Some Observations on the Nature of Bone Mineral 



141 



In each case there are high values in the young which fall to a minimum around the 

 age of 40 before a further rise in later years. The general trend is highly suggestive. 



It is interesting to speculate that in osteomalacia where the mineralisation of 

 osteoid is retarded, the observed low phosphate concentrations may be the result 

 rather than the cause of the condition, although in several cases secondary hyper- 

 parathyroidism with increased renal clearance of phosphate may also be a factor. 



In carefully controlled experimental conditions, however, it may be that a 

 critical evaluation of plasma phosphate levels will indicate the trend in the rate of 

 new bone formation. 



In conclusion, I would like to commend caution to those workers who assess 

 "calcification" and "calcifiability" in vitro. Dr. H. Fleisch has kindly made available 

 his data on the "formation 

 products" of calcium phos- 

 phate precipitation on to 

 collagen in vitro. The ap- 

 propriate calculations for de- 

 termining the stoichiometry 

 of the equilibrium at the 

 point oi precipitation have 

 been made using the chemi- 

 cal potential method as de- 

 scribed in MacGregor and 

 Brown (1965). The results 

 are shown in Fig. 3. A good 

 fit was obtained and the 

 regression coefficient was 

 such that the Ca : P ratio of 

 the equilibrium was not si- 

 gnificantly different from 1 

 (in fact Ca : P = 6.6 : 6). In 

 this case, therefore, CaHP04 

 must have been the dominant 

 phase, and not the OCP or 



HA found when bone is present. It may be of interest here to mention in parenthesis 

 that an unselected group of renal calculi equilibrated at pH 7 in vitro have also 

 shown the presence of OCP (MacGregor et al., in press). 



It appears probable that the precipitation of inorganic Ca and P in the absence 

 of formed bone or an "active" template does not represent either the process of 

 in vivo calcification nor even the physical-chemical equilibrium conditions for bone 

 mineral and, therefore, appropriate limitations should be set on the mterpretation of 

 such data. 



References 



chemical potent 



;ium phosphate (precipita 



Dr. H. Fleisch) 



AsTwooD, E. B.: Growth hormone and corticotropin. In The Hormones. Pincus and Thim- 



MAN (eds.). New York: Academic Press 1955, p. 235. 

 Frost, H. M.: Bone Remodelling Dynamics. Springfield: Charles C. Thomas 1963. 

 — Bone Biodynamics. Springfield: Charles C. Thomas 1964, p. 315. 



