J Oral Maxillofac Surg 53:924-929, 1995

Normal Cartilage Structure, Biochemistry, and Metabolism:

A Review of the Literature

LEONORE C. DIJKGRAAF, DDS,* LAMBERT G.M. DE BONT, DDS, PHD,1- GEERT BOERING, DDS, PHD,:I: AND ROBERT S.B. LIEM, PHD§

Purpose: To understand the possible significance of the presence of prote- ases, cytokines, growth factors, and arachidonic acid metabolites in the osteo- arthritic temporomandibular joint (TMJ), a review of the normal physiologic processes and participating factors in the normal TMJ is established, based on knowledge of structure, biochemistry and metabolism of normal cartilage in general.

Osteoarthritis (OA) is a slowly progressive, usually monoarticular disorder, that is inherently noninflam- matory. ~'2 It is characterized by progressive degrada- tion of the components of the extracellular matrix (ECM) of articular cartilage. 3 Initially, research of OA was confined to histopathologic and electron micro- scopic changes in human osteoarthritic cartilage. Dur- ing the last decade, however, research of OA has fo- cused on the biochemical and metabolic changes in osteoarthritic cartilage. Because of the scarce availabil- ity of human cartilage, especially in the early stage of the disease, most data on these changes derive from studies of animal models and in vitro cartilage explant studies. First, an acid lysosomal protease was identified that was capable of degradation of proteoglycans. 4 Subsequently, a neutral proteoglycanase was identified that was active at neutral pH. 5'6 Several other proteases present in osteoarthritic cartilage have been identified since then. Therefore, the osteoarthritic process was

Received from the TMJ Research Group, Department of Oral and Maxillofacial Surgery, University Hospital Groningen, The Nether- lands.

* TMJ Research Group, Department of Oral and Maxillofacial Surgery.

t Professor and Chairman, Department of Oral and Maxillofacial Surgery.

:~ Professor Emeritus, Department of Oral and Maxillofacial Sur- gery.

§ TMJ Research Group, Department of Medical Physiology. Address correspondence and reprint requests to Dr Dijkgraaf: TMJ

Research Group, Department of Oral and Maxillofacial Surgery, University Hospital, PO Box 30.001, 9700 RB Groningen, The Neth- erlands.

© 1995 American Association of Oral and Maxillofacial Surgeons

0278-2391/95/5308-001053.00/0

believed to be caused by a cascade of enzymatic events in which proteases from several sources were involved. An experiment by Fell and Jubb, 7 in which traumatized synovial membrane kept at a distance from either liv- ing or dead (freeze-thawed) autologous articular carti- lage was shown to be able to induce proteoglycan loss in living but not in dead cartilage, led to the discovery of interleukin- 1 (IL- 1) and subsequently of other cyto- kines. Over the last decade a whole network of inter- acting enzymes and cytokines involved in the cartilage degradation process has been discovered, and undoubt- edly more factors will be discovered in the future. The most intriguing question currently being explored is which factor initiates the cartilage degradation process, possibly providing an adequate therapeutic answer by blocking this factor and thereby preventing the start or progression of the degradation process.

The concept that in many cases temporomandibular joint (TMJ) signs and symptoms may be attributed to OA is gaining increasing support. 8 The TMJ is a syno- vial joint and obeys the same laws as other synovial joints. 9 Therefore, knowledge of OA in other synovial joints may be applied to the TMJ. Several TMJ re- searchers have already succeeded in identifying factors in TMJ synovial fluid known to play an important role in the pathogenesis of OA of other synovial joints. 1°'11 However, the choice of factor to be identified in the synovial fluid appears to be dictated by the readily availability of antibodies to this factor rather than by profound insight into the pathogenesis of OA. More- over, to justly interpret the significance of the presence of various factors in the osteoarthritic TMJ, an outline of normal physiologic processes and participating fac- tors in the normal TMJ should be established first.

