceQueen Mary University of LondonLondon, UK

Naomi C. CheslerDepartment of Biomedical EngineeringUniversity of Wisconsin–MadisonMadison, WI, USA

Yanan DuDepartment of Biomedical EngineeringSchool of Medicine,Tsinghua UniversityBeijing, China

Kian F. EichholzTrinity Centre for BioengineeringTrinity Biomedical Sciences Instituteand Department of Mechanical andManufacturing EngineeringSchool of EngineeringTrinity College Dublin, DublinIrelandDepartment of Mechanical, Aeronauticaland Biomedical EngineeringCentre for Applied BiomedicalEngineering ResearchMaterials and Surface Science InstituteUniversity of Limerick, LimerickIreland

Alicia J. El HajInstitute for Science and Technology inMedicine, University of KeeleStaffordshire, UK

Winston ElliottDepartment of Mechanical EngineeringUniversity of Colorado at BoulderBoulder, CO, USA

Nicholas D. EvansCentre for Human Development, StemCells and Regeneration, Institute for LifeSciences, University of SouthamptonFaculty of Medicine, Institute forDevelopmental SciencesUniversity of SouthamptonSouthampton, UKBioengineering Sciences GroupFaculty of Engineering and theEnvironment, University ofSouthampton, Southampton, UK

Gabriel L. GaleaDevelopmental Biology of Birth DefectsUCL Great Ormond Street Institute ofChild Health,London, UK

Ulysse GaspardDepartment of GynecologyLiège University HospitalLiège, Belgium

Julien E. GautrotInstitute of Bioengineering and Schoolof Engineering and Materials ScienceQueen Mary University of LondonLondon, UK

Hamish T. J. GilbertWellcome Trust Centre for Cell‐MatrixResearch, Faculty of BiologyMedicine and HealthUniversity of ManchesterManchester, UK

K. Jane Grande‐AllenDepartment of BioengineeringRice University, Houston, TX, USA

James R. HenstockInstitute for Ageing and Chronic DiseaseUniversity of Liverpool, LiverpoolUK

Karin A. HingInstitute of Bioengineering andSchool of Engineering andMaterials ScienceQueen Mary University of LondonLondon, UK

David A. HoeyTrinity Centre for BioengineeringTrinity Biomedical Sciences Instituteand Department of Mechanical andManufacturing EngineeringSchool of Engineering, Trinity CollegeDublin, Dublin, IrelandDepartment of Mechanical, Aeronauticaland Biomedical EngineeringCentre for Applied BiomedicalEngineering Research, Materials andSurface Science InstituteUniversity of Limerick, LimerickIreland

Chao‐Kai HsuDepartment of Dermatology, NationalCheng Kung University HospitalCollege of Medicine, National ChengKung University, Tainan, TaiwanInstitute of Clinical Medicine, Collegeof Medicine, National Cheng KungUniversity, Tainan, TaiwanInternational Research Center ofWound Repair and RegenerationNational Cheng Kung UniversityTainan, Taiwan

Chenyu HuangDepartment of Plastic, Reconstructive,and Aesthetic Surgery, Beijing TsinghuaChanggung Hospital, Beijing, ChinaMedical Center, Tsinghua UniversityBeijing, China

Philippe HumbertUniversity of Franche‐Comté, BesançonFranceDepartment of Dermatology, UniversityHospital Saint‐Jacques, Besançon, FranceInserm Research Unit U645, IFR133Besançon, France

Hanna IsakssonDepartment of Biomedical EngineeringLund University, Lund, Sweden

Amir KeshmiriEngineering and Materials ResearchCentre, Manchester MetropolitanUniversity, Manchester, UKSchool of Mechanical, Aerospace andCivil Engineering, the University ofManchester, Manchester, UK

Hanifeh KhayyeriDepartment of Biomedical EngineeringLund University, Lund, Sweden

Franziska LauseckerWellcome Trust Centre for Cell‐MatrixResearch, Faculty of Life SciencesUniversity of ManchesterManchester, UKInstitute of Human DevelopmentFaculty of Human SciencesUniversity of ManchesterManchester, UK

