The first expandable human embryonic stem cell (hESCs) culture, was successfully derived from the inner cell mass of blastocysts in 1998 [41] and represents a potentially unlimited source of cells for regenerative medicine and tissue engineering strategies. These cells maintain their undifferentiated state for at least 80 passages in vitro when grown using current published protocols [41,42]. They can be differentiated into cells from all three embryonic germ layers: (i) ectoderm: skin and neurons [43,44,45,46]; (ii) mesoderm: blood, cardiac cells, cartilage, endothelial cells, and muscle [47,48,49]; (iii) endoderm: pancreatic cells [50,51,52]. Interestingly, while ESCs, can form teratomas in vivo, in vitro generate embryoid bodies, which are cell aggregations that contain all three embryonic germ layers [53,54]. Cyclin-dependent kinase 1 (Cdk1) is indispensable for the early development of the embryos. Cdk1 expression is tightly correlated with the undifferentiated state of ES cells by maintaining the unique undifferentiated and self-renewing state of mouse ES cells [55], whereas Cdk1 has a crucial role in orchestrating a fine balance between cellular proliferation, cell death and DNA repair in hESCs [56]. Recently, it was shown that the transition of ES cell differentiation from the epiblast state into neuroectodermal progenitors specifically depends on the expression and activator functionality of Zfp521 [57].

Adult stem cells (ASCs) are multipotent stem cells that, under controlled conditions, may differentiate into various cells in vitro and in vivo [58,59,60,61,62]. ASCs have been isolated from bone marrow, cord blood, skeletal muscle, brain, cornea, tooth and skin among other tissues [63,64,65,66]. ASCs can self-renew and undergo multipotential differentiation, however they show a more restricted differentiation potential compared to ESCs [67,68,69,70]. The main function of ASCs, within the body, is their involvement in tissue repopulation under physiological and pathological conditions [71,72]. Cell-fate decisions in the developing embryo are orchestrated by a complex balance between cell-autonomous signals and stimuli from the surrounding micro-environment. Within the stem cells niche these processes control the birth and maturation of stem cells that replenish mature cells in adult tissues [73,74,75]. For instance, satellite cells are considered the main progenitors of adult skeletal muscle and present several stem cell properties [76]. Recently, d'Aquino et al. [77] isolated human neural crest derived postnatal cells, from the dental follicle, that exhibit remarkable embryonic features both in vitro and in vivo.

Notably, ASCs respond to microenvironment changes which, in turn, may alter their fate [73,74,75], suggesting ASCs useful for regenerative medicine applications. In this regard, mesenchymal stem cells (MSCs) remain the most promising type of adult stem cells for regenerative medicine in cell therapy and tissue engineering. Their most common sources are bone marrow, fat, amniotic fluid, amniotic membrane, and umbilical cord matrix [78,79,80].

Potential clinical applications of ES cells raise many practical and ethical concerns. In this regard, a major breakthrough was achieved in 2006, when it was shown that pluripotent stem cells could be obtained by transducing either mouse embryonic or adult fibroblasts with a limited set of specific transcription factors [81]. These reprogrammed cells, named induced pluripotent stem (iPS) cells, resembled ES cells in many of their characteristics. To date, iPS cells have been generated from cells of several species using different sets of reprogramming factors. For instance, reprogramming blood cells to iPSCs provides a novel tool for modeling blood diseases in vitro. Hu and co-workers [82] demonstrated that iPSCs free of transgene and vector sequences could be efficiently generated using non-integrating episomal vectors from human bone marrow and cord blood mononuclear cells of healthy donors and the bone marrow of a patient with chronic myeloid leukemia. This approach provides an opportunity to explore normal and diseased cord blood and bone marrow samples without any limitations associated with virus-based methods [82]. Moreover, somatic cells reprogramming to iPSCs can also be achieved using poly(β-amino ester)s as the transfection reagent for the delivery of a single CAG-driven polycistronic plasmid expressing Oct4, Sox2, Klf4, c-Myc and a GFP reporter gene (OSKMG) [83].

