The chondrocyte

The International Journal of Biochemistry & Cell Biology 35 (2003) 401–404

Cells in focus

The chondrocyte Charles W. Archer a,∗ , Philippa Francis-West b a

Cardiff Institute of Tissue Engineering and Repair and School of Biosciences, Cardiff University, Museum Avenue, Cardiff CF10 3US, UK b Department of Craniofacial Development, King’s College, Guy’s Tower, Floor 28, London Bridge, London SE10 9RT, UK Received 18 July 2002; received in revised form 25 October 2002; accepted 30 October 2002

Abstract The chondrocyte is the resident cell of cartilage that is a prominent tissue in the embryo acting as a template for the development of skeletal elements. In the adult, the distribution of permanent cartilage is much more restricted and is necessary for mechanical support, growth and movement. The cell is isolated within a voluminous extracellular matrix (ECM) that is neither vascularised nor innervated. As a result, nutrient/waste exchange occurs through diffusion and, consequently, under normal and pathological conditions, the cell is unique in its ability to exist in a low oxygen tension environment. Partly as a result of these properties, the tissue has a low reparative potential that, in the case of articular cartilage, predisposes the tissue to degenerative conditions such as arthritis that is a significant clinical problem. Cellfacts • • • • •

Cytoplasmically isolated. High matrix/cell volume ratio. Do not divide after skeletal maturity unless during pathology. Major contributor to growth of the body. Most energy requirements obtained through glycolysis.

© 2003 Elsevier Science Ltd. All rights reserved. Keywords: Cartilage; Joint; Matrix; Arthritis

1. Introduction The chondrocyte is the sole constituent cell of cartilage that is unique within the body in that in most cases, it is cytoplasmically isolated from its neighbouring cells, it has no ready access to the vascular ∗ Corresponding author. Present address: School of Molecular and Medical Biosciences, University of Wales College of Cardiff, 911 Museum Avenue, Cardiff CF1 3US, UK. Tel.: +44-292-0875206; fax: +44-292-0874594. E-mail addresses: [email protected], [email protected] (C.W. Archer).

system and the tissue is not innervated. Cartilage is often considered an ‘embryonic’ tissue due to its extensive distribution within the foetus providing templates for skeletal elements that develop through endochondral ossification. Indeed, the chondrocyte is responsible for longitudinal growth within the epiphyseal growth plates. However, in the adult, cartilage distribution is more restricted but is still located in a variety of anatomical sites (e.g. joints, trachea, nasal septum) where the major function is mechanical support. In joints, cartilage has an additional role that allows for smooth pain-free articulation. It is within the articular cartilage that the chondrocyte rises to

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Fig. 1. Electron micrograph of a typical articular chondrocyte. The cell is typically rounded containing ordered rough endoplasmic reticulum, juxtanuclear Golgi apparatus and conspicuous deposits of glycogen including the remnants of lipid droplets.

clinical prominence where the tissue is susceptible to a host of degenerative conditions the most common being osteoarthritis. Because of space constraints, we will focus on the articular chondrocyte but also refer to chondrocytes of other tissues where relevant. The main morphological features of chondrocytes are that they are normally rounded (Fig. 1) or polygonal except at tissue boundaries where they may be flattened or discoid such as at the articular surface of joints. Intracellularly, the chondrocyte shows typical features of a metabolically active cell that has to synthesise and turnover a large matrix volume comprising collagens, glycoproteins, proteoglycans and hyaluronan. Specific features that are often present are the deposition of glycogen within the cell and the possession of a primary cilium. Another feature of the chondrocyte is the relatively high matrix to cell volume ratio. This ratio can be correlated to function with mammalian articular cartilages having a high ratio; the chondrocytes occupying about 10% of the total tissue volume (Stockwell, 1979). 2. Cell origin and differentiation Chondrocytes arise in the embryo either from mesodermal origin such as the elements of the limb or from