Rachel LennonWellcome Trust Centre for Cell‐MatrixResearch, Faculty of Life SciencesUniversity of ManchesterManchester, UKInstitute of Human DevelopmentFaculty of Human SciencesUniversity of ManchesterManchester, UKDepartment of Paediatric NephrologyCentral Manchester University HospitalsNHS Foundation Trust (CMFT)Manchester Academic Health ScienceCentre (MAHSC), Manchester, UK

Stefania MarcottiDepartment of Materials Science andEngineering, INSIGNEO Institute forIn Silico Medicine, Sheffield, UK

Lee B. MeakinSchool of Veterinary SciencesUniversity of Bristol, Bristol, UK

Keiji NaruseDepartment of CardiovascularPhysiology, Graduate School of MedicineDentistry and Pharmaceutical SciencesOkayama University, Okayama, Japan

Andromeda M. NauliDepartment of Pharmaceutical SciencesMarshall B. Ketchum University,FullertonCA, USA

Surya M. NauliDepartment of Biomedical &Pharmaceutical SciencesChapman University, IrvineCA, USA

Rei OgawaDepartment of Plastic, Reconstructive,and Aesthetic Surgery, Nippon MedicalSchool, Tokyo, Japan

Hulin PiaoDepartment of CardiovascularPhysiology, Graduate School of MedicineDentistry and Pharmaceutical SciencesOkayama University, OkayamaJapanDepartment of Cardiovascular SurgeryThe Second Affiliated Hospital of JilinUniversity, Changchun, China

Gérald E. PiérardLaboratory of Skin Bioengineering andImaging (LABIC), Department of ClinicalSciences, University of LiègeLiège, BelgiumUniversity of Franche‐Comté, BesançonFrance

Sébastien L. PiérardTelecommunications and ImagingLaboratory INTELSIG, MontefioreInstitute, University of Liège, LiègeBelgium

Claudine Piérard‐FranchimontDepartment of DermatopathologyUnilab Lg, Liège University HospitalLiège, BelgiumDepartment of DermatologyRegional Hospital of Huy, HuyBelgium

Andrew A. PitsillidesComparative Biomedical SciencesThe Royal Veterinary CollegeLondon, UK

Andrea S. PollardComparative Biomedical SciencesThe Royal Veterinary CollegeLondon, UK

Patrick J. PrendergastTrinity Centre for BioengineeringSchool of Engineering, Trinity CollegeDublin, Dublin, Ireland

Daniel PuperiDepartment of BioengineeringRice University, Houston, TX, USA

Prashanth RavishankarDepartment of Biomedical EngineeringUniversity of Arkansas, FayettevilleAR, USA

Simon C. F. RawlinsonCentre for Oral Growth andDevelopment, Institute of DentistryBarts and The London School ofMedicine and DentistryLondon, UK

Caretta J. ReeseDepartment of Biomedical &Pharmaceutical SciencesChapman University, IrvineCA, USA

Gwendolen C. ReillyDepartment of Materials Science andEngineering, INSIGNEO Institute forIn Silico Medicine, Sheffield, UK

Hitomi SanoDepartment of Plastic, Reconstructiveand Aesthetic Surgery, Nippon MedicalSchool, Tokyo, Japan

Rinzhin T. SherpaDepartment of Biomedical &Pharmaceutical SciencesChapman University, IrvineCA, USA

Joe SwiftWellcome Trust Centre for Cell‐MatrixResearch, Faculty of BiologyMedicine and HealthUniversity of ManchesterManchester, UK

Ken TakahashiDepartment of CardiovascularPhysiology, Graduate School of MedicineDentistry and Pharmaceutical SciencesOkayama University, OkayamaJapan

Wei TanDepartment of Mechanical EngineeringUniversity of Colorado at BoulderBoulder, CO, USA

