Exploring the Remarkable Properties and Architecture of Articular Cartilage

In the last decade, tissue engineering has emerged as a concept for replacing or facilitating repair of worn-out and functionally compromised tissues, including cartilage. Indeed, biotech companies are working hard to design "tissue" that can be placed into osteoarthritic joints. They are focusing mainly on how cells in engineered tissues make collagen and aggrecan, which will solve part of the problem.

However, for long-term success, they will probably also need to "engineer" the architecture of the cartilage matrix, a much more difficult challenge. After all, the final structure and mechanical properties of articular cartilage are the result of eons of genetic "engineering," responding to the subtle mechanical demands that shape our bones.

Articular cartilages are remarkable tissues, critical for adult skeletal function. They protect the articulating surfaces of bones with a composite material that resists compressive skeletal loads during normal activities. They are also sculpted with smooth, congruent surfaces to match their partners on adjacent bones in articulating joints. When either of these properties is compromised, the result is impaired mobility and pain, manifest in arthritic diseases or after acute injury.

The material properties result from two macromolecules in the extensive matrix around the cells (chondrocytes). One is the protein collagen, which forms fibrillar meshworks and defines the shape and tensile properties of the cartilage. Collagen networks cannot bear weight. For example, tendons are composed almost exclusively of collagen fibrils and have excellent tensile properties, but readily bend. These properties are ideal, however, for transmitting forces between muscle and bone into joint movements.

The other macromolecule is aggrecan (see illustration). This complex macromolecule has a large core protein divided into subdomains. Aggrecan is retained within the collagen network as an aggregate (hence the name) by interaction between a binding site in the core protein and hyaluronan, a very long polysaccharide with several thousand sugar units. The strand of hyaluronan weaves into the collagen network and docks many aggrecans.

Foreground: 3-D model illustrates how a short section of aggregate, with six individual aggrecan molecules anchored to a central hyaluronan strand, would appear when fully extended in solution. Side-chain bristles radiate away from the protein cores.

Background: This electron micrograph shows a portion of a proteoglycan aggregate isolated from bovine articular cartilage. Each cigar-shaped molecule extending from a central filament represents one aggrecan molecule, with side chains collapsed onto the protein core to enhance visualization.

The central region of the aggregan macromolecule contains approximately 200 polysaccharide chains of chondroitin sulfate and keratan sulfate, each with a sugar unit motif repeated up to 100 times. Each chain contains negatively charged sulfates, which makes them highly polyanionic. The aggregan macromolecule assumes a bottlebrush structure - with the chains extended as much as possible from the core protein to minimize interactions between negative charges.

When the skeleton responds to gravity, aggrecan molecules compress by packing the chains closer, with loss of solvent inside the domain. This increases negative-charge density, and hence the repellant forces inside the macromolecule. When the load is released, aggrecan expands to the extent allowed by the collagen network.

Aggrecan molecules, like shock absorbers, are preloaded by compression to approximately one-fifth their size. Thus, they compress far less under an equivalent load than they would if fully expanded.

Cartilage, then, is a composite material reflecting its molecular organization. What can go wrong? Chondrocytes must maintain their matrix throughout an adult's lifetime if cartilage function is to be preserved.

While chondrocytes continue to synthesize and remove aggrecan, once cartilage assumes its final architecture at skeletal maturity, they typically stop synthesizing and removing the collagen network. Furthermore, chondrocytes cannot repair significant tissue damage, which in the knee can result from injury to the cruciate ligament or meniscus, and predispose individuals to osteoarthritis 10 to 15 years earlier than individuals with uninjured knees. Although chondrocytes try, by reinitiating collagen synthesis and increasing aggrecan synthesis, they somehow cannot restore optimal tissue architecture.

What is this architecture?
Until skeletal maturity, articular cartilages must serve two functions for smooth joint articulation and bone growth. They establish a columnar architecture in which chondrocytes arise by cell division at the outer edge, progress through a columnar zone while con-structing the matrix, and undergo hypertrophy and mineralize. They are then removed by invading vasculature, and the mineralized cartilage is replaced by bone.

At skeletal maturity, the cartilage stabilizes with collagen fibrils that arrange themselves in predominantly parallel fashion around columns of chondrocytes. These then "arcade" into a meshwork at the surface, parallel to the exposed cartilage in the joint, creating a strong tensile covering for the tissue. This final tissue architecture, precisely sculpted, is ideally suited in shape and strength for the loads and motions of each particular joint.

What fails in osteoarthritis?
Many factors lead to compromised function in an articular joint. Cumulative wear and tear gradually degrade aggrecan molecules. This damage can come from mechanical input, such as injury, or biological processes, such as cell death and release of proteases. If chondrocytes cannot restore aggrecan successfully, the cartilage will become more susceptible to functional demands placed upon it.

At a critical point, chondrocytes can progress from normal maintenance functions to repair functions, manifest by cell division (cloning) and reinitiation of collagen synthesis. However, they apparently cannot reconstruct normal architecture, and eventual degradation of the matrix exceeds repair with net loss of tissue, bone rubbing on bone, and pain that brings patients to their orthopaedic specialist.