ROLE OF CHELATION IN DECALCIFICATION SYSTEMS 645 



The Dissolving of Apatite within Osteoclasts 



Four groups of workers (Cameron and Robinson, 1958; Hancox 

 and Boothroyd, 1961; Gonzales and Karnovsky, 1961; Scott and 

 Pease, 1956 ) have now demonstrated, in electron micrographs, bone 

 salt crystals within the osteoclast. The crystals enter by pinocytosis 

 in the "ruffled border" and can be seen in the cytoplasmic vacuoles. 

 There is at present no means of knowing whether all the crystals 

 removed by osteoclasts enter the cell. The crystals presumably dis- 

 solve in the cytoplasmic vacuoles and the dissolved products are 

 shed into the tissue fluid. The question now becomes: What is the 

 nature of the intracellular mechanism for dissolving the crystals and 

 discharging the salts in a form in which they will remain soluble 

 in the tissue fluids? The contents of cells are stated to be acid ( Cald- 

 well, 1954); much of the intracellular calcium is likely to be bound 

 to protein, and the phosphate is also largely organically bound. 

 These conditions of acidity and low concentrations of calcium and 

 phosphate ions would favor the dissolving of the crystals perhaps 

 without the intervention of chelating sul^stances. However, since 

 the osteoclasts are presumably taking in many apatite crystals, the 

 capacity for binding the calcium and phosphate would become 

 rapidly used up — unless the organization of the cell can prevent the 

 contents of the vacuoles from having access to the protein and en- 

 zymes of the cytoplasm. The probability is, then, that the cell needs 

 chelating agents to dissolve the crystals as well as to discharge the 

 calcium into the circulation in a form in which it will remain soluble. 



Possible Importance of Chelating Agents 

 Released from the Matrix 



There remains the problem of how the crystals are dislodged 

 from the bone matrix. The electron micrographs of Hancox and 

 Boothroyd showed naked collagen libers, from which they concluded 

 that the removal of the apatite crystals occurs first, and is followed 

 by the disintegration of the matrix. An extracellular solvent, perhaps 

 citrate or lactate, may dissolve some of the crystals and thus expose 

 the matrix. Scott and Pease (1956) and Gonzales and Karnovsky 

 (1961), on the other hand, saw no matrix in their micrographs and 



