Mechanical regulation of chondrogenesis

Abstract

Mechanical factors play a crucial role in the development of articular cartilage in vivo. In this regard, tissue engineers have sought to leverage native mechanotransduction pathways to enhance in vitro stem cell-based cartilage repair strategies. However, a thorough understanding of how individual mechanical factors influence stem cell fate is needed to predictably and effectively utilize this strategy of mechanically-induced chondrogenesis. This article summarizes some of the latest findings on mechanically stimulated chondrogenesis, highlighting several new areas of interest, such as the effects of mechanical stimulation on matrix maintenance and terminal differentiation, as well as the use of multifactorial bioreactors. Additionally, the roles of individual biophysical factors, such as hydrostatic or osmotic pressure, are examined in light of their potential to induce mesenchymal stem cell chondrogenesis. An improved understanding of biomechanically-driven tissue development and maturation of stem cell-based cartilage replacements will hopefully lead to the development of cell-based therapies for cartilage degeneration and disease.

Figure 1

Delayed dynamic compressive loading improves mechanical properties and extracellular matrix distribution without increasing biochemical content. Following 3 weeks of chondrogenic preculture, dynamic compressive loading was applied daily to human mesenchymal stem cell (MSC)–agarose constructs for 3 weeks. (A) The equilibrium modulus of MSC-seeded constructs was higher in medium containing transforming growth factor beta (TGFβ; CM+) compared with culture without TGFβ (CM–) at 3 and 6 weeks; dynamic loading (DL) in CM + for 3 weeks further improved mechanical properties. (B) Biochemical content of dynamically loaded constructs at week 6 was not different compared with CM + controls. (C) to (E) Alcian Blue staining at week 6 showed equal distribution of proteoglycans between CM + controls and loaded constructs with weak staining in CM– controls. (F) to (H) Picrosirius Red staining and (I) to (K) collagen type II immunostaining showed more homogeneous distribution of collagen in loaded constructs compared with controls. Scale bar: 100 μm. *Greater than CM– controls (P <0.05). **Greater than CM + controls (P <0.05). FS, free swelling. Reproduced from [19] with kind permission from eCM journal [32].

Figure 2

Mechanical loading using a multimodal bioreactor enhances mesenchymal stem cell chondrogenesis. (A) The bioreactor can apply both compression and shear to the cell-seeded construct through rotation of the ceramic hip ball in contact with the surface of the construct and through vertical movement of the ball perpendicular to the construct surface. Relative (B) Sox9 and (C) Col2 mRNA expression of human mesenchymal stem cells after culture for 21 days in fibrin/polyurethane constructs without exogenous growth factors. Although either compression or shear loading alone increased these chondrogenic markers above free swelling levels, the combination of shear and compression loading further enhanced the response. ^#P <0.05, ^##P <0.01, ^###P <0.001. Reproduced from [22] with kind permission from eCM journal [32].

Figure 3

Mechanisms of mechanically-induced chondrogenesis. Joint loading produces complex tissue strains, which lead to direct cellular and nuclear deformation, and generates indirect biophysical factors, including osmotic and hydrostatic pressure and fluid flow. Mechanical loading of isolated chondrocytes or mesenchymal stem cells (MSCs) seeded into hydrogels or polymeric scaffolds may recapitulate many of the changes that occur in native cartilage. Candidate mechanical signal transducers in chondrocytes and MSCs include ion channels, the primary cilium, the nucleus, and the cytoskeleton.

Figure 4

Culture in an oscillating bioreactor enhances tissue mechanical properties and collagen content. (A) Aggregate modulus and (B) total collagen content in human mesenchymal stem cell–poly(ϵ-caprolactone) (hMSC-PCL) constructs. *Significant difference due to scaffold structure (P < 0.05); **Significant difference due to culture vessel (P < 0.05). (C), (D) Histological (top) and immunohistological (bottom) appearance of day 21 hMSC-PCL constructs cultured (C) statically or (D) in a bioreactor. Tissue sections were stained for safranin-O (top, scale bar: 20 μm) and double immunostained (bottom, cellular DNA counter-stained, scale bar: 100 μm) for collagen I (red, not seen) and collagen II (green). GAG, glycosaminoglycan. Adapted from [80].