Engineering custom-designed osteochondral tissue grafts

Abstract

Tissue engineering is expected to help us outlive the failure of our organs by enabling the creation of tissue substitutes capable of fully restoring the original tissue function. Degenerative joint disease, which affects one-fifth of the US population and is the country's leading cause of disability, drives current research of actively growing, functional tissue grafts for joint repair. Toward this goal, living cells are used in conjunction with biomaterial scaffolds (serving as instructive templates for tissue development) and bioreactors (providing environmental control and molecular and physical regulatory signals). In this review, we discuss the requirements for engineering customized, anatomically-shaped, stratified grafts for joint repair and the challenges of designing these grafts to provide immediate functionality (load bearing, structural support) and long-term regeneration (maturation, integration, remodeling).

Figure I

Native cartilage and a customized graft. Image of native tissues reproduced, with permission, from [69].

Figure 1

Key steps in custom-designing osteochondral grafts. One approach to replace terminally damaged joint tissues is to use custom-designed grafts with high structural and functional fidelity for a large region or an entire joint surface. The example shown here is for engineering the temporomandibular joint (TMJ) condyle, the only articulating joint in the head, but the same approach could, in theory, be extended to other joints in the body. A clinical image is used first to obtain the exact graft geometry for the manufacture of an anatomically shaped biomaterial scaffold and culturing system. Cells are obtained from the patient and expanded in culture to form a cellularized construct. Scaffold seeded with cells is cultured in a bioreactor capable of providing the multi-parametric signals needed for graft development and maturation. Upon achieving targeted properties, the graft is implanted into the patient, where it will support the joint function and remodel, integrate and mature.

Figure 2

Potential of human mesenchymal stem cells to form osteochondral grafts. hMSCs can be isolated from the patient’s tissue, expanded in culture and used to form osteochondral composites. In one notable example, hMSCs were chondrogenically induced in pellets (medium supplemented with dexamethasone, L-ascorbic acid and transforming growth factor-β) and press-coated on porous polylactic blocks [30]. Confluent monolayers of the same cells were osteogenically induced (culture medium supplemented with dexamethasone, L-ascorbic acid and β-glycerophosphate) and seeded into the porous regions of the scaffolds from the opposite side. (a) Osteochondral composites with a 2 mm thick cartilaginous layer (CL) integrated with the underlying osteogenic layer (OL) were formed by cultivation in medium containing both chondrogenic and osteogenic factors. Scale bar represents 1.5 mm. (b) Deposition of cartilaginous matrix (blue glycosaminoglycans stain) was limited to the CL. Arrows indicate the boundary between CL and OL. (c) Mineralized matrix (red stain) was limited to the osteogenic layer. Images were reproduced with permission from [30].

Figure 3

Scaffold design: composite biomaterials for complex grafts. (a) Composite scaffold for cartilage tissue engineering made by microscale weaving of polycaprolactone fibers. Top: a schematic of the weave; bottom left: surface view of the weave by scanning electron microscopy (scale bar represents 500 μm); bottom right: fluorescence image of articular chondrocyte seeded weave within a 2% agarose gel, labeled with calcein-AM (scale bar represents 100 μm) (reproduced, with permission, from [32]). (b) Scaffolds for ligament and tendon engineering that were formed through the use of silk fibroin yarns in hierarchically organized forms of bundles, strands and cords. Top: a schematic of the yarn organization; bottom arrows show the twisting direction of the yarns at each level of organization (reproduced, with permission, from [43]). (c) Electrospun polycaprolactone fibres were used to generate scaffolds for engineering an intervertebral disc. Insert: this shows sections that were cut from mats of electrospun fibers to provide different fiber orientations within the sample for mechanical testing; graph: predictions of mechanical properties (modulus) versus experimental data from the samples of electropsun mats, where the modulus of the material was related to the fiber orientation. These relationships are important in terms of replicating the fiber orientations found in tissue structures, such as the intervertebral disc, to provide sufficient resistance to mechanical compression. (reproduced, with permission, from [33]). (d,e) Examples of key component parts for osteochondral systems. (d) Hydrogels for cartilage – the samples shown were generated from silk fibroin protein by controlling water content during self-assembly into gel states. (e) Porous sponges formed by porogen leaching from silk fibroin after solidification; these sponges are used for bone formation. This system can be pre-mineralized as needed to alter initial mechanical properties.

Figure 4

Engineering anatomically shaped tissue grafts. (a) Anatomically shaped osteochondral patella formed with a cartilage layer over unseeded trabecular bone substrate (reproduced, with permission, from [55]). (b) Digital image of a patella formed by using a two-part mold. The upper part has the ‘negative’ shape of the patella and the lower part has a ‘positive’ geometry. The spacing between these parts can be used to control the thickness of the cartilage layer (shown in white) (reproduced, with permission, from [55]). (c) Precisely shaped scaffold machined from decellularized trabecular bone using digital imaging data for a human temporomandibular joint (TMJ). (d) Schematic of the bioreactor with spatial control of lineage-specific stimuli to an anatomically shaped graft with cartilage and bone regions interpenetrating to form an ‘interface’. The system can provide dynamic loading of the cartilage layer (red arrows) and shear stress (via medium perfusion) to the bone region (blue arrows).