However, many questions regarding the molecular process of induced reprogramming remain unanswered and need to be addressed before iPS cells can be employed in the clinics. In fact, iPSCs cell-line-specific differences and the mechanisms regulating pluripotency must be better understood. Despite the common ability of hiPSCs and hESCs to differentiate into all 3 germ layers, their functional equivalence at the single cell level remains to be demonstrated. Comparison between single hESCs and single hiPSCs have indeed revealed a much higher heterogeneity in gene expression levels in affecting hiPSCs, suggesting that these cells feature an alternate, less stable pluripotent state [84,85].

Currently, patient-specific iPSCs are extensively studied for translational research applications. Yazawa and co-workers generated iPSCs from fibroblasts from Timothy syndrome patients, and differentiated these cells into cardiomyocytes. This study provides new opportunities for studying the molecular and cellular mechanisms of cardiac arhythmias in humans, and provides a robust assay for developing new drugs to treat these diseases [86].

Recently, it was reported the generation of iPS cells from peripheral blood CD34⁺ cells of two patients with myeloproliferative disorders (MPDs). The MPD-derived iPS cells, despite unchanged phenotypes, karyotype, and pluripotency, showed increased erythropoiesis and recapitulated features of primary CD34⁺ cells of the corresponding patient from whom the iPS cells were derived. These iPS cells provide a renewable cell source and a prospective hematopoiesis model for investigating MPD pathogenesis [87].

A comprehensive report published by Park et al. in 2008 [88] showed for the first time the feasibility of generating iPS cells from fibroblasts of patients with complex genetic disorders including Huntington and Parkinson disease, diabetes mellitus and Down syndrome [88]. Another example is spinal muscular atrophy (SMA), a neuromuscular disorder caused by mutations in the SMN1 gene that result in the degeneration of selected motorneurons. iPS cells established from a patient suffering SMA (iPS-SMA) maintain the disease phenotype and are capable of differentiating into motorneurons initially, however, these cells degenerate with time, unlike their counterpart derived from the patient's healthy mother [89,90].

Integrins, acting as mechanosensors alone or in concert with cytoskeletal proteins, are one of the major components involved in mechanotransduction [116,117,118,119,120]. Integrins, of adherent cells (i.e., MSCs), are considered direct mechanosensors that physically connect the ECM to the cytoskeleton [121], thus acting as signaling receptors. More in particular, cell integrins bind the ECM externally, while linking the focal adhesion to actin cytoskeleton [122]. Moreover, mechanical forces promote the assembly of focal adhesion spots (FAs), by altering the relative positions of specific internally FAs components (such as vinculin and fibronectin) and their conformations, and trigger intergrin-dependent signaling and MAPKs activation [92,122,123,124].

Tension transmission to the nucleus is due to the actin cytoskeleton, and those tensional forces along the actomyosin contractile system are also regulated by the degree of phosphorylation of the myosin light chain [125,126]. In fact, as demonstrated by Chrzanowska-Wodnicka and Burridge in 1996 [127], the activation of RhoA controls the development of FAs and stress fibers, whereas the influence of RhoA on actin cytoskeleton is mediated by its downstream effector ROCK, which inactivate myosin phosphatase, thereby on inducing the stabilization of filamentous actin and the stress fiber formation [128]. The remodeling of the microfilament and microtubule networks occur as consequence of mechanical stretching, while the prevention of cytoskeletal remodeling mitigates stretch-induced increase in gene transfer and expression [129], thereby eliciting “local” cellular events. A model of cell structure suggests that this dynamic remodeling of the cytoskeleton is also a ‘hard-wired’ tensegrity [130]. This model takes into account the observation that individual cytoskeletal filaments can bear significant tensile and compressive loads in living cells because their structural integrity is maintained for longer than the turnover time of individual protein monomers [131,132,133]. In particular mechanical forces applied to the cell surface mainly involve integrins and cadherins that are physically coupled to cytoskeletal filament networks and, in turn, are linked to nuclear scaffolds, nucleoli, chromatin and DNA inside the nucleus [134,135]. This raises the intriguing possibility that mechanical forces applied the cell surface might act by promoting mechanochemical conversion in the nucleus [120], in addition to mechanochemical transduction in the cytoplasm [136].

