CONSERVATION OF SKELETAL CALCIUM ATOMS THROIOH LIFE 219 



ral salts. Since the major part of the tracer is found in limited areas, 

 the initial rapid uptake of labelled phosphate — according to 1heir 

 view — cannot be due 1o ion exchange on the crystal surface of the bone 

 minerals, such an exchange being prevented by the organic constilu- 

 cnts of the bone. 



While there can hardly be any doubt that the main part of the renewal 

 of the bone apatite of skeleton is due to a recrystallization process, 

 to a degradation and new formation of the mineral constituents of the 

 skeleton, objections may be raised against the view that the initial 

 uptake of ^^p is due exclusively to some kind of recrystallization. 



Uneven distribution of radioactive phosphate as shown in auto- 

 radiographic patterns of cross sections from long bones cannot be inter- 

 ])reted as an absence of surface interchange. According to Paneth*^^- ^"^ 

 th(> whole uppermost molecular layer of crystalline salt powders inter- 

 changes with the ions of a surrounding solution, while properly crystal- 

 lized surfaces like those of natural crystals fail to do so. He states that 

 his investigations suggest that the radioactive method of determining 

 surfaces, based on the assumption that the whole uppermost layer 

 molecular interchanges, should be employed in those cases only for 

 which it is established that the fundamental supposition of kinetic 

 exchange of the entire surface is valid. If we assume the bone apatite, 

 or part of it, as occurring in vivo, to be a properly crystallized substance, 

 Ave arrive at another explanation than that of Engfeldt, Engstrom 

 and Zetterstrom, according to which the organic constituents of 

 fresh bone are responsible for preventing surface exchange. This alter- 

 native explanation is that the bone apatite, or large parts of it, behaves 

 like a properly crystallized substance in Panetli's experiments and not 

 like a crystal powder. 



The exchange of ions on a crystal surface is thus far from being absent 

 and, though restricted to a fraction of that surface, is responsible for 

 an appreciable part of the early uptake of labelled ions by the mineral 

 constituents of the skeleton. As shown by Armstrong and assoc.^^"\ 

 in the course of the first ten minutes, 2% of the skeletal calcium of the 

 dog are replaced by labelled calcium of the plasma; this is Yio oi^ly o^ the 

 amount which, according to Falkenheim's^^*^' ^^''^ calculations, would 

 be necessary to replace the whole uppermost molecular layer of the bone 

 apatite, or i/g of the amount estimated by Hendricks and Hill^^'". 

 A large part of these 2% — or even 2% — could be due to a surface inter- 

 change in spite of the uneven autoradiographic patterns of cross sections 

 of long bones observed by Engfeldt, Engstrom, and Zetterstrom. 

 Tn view of the high specific activity of the plasma calcium in an early 

 stage of the experiment, to a 2% interchange corresponds a verj^ much 

 higher percentage decrease in the ^^Ca content of the plasma in the 

 course of the first two minutes, more than 50% of the injected radio- 



