472 R. W. YOUNG 



areas of normally active osteogenesis, which is preferentially re- 

 moved ( Jaffe et at, 1931a, 1931&; Heller et ah, 1950; Talmage et ah, 

 1953; Lotz et al., 1954; Bronner, 1961). The rapid destruction of 

 metaphyseal trabcculae is often noted, and this is a region of rela- 

 tively low mineralization (Weidman and Rogers, 1950; Engstrom 

 and Bergendahl, 1958; Arnold, 1960). 



Indeed, it has been reported that matrix need not be calcified at 

 all to yield to resorption (Carnes, 1950; Follis, 1952), although some 

 studies indicate that uncalcified matrix is refractory to resorption 

 (Weinmann and Schour, 1945; Reitan, 1951), and other experiments 

 indicate that matrix need not be calcified at the time of resorption, 

 so long as it was calcified at some time in the past (Irving and 

 Dale, 1962). 



Finally, there are some reports (Engfeldt and Zetterstrom, 1954) 

 indicating that both old and new bone are removed, following sys- 

 temic acceleration of resorption; and published microradiograms 

 indicate that resorption may extend through areas with varying 

 degrees of calcification (e.g. Engfeldt and Zetterstrom, 1954; Sogn- 

 naes, 1959; Jowsey, 1960). 



In the light of these inconsistent findings, it appeared worth while 

 to summarize a number of previously unpublished observations 

 which bear on the relation of the state of the intercellular matrix 

 to preferential sites of resorption. Furthermore, since it appears to 

 be generally accepted that bone resorption is an active, cellular 

 process (McLean, 1954; Goldhaber, 1961; Talmage, 1962), attention 

 has also been directed to the cellular element in resorption, and to 

 controlling mechanisms which operate at the cellular level. 



Materials and Methods 



Young rats of the Long-Evans strain have been used throughout. 

 The bones of normal animals have been analyzed by a number of 

 techniques, and have been compared with those of animals in which 

 bene resorpticn had been accelerated by the prior intraperitoneal 

 injection of parathyroid extract ( PTE ) . * Generally, a total of 75 to 



** Injection parathyroid U.S. p., Lilly; in part donated by Eli Lilly Company, Indian- 

 apolis, Indiana. 



