A Scientific essay in Medical Sciences DOCTORAL THESIS To obtain the degree of doctor from the Radboud University of Nijmegen on the authority of Rector Magnificus, Prof. dr. C.W.P.M Blom, according to the decision of the Council of Deans to be defended in public on Monday 9th May 2005 at 1.30 a.m. By: Myriam Delatte Born in Brussel on June 26th, 1967

Chapter 1 Explains the background of the study and provides an outline of the thesis.

Chapter 2 [contents]

Chapter 2 is a literature review on the general composition of cartilage, the different cartilage types, and  their major functions in the human body. The relation between the composition of a certain cartilage and its specific function is described.

Chapter 3 deals with the complex regulatory mechanisms controlling cartilage growth and development. Mechanical factors and systemic mediators like hormones are key regulators of growth and metabolism of cartilage.  Growth factors like FGF-2, IGF-I, and TGF-b₁ play a crucial role in the local regulation of cartilage metabolism. Numerous interactions between growth factors and other regulatory factors occur at different levels in their signalling pathways. In contrast with the abundant literature on the effects of growth factors on primary cartilage, relatively few studies exist on their effects on secondary cartilage such as that of the mandibular condyle. There are indications that the response to growth factors differs in primary and secondary cartilages.

Chapter 4 [contents]

Chapter 4 presents a comparison of certain histological and biochemical parameters of the primary cartilage of the femoral head and the secondary cartilage of the mandibular condyle from four-day-old rats. Major differences in tissue organisation, mode of growth, and matrix composition were found between both cartilages. Higher amounts of GAGs and collagen type II were detected in femoral heads which is probably related to the higher resilience and higher resistance to compressive forces of this cartilage. This study shows that the matrix composition of the two types of cartilage is related to their specific functional requirements. Further, similar growth factors seem to be involved in the physiology of the two cartilages since receptors for IGF-I, TGF-b₁ and FGF-2 were found in both, albeit in different locations.  

Chapter 5 [contents]

In chapter 5 the growth of mandibular condyles and femoral heads is compared in vitro, and the effects of IGF-I are investigated. In this study, cartilages were cultured for up to two weeks with 0, 5, and 25 ng/ml IGF-I. IGF-I increased the GAG synthesis and stimulated the dimensional growth of both the primary and the secondary cartilage. However, higher concentrations were required to stimulate the dimensional growth of the femoral heads, and cell proliferation was stimulated only in the mandibular condyles. Therefore, the mandibular condyle seems to be more sensitive to IGF-I than the femoral head. The results show that this organ culture model is suitable to analyse the effects of growth factors on the main processes determining growth in both cartilages.  

Chapter 6 [contents]

Chapter 6 describes the effects of varying concentrations of IGF-I, TGF-b₁ and FGF-2 on cell proliferation and GAG synthesis in the two cartilages. In addition, interactions between growth factors were studied. Both types of cartilage were more sensitive to IGF-I than to TGF-b₁ and FGF-2 in the concentrations used. The stimulation of cell proliferation by IGF-I was much stronger in the mandibular condyles than in the femoral heads. This effect was dose-dependent. TGF-b₁ and FGF-2 were further shown to antagonise the effects of IGF-I in both mandibular condyles and femoral heads.
The highest concentrations of TGF-b₁ and FGF-2 were used to evaluate their effects on growth and matrix composition. FGF-2 reduced the GAG and collagen contents of the mandibular condyles more than that of the femoral heads. The dimensional growth of mandibular condyles was not affected by TGF-b₁ and FGF-2 while that of femoral heads was impaired by TGF-b₁ and even more by FGF-2. This study shows that the metabolism of mandibular condyles is more sensitive to IGF-I, TGF-b₁ and FGF-2 than that of femoral heads. In spite of this, the growth of the mandibular condyle was not reduced by TGF-b₁ and FGF-2. The regulatory processes of growth and development therefore seem to differ in the two types of cartilages.

Chapter 7 [contents]

In chapter 7, the effects of TGF-b₁ on the primary and secondary cartilage were investigated after varying culture periods. Mandibular condyles and femoral heads were cultured for 0, 7 and 14 days with 0 and 10 ng/ml TGF-b₁. TGF-b₁ decreased cell proliferation of both cartilages but only in femoral heads GAG synthesis was reduced. These effects were stronger after two weeks of culture than after one week. The dimensional growth of the femoral heads was strongly reduced by TGF-b₁ whereas that of the mandibular condyles was not affected. This was due to a stronger inhibition of chondrocyte hypertrophy in femoral cartilage. It is concluded that chondrocyte maturation in primary cartilage depends more on growth factors such as TGF-b₁ than that in secondary cartilage.

Chapter 8 [contents]

Chapter 8 presents a study into the effects of FGF-2 on both cartilages after increasing culture periods. The cartilages were cultured for one day, one week and two weeks without FGF-2 or with 100 ng/ml FGF-2. FGF-2 reduced the amount of GAG and collagen in both cartilages but the effects were stronger in the mandibular condyles. Moreover, the morphology of the mandibular condyles changed extensively. The dimensional growth of the mandibular condyles was not affected by FGF-2 while that of the femoral heads was much reduced. In both cartilages, FGF-2 stimulated cell proliferation in short-term cultures but reduced it in long-term cultures. In conclusion, FGF-2 showed a larger effect on the metabolism of the mandibular condyles as compared to the femoral heads. The terminal differentiation of chondrocytes in primary cartilage in vitro was more sensitive to FGF-2 than that of secondary cartilage.

  Chapter 9 [contents]

In chapter 9 the results from the different studies are discussed. General conclusions are drawn and suggestions are made for future research.