Although the molecular mechanism by which a mechanical stimulus is translated into a chemical response in biological systems is still unclear, the mechanical stretching of single cytoplasmic proteins is known to activate binding of other molecules. For example, the application of physiologically relevant forces cause stretching of single talin rods that expose cryptic binding sites for vinculin. Thus, in the talin-vinculin system, molecular mechanotransduction can occur by protein binding after exposure of buried binding sites in the talin-vinculin system. Such protein stretching may be a more general mechanism for force transduction [94].

Compressive stimulation increases the level of phosphorylated focal adhesion kinase (FAK) and prostaglandin E(2) production. The FAK-integrin complex plays a role in mechanoreception and mechanotransduction in human periodontal ligament cells [137]. In these, it was demonstrated that strain-dependent mechano-/signal-transduction also involves MAP-kinases p42/44, and p38 stress kinase in conjunction with the amount of MMP-13, and integrin subunits beta1 and beta3 [138].

Mammographically dense breast tissue is one of the greatest risk factors for developing breast carcinoma, but the molecular mechanisms still remain largely unknown. Recently, it was proposed that chronically elevated signaling loop (FA-FAK-Rho) is necessary to generate and maintain the invasive phenotype. Moreover, this signaling network resulted in hyperactivation of the Ras-mitogen-activated protein kinase (MAPK) pathway, which activated a clinically relevant proliferation signature that predicts patient outcome. In this scenario, these findings provide compelling evidence of the importance of mechanical features of the microenvironment, and suggest that mechanotransduction in these cells occurs through a FAK-Rho-ERK signaling network with extra cellular signal-regulated kinase (ERK) as a bottleneck through which much of the response to mechanical stimuli is regulated [139].

Consequences of mechanical forces applied to cell surface are changes in Ca²⁺ influx through stretch-activated channels [140]. This alteration in the Ca²⁺ influx could result in intracellular activation of several molecules, such as NF-κB, cAMP-response element binding protein (CREB), membrane kinases and EGFR, leading to the activation of MAPK signaling pathway [141,142,143].

Both mechano-responsive elements (adhesion-dependent and ion channel-based mechanism) are linked via a motor protein (such as myosin II) to the cytoskeleton inside the cell and to an extra cellular anchor (usually the ECM).

Mechanotransduction could also act at the cell membrane level, through the involvement of G-protein-coupled receptors (GPCRs) [144,145,146].

Mechanical forces with a clear direction (such as the circumferential stretch of the arterial tree) cause only transient molecular signaling of pro-inflammatory and proliferative pathways, which become down-regulated when such directed mechanical forces are sustained. In contrast, mechanical forces without a definitive direction (e.g., disturbed flow and relatively undirected stretch) cause sustained molecular signaling of pro-inflammatory and proliferative pathways. The vascular endothelial cells (EC) responses to directed mechanical stimuli involve the remodeling of EC structure to minimize alterations in intracellular stress/strain and elicit adaptive changes in EC signaling as a result of sustained stimuli; these cellular events constitute a feedback control mechanism to maintain vascular homeostasis and are atheroprotective. Such a feedback mechanism does not operate effectively in regions of complex geometry, where the mechanical stimuli do not have clear directions, thus placing these areas at risk for atherogenesis [92,147].

Mechanical forces associated with blood flow are determinants of vascular morphogenesis and physiology. Recent data highlighted the endothelial mechanotransducers that might mediate responses to blood flow, the effects of atheroprotective rather than atherogenic flow, the mechanisms that contribute to the progression of the disease and how systemic factors interact with flow patterns to cause atherosclerosis [148].

The immunoglobulin family receptor platelet endothelial cell adhesion molecule (PECAM)-1, vascular endothelial cell cadherin (VE-cadherin) and vascular endothelial growth factor receptor 2 (VEGFR2) comprise a mechanosensory complex capable of conferring responsiveness to flow in heterologous cells. In support of the relevance of this pathway in vivo, PECAM-1-knockout mice do not activate NF-κB and downstream inflammatory genes in regions of disturbed flow. Therefore, this mechanosensing pathway is required for the earliest events associated with the development of atherogenesis [149].

