Page 389 - Advances in Biomechanics and Tissue Regeneration

386 19. IMPACT OF MECHANOBIOLOGICAL PERTURBATION IN CARTILAGE TISSUE ENGINEERING

the simultaneous application of TGFβ and mechanical loading inhibits chondrogenesis [107, 108]. Delayed application
of dynamic loading provided after a preculture period of chondrogenic induction was instead shown to be beneficial
for MSC chondrogenesis [109, 110]. The formation of sufficient pericellular matrix (PCM) by chondro-differentiating
MSCs was speculated to be an important factor in determining MSCs’ response to compression. PCM is both biochem-
ically and biomechanically distinct from the surrounding ECM that can influence the stress-strain microenvironment
of the cells and serves as a direct transducer of biochemical and biomechanical signals to the cells [111]. MSCs seeded in
the stiffer hydrogels, with increased PCM formation, displayed a more prochondrogenic response to the application of
DC [112] and hydrostatic pressure [113], compared with those seeded in softer hydrogels with lower level of PCM
formation. Cytoskeletal organization and focal adhesion formation were observed to be altered in the stiff gels relative
to the soft gels in which cell-matrix interaction via integrin could have played a critical role in distributing and trans-
mitting the exogenous mechanical signal [112, 114].
Stabilization of chondrogenic phenotype by DC [110, 115] and HP [116] has also been reported. Inhibition of hyper-
trophic development by mechanical compression correlated with the outcomes of an in vivo study involving ortho-
topic transplantation of MSC, wherein collagen type X–positive chondrocytes were found only at the osteochondral
interface [117]. Study with elastomeric poly-L-lactide-co-ε-caprolactone (PLCL) scaffolds suggested that compression
led to hypertrophic development (Zhang et al., 2015). However, long-term dynamic loading has also been shown to
induce hypertrophy development [108]. These discrepancies in compression associated-hypertrophy development
could depend on the loading strain, frequency, and also the existence of pericellular matrix deposition to provide prox-
imal matrix-cell interaction that is essential for mechanotransduction [113].

19.4.2 Shear Stress
Apart from compression loading, articular cartilage experiences mechanical shear deformation at the tissue surface
[118, 119]. The combined interplay between load and articular oscillation shearing and their relationship to chondro-
cytes and MSC responses were explored with the employment of multiaxial bioreactor, which superimposed surface
shearing on cyclic axial compression, mimicking in vivo articulation [120]. Adding a component of shear to compres-
sive loading was shown to be superior to when only compressional loading was provided at inducing matrix biosyn-
thesis in chondrocytes. Specifically, expression of the superficial zone–specific PRG4 was induced [72, 119].The
application of shear superimposed upon dynamic compression also led to significant increases in MSC chondrogenic
gene expression, including PRG4, resulting in a tissue of improved tensile modulus [102, 121, 122], with the phenotype
of the derived neocartilage tissue dependent on the frequency and amplitude of the compression and shear stress.

19.4.3 Fluid Flow

With prolonged and/or higher loading magnitude, interstitial fluid within the extracellular matrix is eventually
exuded, generating secondary biophysical signals in the form of fluid shear stress and changes in osmolarity across
the cellular membrane. Fluid flow in response to joint loading is complex and challenging to recapitulate for isolated
chondrocytes in in vitro three-dimensional cultures. Controlled medium flow has been used to culture chondrocyte-
seeded constructs, as culture with dynamic fluid flow provides several advantages over static culture including
enhanced mass transport, improved nutrient delivery, and suitable hydrodynamic stimuli. Perfusion and rotating wall
bioreactors have been shown to enhance extracellular matrix accumulation by chondrocytes seeded in porous poly-
meric constructs [123–125]. Perfusion of chondrocytes induced superficial zone–specific phenotype [126, 127] that was
dependent on intracellular calcium elevation [126, 128]. Perfusion and oscillating bioreactors have similarly been
found to enhance the biochemical and functional properties of MSC-seeded scaffolds in TGFβ-supplemented chondro-
genic medium [129–131]. However, the effect of fluid flow is dependent on flow rate and the porosity of the tissue
construct. Mechanical signaling induced by fluid flow likely acts through calcium response that is dependent
on the fluid flow rate [128]. At high fluid flow rate, reduced chondrogenesis with decreased GAG and diminished
Col II was reported [132]. Shear stress as a result of increased fluid flow has also been associated with increased hyper-
trophy [133].

19.4.4 Mechanotransduction of External Mechanical Stimulation
A number of key transduction mechanisms have been identified that facilitate the mechanically driven enhance-
ment of cartilage ECM biosynthesis and functional properties, including mechanosensitive ion channels [7] and

II. MECHANOBIOLOGY AND TISSUE REGENERATION