Lian TianDepartment of Medicine, Queen’sUniversity, Kingston, ON, Canada

Camelia G. TusanCentre for Human Development, StemCells and Regeneration, Institute for LifeSciences, University of SouthamptonFaculty of Medicine, Institute forDevelopmental SciencesUniversity of SouthamptonSouthampton, UKBioengineering Sciences GroupFaculty of Engineering and theEnvironment, University ofSouthampton, Southampton, UK

Zhijie WangDepartment of Biomedical EngineeringUniversity of Wisconsin–MadisonMadison, WI, USA

Preface

Mechanobiology is the study of how tissues and cells interact with, and respond to, the physical environment, either through direct contact with a substrate via cell attachments or through cell‐surface perturbation by a varying extracellular situation/climate.

The vast majority of cells are subjected to a fluctuating physical environment – and this is not restricted to the animal kingdom. In response to increased loading conditions (bending), the branches of trees compensate with new wood formation. Interestingly, though, conifers and hardwoods respond to this increased bending differently: conifers tend to produce “tension wood” on the upper part of the bough, whereas hardwoods produce “compression wood” on the lower surface – two distinct solutions to one problem.

This volume attempts to briefly introduce the topic of mechanobiology in humans to a broad audience, with the intention of making the phenomenon more widely recognized and demonstrating its relevance to medicine. It covers three broad topics: (i) recognition of the mechanical environment by extracellular matrix (ECM) and primary cilium, (ii) selected tissue types, and (iii) physical, computational/substrate models and the use of such findings in practice.

Obviously, the list of chapters for each topic is not exhaustive – there are too many examples, and this volume therefore can only be an introduction. The tissue types discussed are some of the more immediately recognizable as being subjected to mechanical forces, though a few are less obvious.

One important question is, given that most biology is subjected to the mechanical environment, how can we best reproduce that in experimental conditions? Would the effect of a compound be influenced if the tissue/cells were subjected to their normal physiological environment at the time of application? Such questions need to be at least acknowledged, if not accommodated within experimental design.

I hope the volume generates interest in, and appreciation of, this emerging field with those considering a career in science or medicine.

Finally, I would like to thank all the contributing authors to this manuscript. They have all devoted their time to writing their chapters and have focused on presenting their ideas clearly and logically to the target audience.

Simon C. F. Rawlinson

1Extracellular Matrix Structure and Stem Cell Mechanosensing

Nicholas D. Evans and Camelia G. Tusan

Centre for Human Development, Stem Cells and Regeneration, Institute for Life Sciences, University of Southampton, Faculty of Medicine, Institute for Developmental Sciences, University of Southampton, Southampton, UK

Bioengineering Sciences Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, UK

1.1 Mechanobiology

An ability to sense the external environment is a fundamental property of life. All organisms must be able to interpret their surroundings and respond in a way that helps them survive – for example, by feeding, moving, and reproducing. The ability to sense also allows organisms to communicate with one another. Communication and cooperation were the primary driving forces that led to the evolution of complex multicellular organisms from simpler unicellular organisms. Evidence of this remains in many of the signaling pathways found in mammals that promote cell arrangements during development, which evolved from primordial chemical signals that unicellular organisms used to communicate with one another (King et al. 2003). Cells in our mammalian bodies are experts at communicating with one another using chemicals, and our physiology is completely dependent on this, from the precisely orchestrated cascades of growth factors during development to the hormones necessary for homeostasis and the immune mechanisms fundamental to repelling microbes.

Cells can also interact with one another by direct contact. Cells express characteristic surface proteins of various types, most prominently the cadherins, which allow them to determine whether they have a close neighbor.

Organisms are not just aggregates of cells – cells also make materials that provide structural support and knit groups of cells together. This material is called “extracellular matrix” (ECM). Again, the ECM is rich in chemical information for cells, provided in the three‐dimensional information encoded in the myriad proteins that may be deposited there. In this way, cells can communicate with one another not only in space, but also over relatively long periods of time, with insoluble ECM having a much longer half‐life that secreted soluble cues (Damon et al. 1989).

