Remodelling of the Bone Matrix 21 



1962; Krane and Glimcher, 1962). This is not a particular property of bone collagen 

 but a function displayed by any collagen in its native type fibrous form (Glimcher, 

 1959). It indicates that besides the collagen system, there must exist in soft and 

 calcified connective tissues some other component controling calcification. We will 

 not deal with this problem although we recognize that its role in the organization of 

 the collagen in calcified tissues might be of the greatest importance. 



In contrast to the loose and commonly random arrangement of the collagen 

 fibers in most soft connective tissue, in bone the collagen framework is organized in 

 tubules (osteons) or in lamellae. These are the units of bone which adapt to external 

 mechanical stresses by a process of remodelling as emphasized by Wolff in 1892 

 (cited by Glimcher, 1959). An understanding of how organization and remodelling 

 of bone is feasible from a biochemical point of view will only be possible when its 

 basic phenomena are known, that is how synthesis of calcified structures occurs and 

 how breakdown is realized. 



Synthesis and organization of collagenous structure in soft and calcified 

 connective tissues 



The collagen in soft connective tissues can be fractionated by varying the salt 

 concentration or the pH of the extracting medium (Harkness et ai, 1954; Jackson, 

 1957; Gross, 1958 a, 1959; Harkness, 1961). These fractions of the tissue collagen 

 belong to different pools of molecules. Their size depends on the metabolic activity 

 of the fibroblasts of the tissue (Gross, 1958 b) and can be related to the age of the 

 molecules (Gross, 1958 c). The experiments of Harkness et al. (1954) of Jackson 

 (1957) and Jackson and Bentley (1960) show that radioactive collagen appears at 

 different rates in the various fractions after injecting a labelled amino acid. These 

 results correlate with the hypothesis of Gross (1959) proposing that the extractability 

 of the collagen depends on its stage of aggregation, which would be partially a func- 

 tion of its age. Immediately after its synthesis the newly formed collagen liberated in 

 the extracellular space is extractable in isotonic saline at neutral pH and low tem- 

 perature (5 °C). After these molecules are organized in the connective tissue, an in- 

 creased ionic strength is required to separate another fraction of the loosely arranged 

 fibers. More collagen molecules can be further separated from the bulk of the remain- 

 ing insoluble fibers by lowering the pH to 3.5. With increasing time the proportion 

 of extractable collagen decreases indicating an increase of organization. 



By a combination of the methods of isotopic labeling of the collagen and frac- 

 tionation of these molecules by diflferent salt solutions we can follow the process of 

 synthesis and organization of the fibers in connective tissues. We have used this 

 technique to investigate the metabolism of the collagen in remodelling tissues (Lapiere 

 and Gross, 1963; Lapiere et al., 1965; Lapiere et ai, in press). These experiments 

 in the tadpole and more recent results provide us with information which can be 

 schematically summarized in Fig. 1. 



The proposed interpretation is the following. The precursor amino-acids are taken 

 from the local pool inside the fibroblast where they are incorporated into the peptide 

 chains of the collagen. In the cell this collagen is bound to the microsomes (Green 

 and Lowther, 1959). At this stage it is not extractable (I,). Shortly after, it is 

 liberated into the extracellular space where we find it in the pool of fibers. At the 



