BONE RESORPTION IN TISSUE CULTURE 619 



In view of our earlier results with chick embr\ o extract-coutaiuiui^ 

 uiedium, wherein it was possible to dciuoustrate regulation of extent 

 of bone resorption by alteration of oxvgen tension, it was significant 

 that the response to parathyroid extract could be altered bv variation 

 in oxygen tension. From Figs. 24 and 25 it mav be seen that whereas 

 increasing the oxygen tension from 10 per cent to 50 per cent had 

 little effect on the gross appearance of 12-da\ control cultures of 

 mouse cal\ aria, there was a distinct effect on the experimental cul- 

 tures containing 0.5 unit of parathyroid extract per ml of medium. 

 Although the hormone-containing group gassed with 10 per cent 

 oxygen showed little diff^erence from its corresponding control group 

 without hormone, the experimental groups gassed with 20 per cent, 

 30 per cent, and 50 per cent ()j showed graded enhancement of bone 

 resorption in response to increasing oxxgen tensions. 



The interdependence of oxvgen tension and parath\roid extract in 

 stimulating bone resorption in this svstem was demonstrated by the 

 observation that despite optimum oxvgen tensions (50 per cent O2) 

 in the gas phase, decreased bone resorption resulted from decreasing 

 the concentration of parathvroid extract in the medium (Fig. 26). 

 Although cultures gassed with 50 per cent O2 and exposed to 0.01 

 or 0.05 unit PTE per ml showed no gross difference from control 

 tubes not receiving hormone, those treated with 0.1 or 0.5 unit PTE 

 per ml (and gassed with 50 per cent Oi) showed enhanced bone 

 resorption after 2 weeks in culture. It should be noted that distinct 

 differences in extent of resorption for the different dose levels of 

 parathvroid extract were determined readilv during the 1st week of 

 culture bv microscopic examination and scoring of the living cul- 

 tures. Another means of comparing the eff^ects of different doses of 

 parathyroid hormone on bone resorption in the previous experiment 

 was to perform calcium determinations (Munson et ah, 1955) on 

 all supernatants throughout the experiment. From Fig. 27 it may be 

 seen that the largest differences in calciimi level occurred in the 

 media collected at the 4th, 6th, and 8th days of culture. This interval 

 corresponded to the period of most rapid bone resorption. Although 

 no statistical analysis was performed, it appears that during this 

 rapid resorption period the calcium levels in the media of tubes 

 containing 0.5, 0.1, and 0.05 unit of PTE per ml were significantly 