But this is not the whole story. The environment is not solely open to sensing by chemical means. Consider what we think of as our own senses: sight, sound, smell, taste, and touch. Smell and taste are perhaps the most analogous to the cellular sensing mechanisms just described, while sight is a somewhat more specialized form of sensing, based on the ability of certain cellular molecules to become altered by the absorption of electromagnetic radiation. Sound and touch are also fundamental sensations, the former a specialized type of the latter, based on our ability to detect the mechanical force of the interaction of matter with our bodies. This property is generally referred to as “mechanosensitivity,” the study of which is known as “mechanobiology.” But despite the importance of these senses, for many years they remained relatively under‐researched in the field of biological sciences, and were limited to some fascinating, specialist examples. One such example is the hair cells of the inner ear, which transduce movement into neural signals that can be interpreted by the central nervous system (CNS) (Lumpkin et al. 2010). These cells not only detect vibrations in materials of particular wavelengths that we understand as sound, but are also able to act as accelerometers – detecting acceleration due to physical movement or the continuous acceleration resulting from the earth’s gravity. In addition, a similar system is thought to be present in the skeleton. Astronauts who experience long periods of reduced acceleration in the microgravity of the earth’s orbit suffer from a reduced bone mass on return to earth (Sibonga et al. 2007). A prevailing hypothesis (yet to be universally accepted) is that osteocytes within the bone matrix, like the hair cells of the inner ear, are able to detect and respond to acceleration (Klein‐Nulend et al. 1995). Evidence for this comes from the observation that bones remodel in response to mechanical stress, tending to increase in density (and strength) in regions where the applied stress is the greatest, an effect unambiguously demonstrated in the forearms of professional tennis players (Figure 1.1), where bone thickness is greater in the dominant arm (Ducher et al. 2005).

Figure 1.1 Bone growth and development are affected by mechanical stress. (a) The response of tissues to mechanical stimulation can clearly be seen in the arms of a professional tennis player. The bone thickness and density are greater in the dominant right arm. (b) On hitting the ball with the racket, the skeletal muscle pulls against the bones, causing them to rebuild and become denser.

Source: x‐ray images reproduced from Krahl et al. (1994) and Taylor et al. (2000).

Aside from these specific examples of mechanosensing, it is increasingly evident that all cells retain intrinsic mechanisms for sensing the mechanical properties of the environment around them. And this property has fundamental repercussions in almost all aspects of physiology and disease. In the context of human health and well‐being, one aspect of mechanobiology that continues to receive special attention is its effect on stem cells.

1.2 Stem Cells

Stem cells are cells that can divide to make more copies of themselves, or which can differentiate into two or more specialized cell types. The concept of the stem cell emerged from ideas about both evolutionary and developmental biology in the late 19th century, generally with the notion that cell lineages, either throughout evolution or in the development of an organism, followed a family tree‐like pattern of descent, with the putative stem cell at the top (Maehle 2011). This concept was brought into sharp focus in the mid‐20th century with the work of a succession of experimental biologists who characterized “haematopoetic stem cells.” These cells were shown to have enormous plasticity and replicative power, and to completely reconstitute the immune systems of animals lacking a working one (the immune systems of these animals had been destroyed with radiation), supporting the early ideas of proponents of the stem cell hypothesis, such as Pappeheim (Figure 1.2a) (Ramalho‐Santos and Willenbring 2007). Today, the concept of the stem cell has spread throughout organismal biology, with stem cells identified in most if not all organs and tissues of the mammalian body. Some are amenable to extraction and culture in in vitro or ex vivo conditions and can be studied relatively easily, but some must be studied in situ. In the latter case, stem cells are known to occupy specific locations where they retain their stem‐like properties. There, they have the correct provision of extracellular signals necessary to keep them in a state primed to divide and produce more functional descendants in normal homeostasis or in case of disease or injury. Such regions are called stem cell “niches,” and the characteristics of such niches are vital to understanding how stem cells are regulated in normal and disease processes (Figure 1.2b).

Figure 1.2