L. J. RicHELLE et al.: Bone Mineral Metabolism in the Rat 123 



Bone Mineral Metabolism in the Rat ' 



L. J. RiCHELLE ■■•■=•', C. OnKELINX, J. -P. AuBERT 



Institut de Therapeutique Experimentale, Universite de Liege, Liege, Belgique; 

 Institut Pasteur, Paris, France 



Introduction 



Numerous attempts have been made to describe the evolution of bone tissue with 

 time. The model which has been applied to such descriptions is quite simple: bone is 

 considered as a mass of a homogeneous material which is being formed and destroyed 

 at finite rates. 



Such models do not allow to take into account existing variations in the pro- 

 portions of organic to mineral material in the microscopic structures of bone, i. e. the 

 existence of a range of specific gravities. A fortiori, possible variations in the chemical 

 composition of the mineral phase in these same structures are ignored. 



Actually, the finding of a range of specific gravities implies that the turnover 

 rate of the organic matrix and the turnover rate of the mineral phase are not identi- 

 cal functions of time. Variations in the Ca/P ratios of the microscopic structures of 

 bone would have the same implications regarding calcium and phosphorus turnover 

 rates. 



It has been shown by Deakins (1942) for teeth, and later by Robinson (1960) 

 for bone, that in a constant volume of calcified tissues, the amount of organic 

 material does not change with time; the specific gravity of this volume increases 

 because water is progressively replaced by mineral. 



One theoretical representation which can be based upon the data of Robinson 

 leads one to consider bone as being made up of a population of elementary volumes. 

 The evolution with time of the number of these elementary volumes is the result of 

 two processes: one corresponds to the appearance of new elementary volumes and 

 the other to the destruction of existing ones. From Rowland et al. (1959), it is known 

 that the amount of mineral present, expressed in g hydroxyapatite per cm^ varies 

 between 1.11 and 1.50, respectively for the least and the most calcified structures 

 (diaphyseal cow bone). One can calculate therefrom the range of expected specific 

 gravities which extends from 1.65 up to 2.25 mg/mm^ (Richelle, 1964). 



In our theoretical representation, we will assume that elementary volumes of bone 

 appear with a specific gravity 1.65 mg/mni'''. This implies that bone does not exist as 

 such below that value. 



The evolution with time of the specific gravity of a given elementary volume is 

 determined by its progressive mineralization raising its value from 1.65 up to a 

 maximum equal to 2.25 mg/mm^, provided it is not destroyed before reaching the 

 maximal value. 



It is indeed considered that resorption can affect elementary volumes irrespective 

 of their degree of mineralization. Moreover, it can be assumed, as opposed to the 

 processes involved in bone formation, that bone destruction bears synchronously on 

 all the constituents. The elementary volume is destroyed as a whole, at the specific 



•'■" This work has received the support of the U.S. Public Health Service, National Institute of Dental 

 Research, Grant nr. DE-01931-Ol-O:. 



"'' Charge de Recherches du FNRS. 