924

DIJKGRAAF ET AL 925

Table 1. Proteases Capable of Degrading ECM Components

Class Protease Protease Inhibitor

Aspartic Cysteine

Serine

Metallo

Cathepsin D a-2-Macroglobulin Cathepsin B Cytastins

c~-2-Macroglobulin Cathepsin L Cytastins

a-2-Macroglobulin Plasminogen activator Protease nexin-I

Plasminogen activator inhibitor

PMN elastase ~wProtease inhibitor Cathepsin G a wProtease inhibitor

a j-Antichymotrypsin a-2-Macroglobulin

Collagenase (MMP-1) TIMP-I TIMP-2 a-2-Macroglobulin

Gelatinase (MMP-2) TIMP- 1 TIMP-2 a-2-Macroglobulin

Proteoglycanase TIMP- 1 (MMP-3, or TIMP-2 stromelysin) c~-2-Macroglobulin

Abbreviations: PMN, polymorphonuclear leukocyte; MMP, ma- trix metalloprotease; TIMP, tissue inhibitor of metalloprotease.

The aim of this article is 1) to discuss the structure, biochemistry, and metabolism of normal cartilage, and the role of proteases, cytokines, growth factors, and arachidonic acid metabolites in normal tissue turnover, based on knowledge of synovial joints in general; and 2) to indicate what is known concerning these factors in the TMJ.

General Review of Proteases, Cytokines, Growth Factors, and Arachidonic Acid

Metabol i tes

PROTEASES

ases are lysosomal enzymes and are most active at acid pH, whereas the serine and metalloproteases are most active at neutral pH. ~2 Proteases can be synthesized by chondrocytes, synovial cells, and inflammatory cells. The synthesis of proteases is mediated by a variety of cytokines, growth factors, and hormones. All proteases are synthesized in inactive proforms that later require activation. ~3 The activated proteases can be inhibited by specific protease inhibitors, presumably synthesized by the chondrocytes (Table 1). 14 Normal articular carti- lage contains large amounts of protease inhibitors, in- cluding tissue inhibitor of metalloprotease (TIMP) and plasminogen activator inhibitor (PAl). An imbalance between protease and protease inhibitor levels has been postulated as a possible pathogenic pathway of OA. ~5

CYTOKINES AND GROWTH FACTORS

Cytokines and growth factors are soluble polypep- tides capable of regulating growth, differentiation, and metabolic activity of cells. H'~6 Generally, in articular cartilage cytokines (including IL-I to XII, tumor necro- sis factor [TNF], and interferon) exert a catabolic ef- fect, whereas growth factors (including insulin-like growth factor [IGF], transforming growth factor [TGF], and fibroblast growth factor [FGF]) exert an anabolic effect (Table 2). Cytokines induce synthesis of proteases, resulting in an increased rate of cartilage ECM degradation and consequently proteoglycan depletion, whereas they reduce the rate of synthesis of proteoglycans and other ECM components. The cyto- kines act primarily through cell surface receptors, whose signal is subsequently mediated by nuclear on- coproteins to activate transcription of the gene, which corresponds with these specific cell activities. ]7 Growth factors can antagonize the cytokine effect by increasing the rate of synthesis of ECM components ~3 and TIMPs. ~8 The net effect depends on the relative con- centration of each mediator present in the cartilage.

Proteases or proteolytic enzymes play an important role both in maintaining normal tissue turnover and in degradation of ECM components of articular cartilage in the osteoarthritic process. They are capable of cleav- ing internal peptide bonds of proteins. 12 Proteases may be found intracellularly, in lysosomes, or extracellu- larly. Four classes of proteases have been identified: 1) aspartic proteases (including cathepsin D), 2) cysteine proteases (including cathepsin B and cathepsin L), 3) serine proteases (including plasminogen activator [PA], polymorphonuclear leukocyte [PMN] elastase, and cathepsin G), and 4) metalloproteases (including collagenase or matrix metalloprotease-1 [MMP-1], gel- atinase or MMP-2, and proteoglycanase or stromelysin or MMP-3) (Table l). The aspartic and cysteine prote-

