Introduction

Stem cells, which can self-renew and differentiate into cells with specialized functions, are usually classified as embryonic stem cells (ESCs) and adult stem cells. ESCs are derived from the inner cell mass of a blastocyst, can self-renew indefinitely, and can give rise to cell types of all somatic lineages from the three embryonic germ layers. Adult stem cells have been found in many types of tissues and organs such as bone marrow, blood, muscle, skin, intestine, fat, and brain. Bone marrow is one of the most abundant sources of adult stem cells and progenitor cells. Mesenchymal stem cells (MSCs), hematopoietic stem cells (HSCs), and endothelial progenitor cells (EPCs) can be isolated from bone marrow. These bone marrow MSCs are pluripotent stromal cells. MSCs can be expanded into billions of folds in culture, and can be stimulated to differentiate into a variety of cell types. HSCs give rise to blood cells. These cells, along with EPCs, are mobilized in response to growth factors and cytokines released upon injury in tissues and organs, and therefore can be isolated from peripheral blood in addition to the bone marrow. Both embryonic stem cells and adult stem cells have tremendous potential for cell therapy and tissue repair.

Introduction

Two essential functions of arterial endothelium are flow-mediated vasoregulation in response to acute changes in blood flow and vascular wall remodeling in response to chronic hemodynamic alterations [1, 2]. Both of these functions require arterial endothelial cells (ECs) to be capable of sensing the mechanical forces associated with blood flow and of transducing these forces into biochemical signals that mediate structural and functional responses. Mechanosensing and -transduction in arterial endothelium also play a critical role in the development and localization of atherosclerosis. The topography of early atherosclerotic lesions is highly focal and correlates with arterial regions that are exposed to low and/or oscillatory shear stress [3, 4]. There is mounting evidence that low and oscillatory shear stress elicit a pro-inflammatory and adhesive EC phenotype, whereas relatively high and nonreversing pulsatile shear stress induce a phenotype that is largely anti-inflammatory [5–9]. In light of the central role of EC inflammation in atherogenesis [9–14], the key to understanding the involvement of flow in the development of atherosclerosis may lie in determining the mechanisms governing the differential responsiveness of ECs to different types of flows.

The current concept of EC mechanotransduction postulates that it involves a sequential progression of events involving sensing of the mechanical stimulus, transduction of the stimulus to a biochemical signal, and cellular reaction and subsequent possible adaptation to the new mechanical environment [15–19]. Consistent with this construct, a number of candidate mechanosensors have been proposed. These include stretch- and flow-sensitive ion channels [20–27], cell-surface integrins at both the luminal and basal cell surfaces [19, 28], the cellular cytoskeletal network [15], subregions of the cell membrane or the entire membrane [29, 30], membrane-associated GTP-binding proteins (or G-proteins) [31, 32] and G-protein–coupled receptors [33], cell–cell junction constituents including platelet–EC adhesion molecule-1 (PECAM-1) [34], and the glycocalyx at the cell luminal surface [35–37]. The rationale for classifying these various structures as candidate mechanosensors is threefold: 1) They are associated with the cell membrane, where the effects of an externally applied force would likely be most immediately felt; 2) they generally respond very rapidly following the onset of the mechanical stimulus; and 3) interfering with the activation of these structures abrogates, or at least significantly diminishes, some of the downstream responses induced by the applied mechanical force. It remains unclear, however, how these various structures interact with one another to potentially form an integrated mechanosensory system.

Introduction

Much like whole organisms, single cells have the ability to “sense” and respond to their surroundings. This “sensing” not only includes monitoring and responding to changes in extracellular chemical messages, but also the physical nature of the cell’s microenvironment, particularly the components of the extracellular matrix (ECM). Anchorage to the surrounding ECM is important for many cellular functions and is mediated primarily by the integrin family, a group of heterodimeric transmembrane proteins that provide physical links of the cell to the external environment. Although integrins were once viewed as structural membrane proteins providing anchor points involved in cell adhesion and movement, they are now known to be centrally important for sensing the external environment and regulating the precise intracellular responses necessary for proper mechanotransduction.

Recent evidence suggests that besides its biochemical composition, the dimensional and rheological properties of the ECM are involved in signaling processes that not only affect cell motility, but also a multitude of intracellular second messenger pathways and gene regulation. In this chapter, we review how cells and their surrounding ECM interact, particularly focusing on integrins and fibronectin, and examine how their points of contact are involved in inside-out and outside-in signaling for setting the stage for mechanotransduction. In addition, a second major focus will be on the most recent findings regarding cellular mechanosensing and its relationship to the ECM. Furthermore, we describe how alterations to matrix components can lead to altered cellular motility, phenotype, and cellular responses.

Introduction

Endothelial cells (EC) lining blood vessel walls are exposed to both the wall shear stress (WSS) of blood flow and the circumferential strain (CS) and associated circumferential stress driven by the wall motion induced by pulsing pressure. Most in vitro studies of EC response to mechanical forces and mechanotransduction have focused on the either the WSS or the CS, but not their interaction. This is in spite of the fact that in the arterial circulation that is most susceptible to disease, the WSS and the CS are imposed concurrently. While there have been relatively few studies of simultaneous WSS and CS, several recent investigations have revealed that the response of endothelial cells to combined stresses is exquisitely sensitive to the temporal phase angle between them, suggesting that when they are applied in a highly out-of-phase manner, a pro-atherogenic response is produced, whereas when they are applied in-phase, the response is more favorable.

