Cartilage Biopsy and Handling

The technology of autologous chondrocyte transplantation is based on the use of autologous cartilage for cell processing. The theoretical background to the autologous harvest and not other cell sources (e.g., bone marrow) is the fact that the joint and the articular cartilage have a unique developmental pathway in comparison to other cells with chondrogenic potential. The articular chondrocytes will have the capacity to form de novo cartilage that is halted before the hypertrophic differentiation and subsequent mineralization.

Theoretical Background

The joint consists of different anatomical structures with different cell types (i.e., bone, ligament, synovium, fibrous capsule, and articular cartilage). The cartilage provides the initial mold of the skeleton in the embryo; in the adult, cartilage provides a mechanical and structural support for breathing, articulation, locomotion, and hearing.

Formation of the Synovial Joint

The understanding of embryonic tissue formation will enable us to understand and control the processes of repair and regeneration in adult tissue. The first sign of joint formation in the embryo is the appearance of an interzone where the cells gives rise to the articular layer of the future long bones. It has been unclear whether the interzone cells derive from transdifferentiation of local prechondrocytes into interzone cells or from migration of mesenchymal cells into the joint site, or a combination.¹ The prechondrocytes in the interzone differentiate to early chondroblasts, thereafter becoming and remaining articular chondrocytes. However, recently studies have shred new light of the role of this population of cells expressing GDF5 (growth and differentiation factor 5; also known as CDMP-1 [cartilage-derived morphogenic protein 1]), a member of the transforming growth factor-beta family. The role of GDF5 expressing cells was demonstrated in ROSA-LacZ-reporter mice where GDF5 reporter expressing cells from the interzone from E13.5 were followed postnatally and remain present in structures that appear to arise from the interzone, namely the articular cartilage, synovium, and other joint tissues.²

This strengthens the hypothesis that there are two populations of cells with distinct phenotypes: the transient chondrocytes forming the epiphysis and the GDF5 expressing cells forming the articular cartilage during interzone formation. These facts support the established method of autologous cartilage biopsy as a source for chondrocytes isolation and subsequent cartilage regeneration in treated patients.

The Adult Articular Hyaline Cartilage

Two questions are often asked: if full thickness biopsies or only superficial biopsies can be taken, and if osteoarthritic cartilage or free bodies can be used.

The articular cartilage tissue is an avascular, noninnervated, and alymphatic tissue where the nutrition of the chondrocytes comes from passive diffusion. The only cell type in cartilage is the chondrocyte, constituting about 2% to 5% of the tissue. The function of chondrocytes is to build, maintain, and remodel the extracellular matrix composed of collagens, proteoglycans, noncollagenous proteins, and water. The articular cartilage has specialized load-bearing properties and the ability to withstand compressive, tensile, and shear forces because of the composition and structural integrity of its extracellular matrix (ECM), which consists of fibrillar and nonfibrillar macromolecules. The principal fibrillar component is collagen type II, which accounts for 90% to 95% of the collagen in articular cartilage; it forms a three-dimensional cross-linked network (together with smaller amounts of other minor collagens such as collagen types IX and XI). The collagen fibrils on the surface are oriented tangentially to create maximal strength, whereas deeper in the cartilage the fibrils are more vertically oriented. Collagen type X is exclusively produced by prehypertrophic and hypertrophic chondrocytes in the calcified layer.

The main nonfibrillar component consists of sulphated aggrecan monomers attached to hyaluronic acid forming large polyanionic aggregates. The collagen network is constructed in a complex three-dimensional network that acts to resist the osmotic pressure induced by the hydrophilic aggrecan monomers. If the collagen network fails, the cartilage will swell and lose its resilient properties. The small proteoglycans include decorin, biglycan, and fibromodulin. They seem not to contribute directly to the mechanical behavior of the tissue. Instead they bind to other macromolecules and probably influence cell function.

Articular cartilage also consists of noncollagenous matrix proteins, which are important for the interaction and assembly of the various macromolecules. Cartilage oligomeric protein (COMP) is a glycoprotein belonging to the thrombospondin family, also named thrombospondin 5. COMP interacts with collagen types I, II and IX, and it has been used as a diagnostic marker in serum for the progress of matrix degradation.³

Depending on matrix composition and cellular appearance, the articular cartilage is divided into several zones with different functional roles: superficial, transitional, radial, and calcified zones. Facing the joint cavity is the tangential layer/superficial zone with small, flattened cells parallel to the surface and mainly collagen type I fibers arranged tangentially to the articular surface and small amounts of proteoglycans. The surface is covered with a thin sheet of fine fibrils and cells lubricated by a thin layer of synovial fluid, sometimes called the “lamina splendens.” The production of the lubricin protein providing the frictionless surface of articular cartilage is a specific property for the cells at the surface of cartilage.⁴