**1. Introduction**

Cartilage degeneration caused by osteoarthritis (OA) and trauma is of great clinical conse‐ quence, given the limited intrinsic healing potential of the cartilaginous tissue. OA is the most common joint disease in world populations. Pain during activities of daily living is a common presenting complaint of individuals with OA and is also associated with a decrease in quality of life for people with OA. Its incidence increases with age, and thus this degenerative disease is a major problem in ageing populations. OA is a multifactorial disease of the joints charac‐ terized by gradual loss of articular cartilage. In the recent years, the mechanism of chondrocyte differentiation has come to be well understood owing to the advancement of molecular biology, and researches have rapidly progressed for bioengineering or tissue engineering technique, where it is aimed to regenerate/reconstruct tissues by simulating the process of cell or tissue differentiation during development. Articular cartilage is composed mainly of collagen/proteoglycan (PG) and water. PG accounts for about 7 - 10% of cartilage tissues, and aggrecan, which is a member of PG representing macromolecules, plays a key role for mitigation of mechanical stress imposed on the cartilage tissues (Maroudas, 1979). A fall in PG concentration is one of the first changes in OA with consequent deleterious effects on the mechanical behaviour of cartilaginous tissues (McDevitt & Muir, 1976, Venn & Maroudas 1977). Among the components of aggrecan, negatively-charged Glycosaminoglycan (GAG) produces a high osmotic pressure in the cartilage tissue, and water is therefore absorbed in the cartilage tissues. As a result, the collagen networks are inflated, and the cartilage tissues acquire elastic resistance characteristic to cartilage tissues to protect from compression force. Thus, the

© 2013 Kobayashi; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

strength of cartilage tissues strongly depends on the density of aggrecan (Kempson et al., 1970, Maroudas, 1979, Maroudas & Bannon 1981). Therefore, in order to produce cartilage tissues that can tolerate mechanical force of about 10-20 Mpa using the tissue engineering technology, it is necessary to generate sufficient PG (Hodge et al., 1986). PG/GAG generation depends on the amount of GAG production, the capacity of GAG retention in the tissues, and the concentration of cells (Kobayashi et al., 2008). There is now an increasing interest in developing biological methods of cartilage repair for these disorders with attainment of the correct biomechanical properties critical for success (Brittberg et al. 1994, Minas, 2001, Risbud & Sittinger 2002, Schaefer et al. 2002 Ochi et al. 2002, Robert et al. 2003). The stiffness of cartilaginous tissues is thus strongly dependent on aggrecan content. Therefore, one of the targets of successful repair is thus that GAG concentration of the tissue-engineered construct should approach that of the native cartilage. First of all, it is important to establish the optimum culture conditions for the generation of cartilaginous tissues. In this study, we examined how physiological levels of extracellular osmolality and cell density influence PG accumulation in chondrocytes in a three-dimensional culture system. And also, we evaluated the influence of transforming growth factor-β (TGF-β) and fibroblast growth factor-2 (FGF-2), which are involved on the metabolism of PGs by cartilage cells cultured under low-osmotic conditions.
