Sequential growth factor stimulation of bone marrow stromal cells in extended culture

Silk Fibroin as a Functional Biomaterial for Tissue Engineering

Tissue engineering (TE) is the approach to combine cells with scaffold materials and appropriate growth factors to regenerate or replace damaged or degenerated tissue or organs. The scaffold material as a template for tissue formation plays the most important role in TE. Among scaffold materials, silk fibroin (SF), a natural protein with outstanding mechanical properties, biodegradability, biocompatibility, and bioresorbability has attracted significant attention for TE applications. SF is commonly dissolved into an aqueous solution and can be easily reconstructed into different material formats, including films, mats, hydrogels, and sponges via various fabrication techniques. These include spin coating, electrospinning, freeze drying, physical, and chemical crosslinking techniques. Furthermore, to facilitate fabrication of more complex SF-based scaffolds with high precision techniques including micro-patterning and bio-printing have recently been explored. This review introduces the physicochemical and mechanical properties of SF and looks into a range of SF-based scaffolds that have been recently developed. The typical TE applications of SF-based scaffolds including bone, cartilage, ligament, tendon, skin, wound healing, and tympanic membrane, will be highlighted and discussed, followed by future prospects and challenges needing to be addressed.

Anterior Cruciate Ligament: Structure, Injuries and Regenerative Treatments

Anterior cruciate ligament (ACL) is one of the most vulnerable ligaments of the knee. ACL impairment results in episodic instability, chondral and meniscal injury and early osteoarthritis. The poor self-healing capacity of ACL makes surgical treatment inevitable. Current ACL reconstructions include a substitution of torn ACL via biological grafts such as autograft, allograft. This review provides an insight of ACL structure, orientation and properties followed by comparing the performance of various constructs that have been used for ACL replacement. New approaches, undertaken to induce ACL regeneration and fabricate biomimetic scaffolds, are also discussed.

Silk fibroin in tissue engineering

Tissue engineering (TE) is a multidisciplinary field that aims at the in vitro engineering of tissues and organs by integrating science and technology of cells, materials and biochemical factors. Mimicking the natural extracellular matrix is one of the critical and challenging technological barriers, for which scaffold engineering has become a prime focus of research within the field of TE. Amongst the variety of materials tested, silk fibroin (SF) is increasingly being recognized as a promising material for scaffold fabrication. Ease of processing, excellent biocompatibility, remarkable mechanical properties and tailorable degradability of SF has been explored for fabrication of various articles such as films, porous matrices, hydrogels, nonwoven mats, etc., and has been investigated for use in various TE applications, including bone, tendon, ligament, cartilage, skin, liver, trachea, nerve, cornea, eardrum, dental, bladder, etc. The current review extensively covers the progress made in the SF-based in vitro engineering and regeneration of various human tissues and identifies opportunities for further development of this field.

Tendon tissue engineering: adipose-derived stem cell and GDF-5 mediated regeneration using electrospun matrix systems

Tendon tissue engineering with a biomaterial scaffold that mimics the tendon extracellular matrix (ECM) and is biomechanically suitable, and when combined with readily available autologous cells, may provide successful regeneration of defects in tendon. Current repair strategies using suitable autografts and freeze-dried allografts lead to a slow repair process that is sub-optimal and fails to restore function, particularly in difficult clinical situations such as zone II flexor tendon injuries of the hand. We have investigated the effect of GDF-5 on cell proliferation and gene expression by primary rat adipose-derived stem cells (ADSCs) that were cultured on a poly(DL-lactide-co-glycolide) PLAGA fiber scaffold and compared to a PLAGA 2D film scaffold. The electrospun scaffold mimics the collagen fiber bundles present in native tendon tissue, and supports the adhesion and proliferation of multipotent ADSCs. Gene expression of scleraxis, the neotendon marker, was upregulated seven- to eightfold at 1 week with GDF-5 treatment when cultured on a 3D electrospun scaffold, and was significantly higher at 2 weeks compared to 2D films with or without GDF-5 treatment. Expression of the genes that encode the major tendon ECM protein, collagen type I, was increased by fourfold starting at 1 week on treatment with 100 ng mL(-1) GDF-5, and at all time points the expression was significantly higher compared to 2D films irrespective of GDF-5 treatment. Thus stimulation with GDF-5 can modulate primary ADSCs on a PLAGA fiber scaffold to produce a soft, collagenous musculoskeletal tissue that fulfills the need for tendon regeneration.

