Growth regulation of the rat mandibular condyle and femoral head by transforming growth factor-β1, fibroblast growth factor-2 and insulin-like growth factor-I

The importance of the mandibular condyle for craniofacial growth and development has often been discussed. Several theories on craniofacial morphogenesis have been proposed, of which some define the mandibular condyle as an important growth centre (Baume, 1961; van Limborgh, 1972); others consider it as an organ without independent growth potential that is mainly adapting to the function of surrounding structures (Moss and Rankow, 1968; Koski, 1968; Petrovic et al., 1975). However, some studies have also demonstrated that the mandibular condyle possesses both an intrinsic and an adaptive growth capacity (Meikle, 1973; Kantomaa, 1984). Cartilage of the mandibular condyle is considered as secondary cartilage, while the epiphyseal growth plate, the synchondrosis, and the cartilage of the femoral head are all considered as primary cartilages. These two types of cartilage have a different embryonic origin. Primary cartilages are part of the primary cartilaginous skeleton, while secondary cartilages are defined as chondral tissues that develop independent of the chondroskeleton and in close association with membranous bones (Beresford, 1981). Primary and secondary cartilages also differ in histological organization and in the composition of the extracellular matrix (ECM) (Takigawa et al., 1984; Copray et al., 1986; Mizoguchi et al., 1990; Ishii et al., 1998). Although few studies have directly compared the growth regulation of primary and secondary cartilages, the general idea is that primary cartilage has a relatively independent growth potential and is more sensitive to hormonal-like factors, while secondary cartilage is more sensitive to functional factors (Copray and Duterloo, 1986; Copray et al., 1986; Rönning, 1991). In recent years, experiments have indicated that growth factors also play an important role in the regulation of the metabolism of the mandibular condyle. Both insulin-like growth factor-I (IGF-I) and transforming growth factor-β1 (TGF-β1) were detected in the mandibular condyle of growing rats, and their localization was shown to be dependent on the stage of development (Li et al., 1998). Fibroblast growth factor-2 (FGF-2) has been detected in cartilage of the mandibular Growth regulation of the rat mandibular condyle and femoral