the neural crest which gives rise to the skeleton of the face. These can be termed primary chondrocytes although it is not usual to do so. However, in the head and clavicular skeleton of higher vertebrates, there are secondary chondrocytes that are unique in the way that they form arising from the ‘periosteal’ layer surrounding the membrane bone in response to mechanical stimulation. Being part of the connective tissue lineage, the chondrocyte can be considered having a primitive precursor ‘the mechanocyte’ that, as its name suggests, is able to respond to mechanical cues (Hall & Miyake, 2000). Like many connective tissues, the first overt signs of expression of the differentiated state is the formation of a cellular condensation at the site where the skeletal elements will form. Whilst there is some uncertainty as to the precise mechanics of the condensation process, the cell adhesion molecules N-CAM and N-cadherin have important roles (reviewed by Delise, Fischer, & Tuan, 2000). The condensation process is followed by commitment to chondrogenesis. Condensation signals the expression of a number of tissue specific molecules. In terms of transcription factors, Sox9 is crucial and becomes evident within the condensation and precedes the expression of type IIa collagen, the alternatively spliced isoform of collagen II synthesised by immature chondrocytes or chondroblasts. Furthermore, the importance of Sox9 is shown by its ability to bind and activate to a consensus sequence in the Col2a 1 enhancer region and the fact that wild type mice chimeras carrying Sox9−/− cells showed no inclusion of the mutant cells into cartilaginous structures. In addition, haplosufficiency of Sox9 in humans results in the syndrome campomelic dysplasia characterised, in part, by smaller skeletal elements. Mice heterozygous for Sox9 have a similar skeletal phenotype which is partly due to the formation of smaller precartilage condensations. Thus, it appears that Sox9 is a major factor in the commitment and development of chondrogenic cells (Lefebvre, Huang, Harley, Goodfellow, & de Crombrugghe, 1997 and reviewed by de Crombrugghe et al., 2000). More recently, both L-Sox5 and Sox6 have also been implicated in the differentiation process by their co-expression with Sox9 (Lefebvre, Li, & de Crombrugghe, 1998). Clearly, there are many other factors suggested to be important in the condensation and differentiation

C.W. Archer, P. Francis-West / The International Journal of Biochemistry & Cell Biology 35 (2003) 401–404

processes and the reader is directed to the comprehensive reviews by Hall and Miyake (2000), Delise et al. (2000). The fates of chondrocytes vary greatly depending on their origin and location. The articular cartilages persist and survive. Those that comprise the epiphyseal growth plates proceed through a differentiation program leading to cell hypertrophy—the terminally differentiated state that facilitates endochondral ossification whereby bone is laid down on the calcified cartilaginous matrix of the hypertrophic chondrocyte. In this context, the chondrocyte seems to have two possible fates. Either the cell dies by apoptosis or it can transdifferentiate or metaplase into an osteoblast thus converting its surrounding matrix from cartilage to bone (Roach, 1997; Zeraga, Cermelli, Bianco, Cancedda, & Cancedda, 1999). What determines the fate choice has yet to be elucidated.

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the entire metabolism of the cell is geared towards operating at a low oxygen tension (ranging from 10% at the surface to <1% in the deep layers) with the majority of the cell’s energy requirements coming from glycolysis and, as a result, chondrocytes normally do not contain abundant mitochondria. Even so, the cells can be remarkably active synthetically. When chondrocytes are cultured in a range of oxygen tensions (<0.1–20%), a number of important anabolic genes are up-regulated at lower tensions such as TGF␤ and connective tissue growth factor (Grimshaw & Mason, 001). The adaptation of chondrocytes to low O2 tensions is also exemplified at the transcriptional level by high expression levels of HIF and AP-1 the latter being elevated in response to reduced O2 (Rajpurohit, Koch, Tao, Teixeira, & Shapiro, 1996).