Table 2. Cytokines and Growth Factors Mediating Degradation and Synthesis of ECM Components

Cytokines (Exerting Growth Factors (Exerting Catabolic Effect) Anabolic Effect)

Interleukins I-XII (IL)

Tumor necrosis factor (TNF)

Interferon (IF)

Insulin-like growth factor (IGF)

Transforming growth factor (TGF)

Fibroblast growth factor (FGF)

Platelet-derived growth factor (PDGF)

Connective-tissue-activating peptides (CTAPs)

926 NORMAL TMJ CARTILAGE

Cytokines can be synthesized by chondrocytes, syno- vial cells, and inflammatory cells. Cytokines can be antagonized by receptor antagonists (eg, IL-1 ra) and by so-called soluble binding proteins (eg, TNF-Bp).19 An imbalance between cytokine and cytokine inhibitor levels has been postulated as a possible pathogenic pathway of OA.

ARACHIDONIC ACID METABOLITES

Arachidonic acid metabolites or eicosanoids are mediators of inflammation. In response to specific stimuli arachidonic acid is released from cell mem- brane phospholipids. Subsequently, the arachidonic acid is converted to several prostaglandins (PGs) and thromboxanes by the cyclo-oxygenase pathway, and to several leukotrienes by the lipoxygenase path- way. 2° Arachidonic acid metabolites can be synthe- sized by synovial cells, mediated by cytokines. PGE2 is a major mediator of inflammation, and its synthe- sis is induced by IL-1. 2~

Normal Cartilage: Structure, Biochemistry, and Metabolism

Articular cartilage is a specialized form of connec- tive tissue composed of ECM and chondrocytes, al- though in the articular zone of TMJ articular cartilage predominantly fibrocytes are found (Figs 1 and 2). 2~ Cartilage is aneural, avascular, and alymphatic. Be- cause cartilage is aneural, pain perception and proprio- ception in synovial joints are dependent on nerve endings in the synovium, capsule, muscles, and sub- chondral bone. 22 Because cartilage is avascular and alymphatic, nutrition and elimination of waste products are dependent on diffusion through the cartilage matrix to and from the synovial fluidY The avasculafity of cartilage is possibly maintained by endothelial cell growth inhibitors and protease inhibitors. 24

EXTRACELLULAR MATRIX

The biochemical composition of the articular carti- lage ECM determines the biomechanical characteris- tics of the tissue, such as resilience and elasticity. 25 The biochemical composition of the ECM varies con- siderably from individual to individual, from site to site, with depth, and with time, because the ECM is a dynamic system that is continuously exposed to ana- bolic and catabolic factors. 25 Moreover, the biochemi- cal composition of the ECM varies per joint. TMJ articular cartilage consists of fibrocartilage, whereas the articular cartilage in most other synovial joints con- sists of hyaline cartilage. The ECM of both hyaline and fibrocartilage consists of water, collagen, proteo- glycans, structural glycoproteins, and small amounts

of lipid and inorganic components. Water constitutes 60% to 80% of the total weight of hyaline cartilageY However, the water content of TMJ fibrocartilage probably is smaller. 26

Collagen constitutes over 60% of the dry weight of TMJ fibrocartilage. 26 The collagen fibrils are organized in sheets and bundles, creating a network (Fig 2B). 27 This collagen network is in general kept together by its basketweave, cross-links, and anchoring proteins, such as chondronectin and fibronectin. 14 The collagen network provides cartilage with its tensile strength and shape, and counteracts the swelling pressure of the highly hydrophilic proteoglycans. 28 Fibrocartilage pre- dominantly contains type I collagen, or a combination of type I and type II collagen, and probably small amounts of type IX and type XI collagen. 29 Type IX collagen, which is diffusely distributed throughout the ECM, is involved in the cross-linking of especially type II collagen fibrils to each other and to other ECM components. 3° Type XI collagen, which is distributed around the chondrocytes, is involved in the organiza- tion of the fibrous components of the ECM and the exocytoskeleton of the chondrocytes. 29