In this chapter we first review the physiological background on WSS and CS in the circulation to focus on those regions where their interaction is significant. In the process we uncover a fascinating pattern that suggests that the WSS and the CS are most asynchronous (out-of-phase temporally) in precisely those regions of the circulation where atherosclerosis is localized. This background is followed by a consideration of the in vitro experiments in which the WSS and the CS have been applied simultaneously to the EC. There we uncover dramatic influences of the phase angle between the WSS and the CS indicating that out-of-phase forces induce a pro-atherogenic EC phenotype. Animal experiments that are consistent with this view are reviewed and possible countermeasures are described.

Introduction

Many studies have shown that mechanical forces inherent in the living body, such as fluid shear stress due to blood flow, play critical roles in regulating cellular physiology and pathogenesis (Davies 1995; Li et al. 2005; Haga et al. 2007). These extracellular forces are sensed at the cellular level, and inside the cells the forces are somehow transduced into changes in gene expression responsible for the cellular responses (Ingber 1997). The mechanism of this force-sensing process or mechanotransduction still remains unclear. Signaling pathways following mechanical force loadings have been identified by means of biochemistry (Li et al. 2005; Haga et al. 2007). In addition to the involvement of these signaling molecules, direct intracellular force transmission from the cell membrane or extracellular matrix to the nucleus via, probably, cytoskeletal filaments may be another possible pathway through which cells respond to extracelullar forces (Wang et al. 1993; Davies 1995; Ingber 1997; Maniotis et al. 1997; Janmey 1998): Loaded forces may cause a deformation of the nucleus that is the principal site of DNA and RNA synthesis, alter the spatial positioning or dynamics of the chromatin that is a complex of DNA and proteins (such as histone) making up chromosomes, and then affect gene expression because such changes in chromatin organization could expose new sites for transcriptional regulation (Dahl et al. 2004; Lammerding et al. 2004).

There is actually considerable evidence that the cell nucleus is deformed or remodeled in response to extracellular forces when the cell adapts to the local mechanical environment by the reorganization of cytoskeletons. Flaherty et al. (1972) demonstrated by in vivo experiments that endothelial nuclei elongate and orient in the direction of blood flow, as do the whole cells (Figure 9.1). Lee et al. (2005) found, based on in vitro observations, that movement of the nucleus in the cytoplasm is enhanced by fluid flow and regulated by mediators of cytoskeleton reorganization. Deguchi et al. (2005a) suggested that not only the endothelial cell cytoskeleton but also its nucleus remodels structure under shear stress applied to the cell. This study suggested that the shear stress applied to the cell might induce structural rearrangement in the nucleus structure, which leads to a permanent alteration in its overall shape and stiffness. Thus, the nucleus deformation and remodeling appear during the force-loading and resultant cellular responses.

Equipped with the basic analytical methods presented in Chapters 9 and 10, the wave-induced motions of floating bodies are discussed in this chapter. In Chapter 12, the final chapter of this book, those methods are applied to the wave-structure interactions of fixed structures. In this book, fixed structures are those that are either resting on the sea bed or directly supported by foundations in the bed. Floating bodies include ships, floating platforms, buoys, and other specialized bodies that are either under way or maintained in position by moorings. The motion of ships in waves is a topic in the field of naval architecture referred to as seakeeping. Thorough coverages of seakeeping are found in the writings of Korvin-Kroukovsky (1961), Newman (1977), Bhattacharyya (1978), Lloyd (1989), and Faltinsen (1990, 2005) among others. Floating bodies discussed in this chapter that are not normally under way are referred to herein as ocean engineering bodies, as opposed to ships. The geometry of an ocean engineering body normally has two vertical planes of symmetry, whereas ships have one, called the centerplane.

In this chapter, the degrees of freedom (surge, sway, heave, roll, pitch, and yaw) of a floating body are introduced and the coupled heaving and pitching motions are analyzed. The stability of a body in calm water is first discussed. Methods of motion analysis are then introduced that lend themselves to both analytical and simple numerical solutions. Body motions in waves are analyzed using the linear strip theory.

Let us begin by stating the term random is synonymous with unpredictable. A truly random phenomenon should by definition defy mathematical analysis. One might question then if random waves can be analyzed. By making certain assumptions, waves that are random in the time domain can be shown to have rather predictable properties in the frequency domain. In this chapter, a brief history of random wave analysis is first presented. This is followed by discussions and illustrations of the various statistical methods of wave analysis.

Introduction

We can safely assume that seafarers down through the ages have been aware of the randomness or unpredictability of ocean waves. The occurrence of “rogue waves” has been documented again and again. These are extremely high waves that occur in the open ocean without warning. The earliest attempts to deal mathematically with random waves were confined to averaging observed wave heights and periods. The data were obtained by visual means in a laboratory setting (as by Weber and Weber, 1825, according to St. Denis, 1969), in lakes, onboard a ship (as by Abercromby, 1888), or in coastal waters. When log-keeping came into being, wave height and period observations were recorded along with wind speeds. The more sophisticated mariners also recorded wavelength estimations obtained by comparing the wavelengths with lengths of their vessels. The accuracy of the wave height estimations from visual observations from ships is discussed by Cornish (1910).