Another example is reported in the study by Shi and collaborators [150] that proposed a conceptual mechanotransduction model for interstitial flow, wherein cell surface glycocalyx HSPGs, in the presence of integrin-mediated cell-matrix adhesions and cytoskeleton organization, sense interstitial flow and activate the FAK-ERK signaling axis, leading to upregulation of MMP expression and cell motility in 3D [150].

The mechanotransduction-induced EC adaptive processes in the straight part of the aorta represent a case of the “Wisdom of the Cell”, as a part of the more general concept of the “Wisdom of the Body” raised by Cannon, regarding the maintenance of cellular homeostasis in the presence of external perturbations [147].

Additionally, a new role for caveolae as a physiological membrane reservoir that quickly accommodates sudden and acute mechanical stresses has been recently proposed by Sinha and co-workers [151]. Acute mechanical stress induced by osmotic swelling or by uniaxial stretching results in a rapid disappearance of caveolae, a reduced caveolin/Cavin1 interaction, and an increase of free caveolins on the plasma membrane. The absences of a functional caveola reservoir in myotubes from muscular dystrophic patients enhance membrane fragility under mechanical stress [151]. Mechanical forces are also critical for fetal lung development, as showed by Wang and co-workers, who demonstrated that Caveolin-1 is present in E19 fetal type II epithelial cells, and is translocated from the plasma membrane to the cytoplasm by mechanical stretch and functions as an inhibitory protein in stretch-induced type II cell differentiation via the ERK pathway [152].

The normal translocation of beta-catenin to the nucleus in osteoblasts that is induced by oscillatory fluid shear stress (OFSS) is enhanced when the nucleocytoplasmic shuttling Nmp4/CIZ (transcription factor nuclear matrix protein-4/cas interacting zinc finger protein) is absent. Furthermore, other aspects of OFSS-induced mechanotransduction, generally associated with the beta-catenin signaling pathway, including ERK, Akt, and GSK3beta activity, as well as expression of the beta-catenin-responsive protein cyclin D1, are also enhanced in cells lacking Nmp4/CIZ. Finally, in the absence of Nmp4/CIZ, OFSS-induced cytoskeletal reorganization and the formation of focal adhesions between osteoblasts and the extra cellular substrate is qualitatively enhanced, suggesting that Nmp4/CIZ may reduce the sensitivity of bone cells to mechanical stimuli. Together these results support the notion that Nmp4/CIZ plays an inhibitory role in the response of bone cells to mechanical stimulation induced by OFSS [153].

Recent findings indicate a stimulating role of Wnt signaling in bone mechanotransduction. In fact, Jansen and co-worker [154] demonstrated a biphasic effect of mechanical loading on beta-catenin in mineralizing human differentiating osteoblasts independent on the ERK pathway. Moreover, the authors hypothesized that the biphasic aspect of Wnt/beta-catenin signaling with a strong decrease up to 40 h after the stretch induction, is important for the anabolic effects of mechanical stretch on bone [154].

Other finding highlighted the involvement of nitric oxide, focal adhesion kinase, and the phosphatidyl inositol-3 kinase/Akt signaling pathway in beta-catenin pathway activation. Authors found that mechanical stimulation by pulsating fluid flow (PFF) induced beta-catenin stabilization and activation of the Wnt/beta-catenin signaling pathway. This stabilization of beta-catenin and activation of the beta-catenin signaling pathway PFF-induced was abolished by adding focal kinase inhibitor FAK inhibitor-14, or phosphatidyl inositol-3 kinase inhibitor LY-294002. Addition of nitric oxide synthase inhibitor L-NAME also abolished PFF-induced stabilization of beta-catenin This suggests that mechanical loading activates the beta-catenin signaling pathway by a mechanism involving nitric oxide, focal adhesion kinase, and the Akt signaling pathway [155].

Liedert and co-workers [156] investigated regulatory mechanisms by which mechanical loading exerts its role in bone mass homeostasis [156]. They demonstrated that estradiol (E2) had a sensitizing effect on mechanically induced cyclooxygenase-2 (Cox-2) expression, which seemed to be ligand-specific in that was abolished by using the anti-estrogen ICI182, 780. However, mechanical strain in the presence of Wnt signaling activators decreased both the E2 sensitizing effect and the stimulatory effect of Wnt signaling in the absence of strain [154,156].