A Biomolecular Survey of Calcification 115 



or of any specific relationship between macromolecule and mineral. We can regard 

 the topographical matrix as a fabric which may, or may not, be mineralized, and 

 which may, or may not, play a direct role in the calcification process. 



Genesis 



Our present ideas about calcification are based largely on studies of bone, and it 

 is understandable that there are numerous theories as to the initiation, growth and 

 control of calcium phosphate crystals with respect to collagen. In many ways, 

 however, the position has changed little from that proposed in the comprehensive 

 survey by Neuman and Neuman in 1953, which examined the problems of 

 calcium deposition in bone in terms of ion exchange and crystal structure, solubility 

 and growth and which laid down certain guiding principles. Not the least of the 

 persistent errors examined in the survey were those of the many calculations of 

 solubility products, which were not considered as applicable to solids of variable 

 composition. The failure to arrive at a satisfactory equation to account for the 

 necessary ions in a lattice of basic calcium phosphate led to the idea of crystalli- 

 zation, as opposed to precipitation, and in the words of Neuman and Neuman (1953) 

 the situation was apocryphal even for acid salt because "It has been shown that the 

 K^,, of CaHP04 must be exceeded for precipitation to occur, yet calcification occurs 

 in individuals whose blood levels of calcium and phosphate are well below this 

 product". 



This conclusion, that apatite could form only by crystallization, principally 

 because of "the blood levels of calcium and phosphate'' led to the suggestion that 

 calcification takes place on sites within the matrix which form specific nucleation 

 centres. As a result, there have arisen several general theories to explain the process 

 of nucleation in terms of bound phosphate and bound calcium, either to collagen or 

 to some closely allied part of the matrix. Mineralization has been thus explained in 

 terms of physical chemical factors causing an epitactic growth of crystals from ions 

 migrating freely through the extracellular organic phase, in contrast to the earlier 

 views of RoBisoN (1932) which supposed that there was some local enzyme mecha- 

 nism which precipitated the calcium salt at the calcification front. When the bulk of 

 the studies on bone salt and bone collagen changed to explore the new ideas proposed 

 by Neuman and Neuman (1953), the emphasis shifted towards the chemistry of the 

 cell-free substances and little attention was paid to the role of the cell in laying 

 down the bone salt. Indeed, there is an inherent fallacy in this neglect, for if the 

 calcium and phosphorus destined to make bone salt goes through the cell en route to 

 the mineralization zone, then the "blood levels of calcium and phosphate" can have 

 no meaning in relation to the inorganic phase in bone other than to decide the rate 

 of transfer. We can find, for instance, the present calculations for serum calcium and 

 phosphorus repeated in a recent review by Hartles (1964) yet we find quoted also 

 the evidence of Whitehead and Weidmann (1959) that some, at least, of the 

 phosphate might go through the cell, since inhibition of the ATP production causes 

 lessening of bone salt formation. It would seem that unless it can be clearly established 

 that all the ions in bone salt accumulate directly from the serum, any attempt to 

 relate salt formation to the serum ion product is meaningless. 



The curious lack of information as to the role of the osteoblast in the transference 

 of ions to the inorganic phase in bone is not reflected in studies elsewhere; a brief 



