174 



ADVENTURES IX RADIOISOTOPE RESEARCH 



phosphate. However, as RobisonI discovered, the cartilage and 

 osteid contain an enzyme, phosphatase, which hydrolyses this ester, 

 thus setting free inorganic phosphate, whereby the concentration of the 

 phosphate ions increases and a supersaturation occurs, followed by a 

 precipitation of the calcium phosphate in the matric of the tissue. With 

 the discovery of the bone phosphatase a second agency (in addition to 

 the acidity change) of great importance was found, regulating the cal- 

 cium phosphate precipitation leading to ossification. Robison found 

 that the enzyme had the greatest activity in ossifying cartilage, bones, 

 and teeth of very young animals, the activity per unit weight of tissue 

 decreasing with age. Although the plasma contains on an average only 

 0.5 mgm of phosphorus present as phosphoric ester per 100 cc. this is 

 completely hydrolysable by the bone phosphatase and thus supplies 

 phosphate ion amounting to about ^/g of the inorganic phosphorus 

 present in the plasma, an amount amply sufficient to bring about a 

 supersaturation and a subsequent precipitation of calcium phosphate, 

 vjr more correctly of the apatite-like bone substance, from the already 

 nearly or fully saturated plasma. The conclusions arrived at in this 

 paper are independent of the special mechanism assumed for the ossifi- 

 cation process. 



DISTRIBUTION OF LABELLED PHOSPHORUS IN THE INCISORS 



OF RATS 



The rapidly growing incisors of rats are very suitable for studying 

 the distribution of phosphorus. According to Friderica and Gudjox- 

 SONS^ the average extrusive incisor growth per week is 2.7 mm. in the 

 case of adults and 3.4 mm. for young 

 rats. As seen in Fig. 1 A the cross sec- 

 tion of the pulpa is very large at the 

 proximal end and gets narrower toward 

 the distal end, the last millimetres of 

 the teeth being free of pulpa. The 

 problem we have to investigate is how 

 the distribution of newly formed cal- 

 cium phosphate in the incisor takes place. 

 Two extreme cases must be envisaged: 



a) the labelled phosphate is deposited Fig. 1. 



1 R. EoBisoN (1912) The Significance of Phosphorus Esters in Metabolism, 

 New York. 



2 L. S. Friderica and S. V. Gudjonsons, Kgl. Danske Vid. Selsk. BinJ. 

 Med. 28, 813 (1931). 



