Mesenchymal Cell Activation by Biomechanical Stimulation and its Clinical Prospects

Eukaryotic cells contain specific structural segments that determine the cellular biomechanical characteristics, with the aid of numerous molecular structures. Cell mechanics is mostly determined by the cytoskeleton dynamics in coordination with the extra- and intra-cellular milieu. Cells can sense mechanical forces and, by signalling system, convert them to biological response through mechanotransduction pathways. Cells of mesenchymal origin are specially sensitive and responsive to mechanical forces because they are involved in building of the biomechanically efficient tissues for force propagation.

The metabolic response of mesenchymal cells in mechanically active environments depends on the interactions between fluid flow, intra and/or extracellular, with the cytoskeletal components, especially microfilaments. Therefore, in vitro research of cellular mechanotransduction requires experimental methods that should include two major components: cultured cells adherent to firm surface that are exposed to controlled mechanical stimulation. The most commonly used mechanical models for the in vitro study of cellular responses to mechanical forces at low frequencies (below 5 Hz) utilize controlled cyclic stretching, when attached to elastic membranes, and exposure of cell cultures to a controlled fluid flow. For higher frequencies of mechanical stimulation (in the infrasonic range) application of external vibration is a preferred method. Methods of mechanical stimulation of mesenchymal cells in vitro are based on their ability to adhere to plastic surfaces. Therefore the anchorage of these cells to a surface provides the possibility to transfer mechanical force from the solid out-surface into the cells. Most of the mesenchymal cell types are metabolically sensitive to the externally applied mechanical force. Osteoblasts are the best example for cellular sensitivity to mechanical stimulation therefore we describe the methods for mechanical stimulation of these cells, but the same methods are also applicable to other mesenchymal cells, such as chondroblasts, fibroblasts, mesenchymal stem cells, etc.

Stem cells’ research reached an advanced stage with clear understanding of the ability of multipotent mesenchymal stem cells to maturate to different metabolically mature cells of the connective tissue. In combination with advancement in genetic research it is now possible to start decoding basic cellular mechanotransduction pathways in stem cells. This novel research field has the potential to lead to the discovery of new and important niche in the biomedicine with a prospective of reaching important clinical advancements. Although important research success has been reached recently the exact mechanisms of the MSC’s determination of the fate are still elusive and require further extensive research efforts. In order to reveal the basic mechanisms of stem cell differentiation two distinct areas of investigation are mostly considered – the biochemical perspective and the physical or biomechanical perspective. In dealing with the biomechanical prospective, an attempt to mimic the mechanically active milieu for the MSCs was introduced using different research methods (see Chapter 2). Because mesenchymal derived cells differentiate into tissues that usually have mechanically supportive function, it is only logical to hypothesize that mechanical stimuli might cause changes on the cellular level. In this chapter we review the effects of mechanical stimulation of mesenchymal stem cells (MSCs) according to different features and parameters of the stimulus: shear stress applied to MSCs by fluid, micro-environmental scale cellular mechanical stimuli, substrate stiffness, cell shape and surface topography effects, the genomic level control of external mechanical force, micro-environmental nano-fibres’ stretch and reorientation with the directional effect of the mechanical stimulation. Clearly the understanding of the numerous mechanical effects with different mechanical parameters of the stimuli that are applied to MSCs has been advanced recently but still requires further extensive and thorough research in order to manipulate these cells biomechanically for a clinical use.

The fibroblast is the most common cell of connective tissue. Fibroblast synthesizes the extracellular matrix and has a crucial role in wound healing. Fibroblast has different morphological characteristics according to their anatomical location and according to their metabolic activities. Although this unremarkable from their shape, when implanted ectopically these cells keep their original tissue characteristics at least over a few generations. Fibroblasts synthesize GAG (glycosaminglycan), different types of collagen, glycoprotein and other fibrous proteins. Additionally they secrete various growth factors, including thymic stromal lymphopoietin (TSLP). Fibroblasts generate the extracellular fibrous matrix as part of their metabolic secretion activity as part of the repair process following tissue damage.

This chapter is divided into two sections i.e. the biomechanical, histomorphological and structural properties of the skin and its behaviour following biomechanical stimulation and the discussion on the cellular mechanotransduction, mechanosensores and mechanosignaling pathways in the fibroblast.

Skeletal system is highly capable to adapt and respond to various mechanical stimuli. The mechanisms for osteoblasts’ differentiation and their cross-talk with osteocytes and osteoclasts are under extensive investigation but the exact interactions of cellular pathways are not fully understood. The mechanisms of osteocytes’ response to the different aspects of a mechanical stimulation remain unclear and so are the mechanisms by which osteoblasts co-ordinate the different intracellular signaling. Further investigation of the molecular principles of mechnotransduction is needed in order to exploit the value of mechanically oriented therapeutics. Therapeutic methods that utilize the use of cellular mechanical stimulation of osteoblasts might provide a non-invasive approach for bone regeneration in osteodegenerative conditions and following traumatic bone damage.

Mechanical load is the cardinal factor in regulating the metabolic activity of chondrocytes. Chondrocytes are sensitive to load pattern and can adjust their metabolism according to the magnitude and frequency of mechanical stress. Quantity and quality characterization of the chondrocyte response to stress is yet to be revealed and may lead to novel therapeutic methods in preventing and treatment of cartilage degeneration diseases.

The process of in vivo bone generation following mechanical stimulation of cells by external distraction force is defined as “distraction osteogenesis”. The events in bone formation and neovascularization in membranous craniofacial bones during the distraction osteogenesis are presented in this chapter.

When distraction force is applied, cellular differentiation in pluripotent tissue, via specific biochemical pathways, is activated with subsequential osteogenesis.

Multifactorial interrelations between osteoblasts and other bone cells occur at the bone forming boundaries. There are several cytokines and growth factors that regulate bone generation and resorption while distraction osteogenesis.

External distraction induces formation of a pool of progenitor cells that dedicated to osteoblastogenesis and local vascularisation.

Recognition of the molecular cellular pathways of membranous distraction osteogenesis is important for the further design of efficient clinical methods for therapeutic bone regeneration.