Remodelling of the Bone Matrix 27 



closer association of calcium salts and collagen fibers (2.1) in a way rather similar to 

 the increase in density and the progressive insolubilization of fibers in non calcified 

 connective tissues (A). The bulk of the osteons would be composed of fully miner- 

 alized structures (2.2) which can be compared to the mass of insoluble fibers in soft 

 connective tissues (I). 



The percent distribution of the collagen in these different fractions of soft and 

 calcified tissues is somewhat analogous. There is little collagen in the isotonic saline 

 (N) extract and in the lightest bone fraction (1.7), some more in the hypertonic 

 extract (NM) and in the 1.9 fraction, even more in the acid and the 2.1. One main 

 fraction contains the bulk of the collagen, the insoluble one in soft connective tissues 

 and the 2.2 fraction of bone. 



It is obvious that only metabolic studies using fractionation techniques will allow 

 us to go further in our understanding of the organization of calcified tissues. The 

 above partial results of a larger experiment designed to study the formation and 

 removal of the bone matrix brings to our attention the similarity in the mechanism 

 of synthesis, organization and distribution of the collagen framework in bone and 

 soft connective tissues. This identity might be based on general properties of the 

 collagen molecules. Bone seems to be in this respect, only a special problem of the 

 molecular biology of the connective tissues. 



Collagen breakdown In soft and calcified connective tissues 



During growth, bone is under constant, rapid remodelling. This function is 

 accomplished by the three types of bone cells, osteoblasts, osteocytes and osteoclasts. 

 This remodelling seems to be required for some part of the homeostatic control of the 

 calcium ion concentration in body fluid (McLean and Rowland, 1963) according to 

 the dual mechanism of the control of calcium balance proposed by McLean and 

 Urist (1961). As a part of this mechanism, bone cells are capable of tunelling com- 

 pact bone to induce the formation of new haversian canals. This function, which 

 continues even in the absence of the parathyroids (Jowsey et al., 1958) seems to be 

 related to the activity of osteoclasts. More recently Belanger et al., (1963) demon- 

 strated that bone can also be degraded by the activity of osteocytes in a process 

 called osteolysis. It is enhanced by parathormone as well as the osteoclastic activity 

 in vivo. 



The best example of fast removal of calcified connective tissue is found in tissue 

 culture (Goldhaber, 1958; Gaillard, 1959). The breakdown of the collagen matrix 

 of bone cultivated in vitro has been shown by Stern et al. (1963) by measuring an 

 increasing amount of solubilized hydroxyproline. That bone cells were capable of 

 secreting an enzymatic system having coUagenolytic activity was demonstrated in 

 tissue culture by Gross and Lapiere (1962) and further studied in detail by Walker 

 et al. (1964). Dissecting the results of these experiments, we find that all the condi- 

 tions for claiming the presence of a collagenase system were fulfilled. The collagen 

 used is In fibrous form in a native state and little of it is degraded by large amounts 

 of trypsin. The pH of the reaction mixture remains at neutrality and a considerable 

 proportion of the breakdown product is dialysable. The reaction goes to completion, 

 breaking down all the collagen present. Finally, the production of this lytic enzyme 

 is enhanced by parathormone proportionally to the duration of the treatment. 