4. Associated pathologies 3. Chondrocyte functions The functions of chondrocytes are two-fold that is again dependent on the cell’s location. The prime function of chondrocytes that occupy supporting structures such as articular cartilage, tracheal cartilage and nasal cartilage is to synthesise and maintain an extracellular matrix (ECM) that is able to withstand physical deformation and will also facilitate tissue function. In the case of articular cartilage this function, as its name suggests, includes joint articulation. The other major function of chondrocytes is in growth, particularly that associated with epiphyseal plates. If we consider that growth constitutes an increase in volume, then the epiphyseal chondrocyte achieves this by three mechanisms. First, through proliferation and the production of more cells. Second, through matrix secretion and third through increased cell volume that occurs during hypertrophy (terminal differentiation). The overall growth contributions of these mechanisms are listed respectively although the precise values may vary both between joints and during the growth period. Clearly, the epiphyseal growth plate also carries out a supporting role being located between the epiphysis and diaphysis. Because articular cartilage is not vascularised, it must rely on diffusion from the articular surface for nutrient and metabolite exchange. Consequently,

There are many human syndromes associated with abnormal development of skeletal structures and rapid progress is now being made in identifying the genes involved (see OMIM, http://www.ncbi.nlm.nih. gov/omim). The major degenerative pathology associated with the chondrocyte is arthritis. Arthritis can be subdivided into two major classes: inflammatory rheumatoid arthritis (RA) and non-inflammatory osteoarthritis (OA). The latter is more common and although may affect juveniles, is more associated with the older population such that most people of 70 years will have some symptoms of the disease. A common feature of both diseases is erosion of the cartilage matrix through two major classes of enzymes, the MMPs (matrix metalloproteinases) and ADAM-TS4 and ADAM-TS5 (a distintegrin and metalloproteinase with a thrombospodin motif) that cleave the collagen and proteoglycan moieties of the extracellular matrix, respectively. Consequently, these enzymes have been targets for therapeutic intervention using inhibitors such as mannosamine and BB-16 that are active against aggrecanase. Interestingly, a new finding has been that the omega-3 class of fatty acids (that is found in cod liver oil) has a remarkable inhibitory effect on both of these classes of enzymes (Curtis et al., 2000) and may form an effective ‘chondro-protective’ prophylactic strategy to disease prevention. Whilst a number of surgical

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procedures such as joint realignment may provide pain relief for patients for a number of years, ultimately most of the major load-bearing joints such as the hip and knees will be treated by joint replacement. In the case of more localised lesions of cartilage, either a result of OA or joint trauma, the implantation of autologous culture-expanded chondrocytes harvested from peripheral regions of the joint is gaining clinical acceptance although the long-term efficacy of this approach is open to debate (Messner & Gillquist, 1996). It is becoming increasingly apparent that genetic mutations in the ECM components of cartilage predispose the joints to OA either through failure of matrix integrity or of whole joint functioning. For example, as with an ‘in-frame’ deletion of the COL9A 1 gene, heterozygotes showed no obvious morphological abnormalities but nevertheless, developed OA whilst homozygotes also showed signs of mild chondrodysplasia (Nagata et al., 1993; and see http://www.ncbi. nlm.nih.gov/omim). In more extreme cases, matrix changes such as those due to deletions in the COL2A 1 gene result in dysmorphogenesis of the joint leading to poor articulation and biomechanical imbalance in addition to loss of matrix integrity (Helminen, Saamanen, Salminen, & Hyttinen, 2002). It appears that any mutation in ECM components that can compromise either matrix integrity or morphogenesis have the potential to predispose a joint to OA.

Acknowledgements The authors would like to thank Dr. Vicky Church for help with the manuscript. References Curtis, C. L., Hughes, C. E., Flannery, C. R., Little, C. B., Harwood, J. L., & Caterson, B. (2000). n-3 fatty acids specifically modulate catabolic factors involved in articular cartilage degradation. The Journal of Biological Chemistry, 275, 721–724.

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