Proteoglycans constitute 20% to 40% of the dry weight of hyaline cartilageY However, the proteogly- can content of TMJ fibrocartilage probably is smaller. 26 Proteoglycans are in general intertwined throughout the collagen network and are not only mechanically but also chemically entangled within this network. 26 In this way the proteoglycans "mask" the collagen fibrils. Proteoglycans are highly hydrophilic macro- molecules with a high water-binding capacity. They are only constrained from full expansion by the tension of the collagen network. 28 Loading of cartilage results in an increase of the internal hydrostatic pressure. When the hydrostatic pressure exceeds the osmotic pressure of the cartilage, water is squeezed out of the ECM, contributing to the lubrication of the joint sur- faces. This so-called weeping lubrication is especially functional under high loads. Under low loads the so- called boundary lubrication by a lubricating glycopro- tein is functional. 22 The proteoglycans in conjunction with the collagen network provide the cartilage with its resilience, elasticity, shear strength, and self-lubri- cation. However, proteoglycans can also function as integral membrane receptors.

Proteoglycans are complex macromolecules, con- sisting of a core protein with many glycosaminoglycan (GAG) side-chains of varying composition and chain length, linked with hyaluronic acid by link protein. In hyaline cartilage the GAG chains consist of 90% chondroitin 6-sulphate and keratan sulphate, and less than 5% of chondroitin 4-sulphate. 31 In TMJ fibrocar- tilage, however, the keratan sulphate content is much smaller, whereas dermatan sulphate is more preva- lent. 32

DIJKGRAAF ET AL 9 2 7

FIGURE 1. A, Section of a healthy mandibular condyle. The subchondral bone (SB) is covered by normal articular cartilage (c) (hematoxylin- eosin stain, original magnification ×64). B, Detail of normal articular fibrocartilage shown in A showing four zones: articular zone (Az), proliferative zone (pz), fibrocartilaginous zone (Fcz), and calcified cartilage zone (ccz). Chondrocytes are clearly visible (arrow) (hematoxylin- eosin stain, original magnification ×200).

The structural glycoproteins, the noncollagen non- proteoglycan matrix glycoproteins, constitute 5% to 15% of the dry weight of hyaline cartilageY However, the structural glycoprotein content of TMJ fibrocarti- lage has, to our knowledge, not yet been determined. Structural glycoproteins in general interact with cellu- lar receptors, mainly of the integrin family, and regu- late adhesion, migration, proliferation, and differentia- tion of the chondrocytes. 33 The two main structural glycoproteins are fibronectin (FN) and laminin. FN is a large, adhesive glycoprotein that aggregates near the chondrocyte cell membrane and in the ECM. FN is also found in serum, where it functions as an opsonin and chemoattractant. FN is a polyvalent molecule with particular binding affinities for fibrin, collagen, hepa- rin, components in bacterial cell coats, and cells. 33 The binding of FN to cells is mediated by the integrins. Laminin is a polyvalent molecule prominent in base- ment membranes and as receptor-bound component of the cell surface. 33 However, laminin has not been detected in the cartilage ECM of the primate TMJ, 34 and its presence in human TMJ cartilage has, to our knowledge, not yet been determined.

Lipids and inorganic components constitute only a very small part of the dry weight of both hyaline and fibrocartilageY

From the preceeding discussion it is obvious that the structure of TMJ articular cartilage is well studied. 26 However, further study of the exact biochemical com- position of the TMJ articular cartilage is needed.

