20 Ch. M. Lapiere 



Remodelling of the Bone Matrix 



Ch. M. Lapiere 



Institut de Medecine, Dcpartcment de Clinique et de Pathologic Mcdicales 



et 



Laboratoire de Dermatologie Experimentale, Universite de Liege, Hopital de Baviere, 



Liege, Belgique 



Introduction 



Bone, the supporting frame of vertebrate animals is made of a two-phase material, 

 the collagen matrix and the hydroxyapatite crystals. The organization of the collagen 

 fibers and of the crystals is such that these two materials with different elastic moduli 

 and strength carry out their mutual function organized as trabeculae and account for 

 the mechanical properties of bone (Currey, 1964). Strongly regulated mechanisms 

 involving synthesis and degradation of the matrix and of the crystal lattice, are 

 probably responsible for such a high degree of organization. 



Until recently, little has been known about the way collagen is synthesized and 

 degraded in bone while some progress has been made in the last few years concerning 

 these processes in soft connective tissues. This difference in the amount of information 

 is mainly due to the difficulty in applying to calcified tissues the techniques of in- 

 vestigation used for the study of collagen metabolism in soft connective tissues. There 

 is, however, no reason to think that the main steps of formation and removal of the 

 collagen fibers are completely different in bone compared to soft collagenous struc- 

 tures. Indeed the osteoblast is only a particular form of fibroblast and the osteoclast 

 a specialized histiocyte. The main difference between soft and calcified connective 

 tissue lies in the composition of the ground substance. Semi-liquid and highly perme- 

 able in soft connective tissues, it is mostly rigid in bone, immobilizing fibers and cells 

 in a crystalline jail. This difference must change the relationship of the collagen 

 framework with its surrounding medium. It surely has some side effect on the me- 

 tabolism of the fibers. It is, however, too early to take this into consideration. In the 

 present discussion we shall only consider the problem related to the synthesis and the 

 breakdown of the matrix in calcified tissues. 



Composition and organization of the bone matrix 



The dry weight of bone is composed of 65 — 70% of inorganic crystals of calcium 

 and phosphate and 30 — 35''/o of an organic matrix in which the collagen accounts 

 for as much as 90 — 95Vo (Eastoe, 1956). It represents so much of the organic phase 

 of the bone that any changes in it must represent modifications of the matrix. The 

 collagen of bone is not different in amino acid composition, except for its hydroxy- 

 lysine content, from this protein found in skin or other organs according to data of 

 PiEZ and LiKiNS (cited in Gross, 1963 b). According to Glimcher (1965), bone 

 collagen would however be composed of only a chains. It does not seem to form as 

 much ft or y sub-units as skin collagen. 



Without considering in detail any concepts of calcification we want to call 

 attention to the fact that the crystals are very closely related to the collagen fibers 

 (S. F. Jackson, 1957; Glimcher et ai, 1957). The nucleation could arise around a 

 phosphate group bound to definite regions of the collagen (Glimcher and Krane, 



