MH K. W. YOUNG 



or in rats in which resorption had been stimulated by the admin- 

 istration of parathyroid extract. In the latter group, resorption ap- 

 peared to represent an acceleration of resorption already in progress, 

 and an extension of this process into adjacent areas. The role of the 

 cellular microenvironment in eliciting the resorptive capacities of 

 bone cells has been emphasized. The cellular response to appropriate 

 microenvironmental modifications apparently involves the complex, 

 coordinated activation of integrated portions of the cell's total ge- 

 netic information, resulting in the onset of resorption, which then 

 proceeds without regard to structural variations in the resorbable 

 tissue. 



Acknowledgments. Parts of this work were conducted in the labora- 

 tories of Professors R. Amprino (Bari, Italy) and A. Engstrom (Stock- 

 holm, Sweden) during the tenure of a National Science Foundation 

 postdoctoral fellowship (1959-1960). The remainder was supported by 

 United States Public Health Service Grants A-4243 and D-1413. 



The technical assistance of Mrs. Mirdza Berzins is gratefully acknowl- 

 edged. 



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