CHONDROCYTES

Chondrocytes occupy only 0.01% to 0.1% of the volume of TMJ articular cartilagefl 6 Each chondrocyte (Fig 2A) is surrounded by a territorial matrix con- taining abundant ground substance but only a few col- lagen fibrilsY Chondrocytes in general have relatively low metabolic activity. Therefore they are able to func- tion under almost anaerobic conditions, but they are sensitive to toxic influences and unable to regenerate after major injury. However, chondrocytes have con- siderable recuperative abilities. 23 Joint loading stimu- lates diffusion of chondrocyte nutrients and waste products through the cartilage matrix and is therefore essential for chondrocyte nutrition. Joint immobiliza- tion impairs matrix diffusion to an extent that chondro- cyte nutrition may stop. 31

Chondrocytes have the capacity to synthesize as well as to degrade all components of the ECM. Although once thought of as an inert tissue, articular cartilage is now recognized to be a dynamic system that is capable

928 NORMAL TMJ CARTILAGE

FIGURE 2. Cells of TMJ articular cartilage. A, Chondrocyte in proliferative zone of fibrocartilage of healthy mandibular condyle. (TEM, original magnification × 11,946). B, Fibrocyte amid network of collagen fibrils in articular zone of fibrocartilage of healthy mandibular condyle (TEM, original magnification × 10,752).

of remodeling under functional demands, and of turn- over of ECM components. The half-life of proteogly- cans, for example, varies from 1 week to 200 daysY This points to the existence of a presumably enzymati- cally mediated "internal remodeling sys tem" by the chondrocytes (Fig 3). Stimulated by balanced levels of

DEGRADATION GFs CYTOs DEGRADATION SYNOVIAL PRODUCTS ~ ~ PRODUCTS FLUID

FIGURE 3. Internal remodeling system of normal articular carti- lage by the chondrocytes. Chondrocytes synthesize all ECM compo- nents, as well as degrade these components through production of precisely regulated amounts of proteases and protease inhibitors. Proteases not only degrade resident ECM components (left), but also newly synthesized ECM components (right). In normal cartilage homeostasis, anabolic (open arrows) and catabolic (arrows) pro- cesses are equated by a balance (gray beam) between proteases and protease inhibitors. Chondrocytes probably maintain this balance by equal amounts (gray beam) of growth factors (GFs) and cytokines (CYTOs).

cytokines and growth factors, chondrocytes produce precisely regulated amounts of proteases and protease inhibitors to induce normal turnover of ECM compo- nents.

Besides the synthesis and degradation of all ECM components, chondrocytes are capable of control over the location of ECM components adjacent to the chon- drocytes through specific receptors in their cell mem- brane. 33 These receptors effectively bind ECM compo- nents such as collagen and fibronectin. Subsequently, these bound polyvalent macromolecules bind other li- gands, thereby extending the linkage between the cell membrane and the pericellular matrix. The intracellular portions of the receptors interact with cytoplasmic structures, thereby providing a direct linkage, physi- cally and functionally, with intracellular events. 33 The cell membrane receptors binding ECM components are of the integrin family, or are integral membrane pro- teins. The integrins are a family of ubiquitous cell- surface ECM adhesion receptors that interact with a specific sequence of three amino acids, arginine-gly- cine-asparagine (RGD), in proteins. 35 Integrins bind many ECM components, including fibronectin. 36'37 Moreover, integrins mediate many cellular processes, including tissue morphogenesis, homeostasis, and re- pair. 35 Integral membrane proteins are receptors, con- sisting of ECM components that are intercalated with

DIJKGRAAF ET AL 9 2 9

the cell membrane, such as syndecan and heparan sul- phate proteoglycan. 33 The extracellular domain of these proteoglycans contains glycosaminoglycan chains ca- pable of binding other ligands including ECM compo- nents and growth factors.

In conclusion, chondrocytes in general are multi- functional cells, capable of synthesis and degradation of all ECM components and, to a certain extent, control over the location of these components in the ECM. Although TMJ articular cartilage chondrocytes pre- sumably also have these capabilities, this remains open to further study.