Ligament-derived matrix stimulates a ligamentous phenotype in human adipose-derived stem cells

Human adipose stem cells (hASCs) can differentiate into a variety of phenotypes. Native extracellular matrix (e.g., demineralized bone matrix or small intestinal submucosa) can influence the growth and differentiation of stem cells. The hypothesis of this study was that a novel ligament-derived matrix (LDM) would enhance expression of a ligamentous phenotype in hASCs compared to collagen gel alone. LDM prepared using phosphate-buffered saline or 0.1% peracetic acid was mixed with collagen gel (COL) and was evaluated for its ability to induce proliferation, differentiation, and extracellular matrix synthesis in hASCs over 28 days in culture at different seeding densities (0, 0.25 x 10(6), 1 x 10(6), or 2 x 10(6) hASC/mL). Biochemical and gene expression data were analyzed using analysis of variance. Fisher's least significant difference test was used to determine differences between treatments following analysis of variance. hASCs in either LDM or COL demonstrated changes in gene expression consistent with ligament development. hASCs cultured with LDM demonstrated more dsDNA content, sulfated-glycosaminoglycan accumulation, and type I and III collagen synthesis, and released more sulfated-glycosaminoglycan and collagen into the medium compared to hASCs in COL (p <or= 0.05). Increased seeding density increased DNA content incrementally over 28 days in culture for LDM but not COL constructs (p <or= 0.05). These findings suggest that LDM can stimulate a ligament phenotype by hASCs, and may provide a novel scaffold material for ligament engineering applications.

Effect of fiber diameter and alignment of electrospun polyurethane meshes on mesenchymal progenitor cells

Effective strategies to guide cell alignment and the deposition of an oriented extracellular matrix are critical for the development of anisotropic engineered tissues suitable for the repair of ligament defects. Electrospinning is a promising means to create meshes that can align adherent cells, but the effect of fiber mesh architecture on differentiation has not been examined closely. Therefore, the goal of this study was to determine the effect of fiber diameter and the degree of fiber alignment on mesenchymal progenitor cell morphology, proliferation, and ligament gene expression. Specifically, a poly(ester urethane)urea elastomer was electrospun onto rigid supports under conditions designed to independently vary the mean fiber diameter (from 0.28 to 2.3 microm) and the degree of fiber alignment. Bone marrow stromal cells--seeded onto supported meshes--adhered to and proliferated on all surfaces. Cells assumed a more spindle-shaped morphology with increasing fiber diameter and degree of fiber alignment, and oriented parallel to fibers on aligned meshes. Expression of the ligament markers collagen 1alpha1, decorin, and tenomodulin appeared to be sensitive to fiber diameter and greatest on the smallest fibers. Concurrently, expression of the transcription factor scleraxis appeared to decrease with increasing fiber alignment. These results suggest that the formation of a ligament-like tissue on electrospun scaffolds is enhanced when the scaffolds consist of aligned submicron fibers.

Sequential biochemical and mechanical stimulation in the development of tissue-engineered ligaments

Application of stimuli in sequence to developing cultures in vitro offers the potential to intricately direct cell development and differentiation by following the template of native tissue behavior. We hypothesize that administration of mechanical stimulation at the peak of growth factor-induced cell activity will differentiate bone marrow stromal cells (BMSCs) along a fibroblast lineage and enhance in vitro ligament development through enhanced matrix ingrowth, matrix metalloproteinase-2 (MMP-2) production, collagen type I production, and extracellular matrix (ECM) alignment. BMSC-seeded silk matrices were cultured in a static growth-factor-free environment for 5 days prior to loading into bioreactor vessels to first establish an appropriate dynamic rotational regime, as determined through assessment of cell activity, histology, and surface topography. Once the regime was determined, seeded matrices initially cultured in basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), or growth-factor-free control medium for 5 days were loaded into the bioreactor for 9 days of mechanical stimulation. Our findings indicated that the sequential application of mechanical stimulation following growth factor supplemented static culture-induced cell differentiation toward a fibroblast lineage, enhancing matrix ingrowth, cell and ECM alignment, and total collagen type I produced compared to respective static cultures. The current results suggest a dynamic culturing regime in the development of engineered tissues.

The effects of GDF-5 and uniaxial strain on mesenchymal stem cells in 3-D culture

Recent endeavors in tissue engineering have attempted to identify the optimal parameters to create an artificial ligament. Both mechanical and biochemical stimulation have been used by others to independently modulate growth and differentiation, although few studies have explored their interactions. We applied previously described fabrication techniques to create a highly porous (90%-95% porosity, 212-300 microm), 3-D, bioabsorbable polymer scaffold (polycaprolactone). Scaffolds were coated with bovine collagen, and growth and differentiation factor 5 (GDF-5) was added to half of the scaffolds. Scaffolds were seeded with mesenchymal stem cells and cultured in a custom bioreactor under static or cyclic strain (10% strain, 0.33 Hz) conditions. After 48 hours, both mechanical stimulation and GDF-5 increased mRNA production of collagen I, II, and scleraxis compared to control; tenascin C production was not increased. Combining stimuli did not change gene expression; however, cellular metabolism was 1.7 times higher in scaffolds treated with both stimuli. We successfully grew a line of mesenchymal stem cells in 3-D culture, and our initial data indicate mechanical stimulation and GDF-5 influenced cellular activity and mRNA production; we did not, however, observe additive synergism with the mechanical and biological stimuli.