•> ] 8 ADVENTURES IN RADIOISOTOPE RESEARCH 



lead ions of the sorrounding solution. In the early investigations mentio- 

 ned above, it was pointed out that the rapid uptake of ^^p during the 

 early phase of the experiment recalls the behaviour of a lead salt placed 

 in the solution containing labelled lead ions, the recrystallization of the 

 mineral constituents of the skeleton reminds of the behaviour of a lead 

 foil immerged into a solution containing labelled lead ions, however, 

 with the difference that, in the latter case, enzymic actions are involved. 

 Or, as it was expressed later*'**, "A restricted extent of renewal of the 

 skeleton is due to the fact that, while the P atoms of the uppermost 

 molecular layer of the bone apatite crystals can promptly interchange 

 with the free P atoms of the plasma (actually not the P atoms, but the 

 phosphate ions interchange), a renewal of the main part of the apatite 

 P can take place only when the crystal is dissolved and when new crys- 

 tals are formed from the plasma; from labelled plasma, labelled crystals 

 are formed". Subsequent experiments confirmed the correctness of these 

 early conclusions, showing that both a surface exchange between plasma 

 phosphate and bone phosphate, and a recrystallization, thus a dissolution 

 of some of the apatite crystals and the formation of new ones, take 

 place in the skeleton. Different workers, however, arrived at divergent 

 results about the share of both processes in the interaction of plasma 

 and bone constituents. 



The introduction of autoradiographic methods into the study of bone 

 formation by Leblond et al.*^^\ was a very important advance, 

 since it became possible to visualize the rapid formation and destruction 

 of some parts of the calcified tissue. Numerous autoradiographic investi- 

 gations such as those by Leblond et al. applying ^^p^ those by 

 (JOMAR etal.^'''^ using 45Ca, ^aSr, and 32p, by Skipper etal.^"^ with i^C, by 

 Kidman et al.*^^"^ with ^^Sr, by Engfeldt et al.^^''* with ^^p^ by Amprino 

 and Engstrom*^^*^ with 45Ca, and by Bauer^^^^ with 22 Na, clearly demon- 

 strate that a great part of the bone salt crystals are more or less unchan- 

 ged until they are reached by the process of resorption. 



Leblond's autoradiographs clearly indicate that the circulating 

 phosphate enters the skeleton either by exchange or by precipitation in 

 definite areas with the formation of new bone. While, in the autoradio- 

 graphs, the exchangeable phosphate is depicted as diffuse reactions 

 disappearing rapidly with time, the precipitated or stable phosphate 

 appears as localized persistent reaction. 



In contradistinction to all workers in this field, Engfeldt, Engstrom 

 and Zetterstrom^^ arrived at the result that even the initial uptake 

 of 32p by the bone is due exclusively to some kind of recrystallization. 

 This conclusion is based on their observation that the autoradiographic 

 patterns of cross sections from long bones show an uneven distribution 

 of radioactive phosphate. They found the fastest uptake of labelled 

 phosphate to take place in Haversian systems with a low content of mine- 