References

1. Mankin HJ: Clinical features of osteoarthritis, in Kelley WN, Harris ED, Ruddy S, et al (eds): Textbook of Rheumatology (ed 3). Philadelphia, PA, Saunders, 1989, pp 1480-1500

2. Hough AJ, Sokoloff L: Pathology of osteoarthritis, in McCarty DJ (ed): Arthritis and Allied Conditions: A Textbook of Rheumatology (ed 11). Philadelphia, PA, Lea & Febiger, 1989, pp 1571-1594

3. Mankin H J, Brandt KD: Biochemistry and metabolism of articu- lar cartilage in osteoarthritis, in Moskowitz RW, Howell DS, Goldberg VM, et al (eds): Osteoarthritis: Diagnosis and Med- ical/Surgical Management (ed 2). Philadelphia, PA, Saunders, 1992, pp 109-154

4. Ali SY: The degradation of cartilage matrix by an intercellular protease. Biochem J 93:611, 1964

5. Sapolsky AJ, Howell DS, Woessner JF Jr: Neutral proteinases and cathepsin D in human articular cartilage. J Clin Invest 53:1044, 1974

6. Ehrlich MG: Degradative enzyme systems in osteoarthritic carti- lage. J Orthop Res 3:170, 1985

7. Fell HB, Jubb RW: The effect of synovial tissue on the break- down of articular cartilage in organ culture. Arthritis Rheum 20:1359, 1977

8. Stegenga B, de Bont LGM, Boering G: Osteoarthrosis as the cause of craniomandibular pain and dysfunction: A unifying concept. J Oral Maxillofac Surg 47:249, 1989

9. de Bont LGM, Stegenga B, Boering G: Hard tissue pathology. A. Osteoarthrosis, in Thomas M, Bronstein SL (eds): Arthros- copy of the Temporomandibular Joint (ed 1). Philadelphia, PA, Saunders, 1991, pp 258-269

10. Quinn JH, Bazan NG: Identification of prostaglandin Ez and leukotriene B4 in the synovial fluid of painful, dysfunctional temporomandibular joints. J Oral Maxillofac Surg 48:968, 1990

11. Holmlund A, Ekblom A, Hansson P, et al: Concentrations of neuropeptides substance P, neurokinin A, calcitonin gene- related peptide, neuropeptide Y and vasoactive intestinal polypeptide in synovial fluid of the human temporomandibu- lar joint. Int J Oral Maxillofac Surg 20:228, 1991

12. Werb Z: Proteinases and matrix degradation, in Kelley WN, Harris ED, Ruddy S, et al (eds): Textbook of Rheumatology (ed 3). Philadelphia, PA, Saunders, 1989, pp 300-321

13. Tyler JA, Bolls S, Dingle JT, et al: Mediators of matrix metabo- lism, in Kuettner KE, Schleyerbach R, Peyron JG, et al (eds): Articular Cartilage and Osteoarthritis. New York, NY, Raven Press, 1992, pp 251-264

14. Howell DS, Treadwell BV, Trippel SB: Etiopathogenesis of osteoarthritis, in Moskowitz RW, Howell DS, Goldberg VM, et al (eds): Osteoarthritis: Diagnosis and Medical/Surgical Management (ed 2). Philadelphia, PA, Saunders, 1992, pp 233-252

15. Dean DD, Martel-Pelletier J, Pelletier J-P, et al: Evidence for metalloproteinase and metalloproteinase inhibitor (TIMP) im- balance in human osteoarthritic cartilage. J Clin Invest 84:678, 1989

16. Trowbridge HD, Emling RC: Inflammation: A Review of the Process (ed 3). Chicago, IL, Quintessence Publishing Co, 1989, p 90

17. Pelletier JP, Faure MP, DiBattista JA, et al: Coordinate synthesis of stromelysin, interleukin-I, and oncogene proteins in experi- mental osteoarthritis: An immunohistochemical study. Am J Pathol 142:95, 1993

18. GUnther M, Haubeck H-D, van de Leur E, et al: Transforming growth factor fll regulates tissue inhibitors of metallopro- teinases-I expression in differentiated human articular chon- drocytes. Arthritis Rheum 37:395, 1994

19. Pelletier J-P, Roughley PJ, DiBattista JA, et al: Are cytokines involved in osteoarthritic pathophysiology? Semin Arthritis Rheum 20:12, 1991

