A Biomolecular Survey of Calcification 119 



electron-dense substances that later become crystalline as the elongated chambers 

 reach their final size. 



Our opinion is that the cell plays a more direct part in the formation of the 

 mineral deposits in bone and enamel than has been accepted hitherto, but we are still 

 left with a need to explain how the salt actually crystallizes in the organic phase. It 

 is possible, even in such widely differing subjects as bone, baleen and Lingula shell 

 that there is some common factor in the organic phase responsible for specific 

 nucleation of the mineral. In baleen, however, we have so far been unable to find 

 any significant evidence for bound phosphorus or serine phosphate in any fraction of 

 cells containing small (2Vo) or large (25''/o) amounts of salt. We have detected a small 

 amount of calcium in the EDTA-soluble, nondialysable portion of cell extracts which 

 increases with increasing mineralization and although this would tend to support the 

 present view of Urist (1964) that calcification is attended by increase in bound 

 calcium, we cannot say that calcification in baleen is closely associated with those 

 features claimed as significant in bone. 



Some conclusions 



A general theory of calcification is not possible without further study of tissues 

 other than bones and teeth. Indeed, there may be no general theory of calcification 

 and each type of deposit may have to be considered separately. It seems likely, 

 though, that there is some biological principal underlying all processes of mineral- 

 ization. A biomolecular survey of calcification produces the surprising conclusion 

 that bone and enamel seem to be the exception to the general rule that mineral- 

 ization is basically an intracellular event which is under the closest control until the 

 mineral substances are occluded, or "pinched off", by the cell. Even in the highly 

 complex silica skeletons of the diatoms, it has been shown recently by Reimann et al. 

 (1965) that every part of the mineral is encapsulated in an organic sheath. 



A tentative general hypothesis for mineralization is illustrated diagrammatically 

 in Fig. 4, the basis of which is to assume that the cell creates the image of the mineral. 

 Improving techniques of studying ultrastructure seem to increase the certainty that 

 the cell is an enormously complicated structure right down to each macromolecule; to 

 suggest that all normal mineralization is the filling of chambers specially created to 

 receive a chosen salt is to ask no more than what we would expect of a cell. Such a 

 prospect solves many problems at once; it explains the regularity of crystal deposits, 

 their sequence of formation and their precise location; it removes the necessity to 

 look for specific arrangements in the topographical matrix — any structure will do, 

 providing it has a hole in it; it spares us the impossible physical chemistry of calculat- 

 ing ion products and it leaves us with new possibilities as to the function of many 

 mineralized tissues. 



If the cell participates directly in the laying down of salt, however, we are left 

 with new problems as to how this is accomplished. The possibility that the cell may 

 "pump" inorganic structures has been enhanced by recent observations (Greenwalt 

 et al., 1964) on mitochondria which accumulate "amorphous" calcium phosphate. Even 

 in pathological conditions, the involvement of mitochondria (see Boyce and King, 

 1963, for a review of the position) has been repeatedly emphasized, and the transition 

 from "amorphous" to crystalline states in such structure may be a matter of met- 

 astable particles (for instance, Eanes et al., in press) rather than metastable solutions. 