20. Goetzl EJ, Goldstein IM: Arachidonic acid metabolites, in McCarty DJ (ed): Arthritis and Allied Conditions: A Text- book of Rheumatology (ed 11). Philadelphia, PA, Lea & Febiger, 1989, pp 409-425

21. Lewis RA: Prostaglandins and leukotrienes, in Kelley WN, Har- ris ED, Ruddy S, et al (eds): Textbook of Rheumatology (ed 3). Philadelphia, PA, Saunders, 1989, pp 253-265

22. Mankin HJ, Radin E: Structure and function of joints, in McCarty DJ (ed): Arthritis and Allied Conditions: A Text- book of Rheumatology (ed 11). Philadelphia, PA, Lea & Febiger, 1989, pp 189-206

23. Fassbender HG: Significance of endogenous and exogenous mechanisms in the development of osteoarthritis, in Helminen HJ, Kiviranta I, Tammi M, et al (eds): Joint Loading: Biology and Health of Articular Structures. Bristol, Wright, 1987, pp 352-374

24. Hamerman D: The biology of osteoarthritis. N Engl J Med 320:1322, 1989

25. Mankin HJ, Brandt KD: Pathogenesis of osteoarthritis, in Kelley WN, Harris ED, Ruddy S, et al (eds): Textbook of Rheuma- tology (ed 3). Philadelphia, PA, Saunders, 1989, pp 1469- 1479

26. de Bont LGM: Temporomandibular Joint Articular Cartilage Structure and Function. Thesis, University of Groningen. Van Dendercn, 1985

27. de Bont LGM, Boering G, Havinga P, et al: Spatial arrangement of collagen fibrils in the articular cartilage of the mandibular condyle: A light microscopic and scanning electron micro- scopic study. J Oral Maxillofac Surg 42:306, 1984

28. Maroudas A: Physicochemical properties of articular cartilage, in Freeman MAR (ed): Adult Articular Cartilage (ed 2). Lon- don, Pitman Medical, 1979, pp 215-290

29. Seyer JM, Kang AH: Collagen and elastin, in Kelley WN, Harris ED, Ruddy S, et al (eds): Textbook of Rheumatology (ed 3). Philadelphia, PA, Saunders, 1989, pp 22-41

30. Dean DD: Proteinase-mediated cartilage degradation in osteoar- thritis. Semin Arthritis Rheum 20:2, 1991

31. Howell DS: Etiopathogenesis of osteoarthritis, in McCarty DJ (ed): Arthritis and Allied Conditions: A Textbook of Rheu- matology (ed 11). Philadelphia, PA, Lea & Febiger, 1989, pp 1595-1604

32. Kopp S: Discussion of: Early diagnosis of osteoarthrosis of the temporomandibular joint: Correlation between arthroscopic diagnosis and keratan sulphate levels in the synovial fluid, by Israel HA, Saed-Nejad F, Ratcliffe A. J Oral Maxillofac Surg 49:712, 1991

33. Trelstad RL: Matrix glycoproteins in Kelley WN, Harris ED, Ruddy S, et al (eds): Textbook of Rheumatology (ed 3). Philadelphia, PA, Saunders, 1989, pp 42-53

34. Milam SB, Klebe RJ, Triplett RG, et al: Characterization of the extracellular matrix of the primate temporomandibular joint. J Oral Maxillofac Surg 49:38t, 1991

35. Woods VL, Schreck PJ, Gesink DS, et al: Integrin expression by human articular chondrocytes. Arthritis Rheum 37:537, 1994

36. Gardner DL: Cells and matrix, in Gardner DL: Pathological Basis of the Connective Tissue Diseases. London, Edward Arnold, 1992, pp 13-64

37. Anderson JC: Biochemical basis of connective tissue disease, in Gardner DL: Pathological Basis of the Connective Tissue Diseases. London, Edward Arnold, 1992, pp 173-226