-4i)0 R. W. YOUNG 



are capable of resorbiiig bone. In the present material, however, 

 cells characterized morphologically as osteoblasts uniformly were 

 engaged in formation (rather than destruction) of bone matrix (cf. 

 Young, 1962/7, 1963r/). The same may be said of immature osteo- 

 cytes, which retain osteoblast characteristics for some time after 

 being trapped in bone. Furthermore, osteoblasts were sparse or ab- 

 sent in PTE-injected rats in which resorption was increased. 



Emlothelial cells have been implicated in resorption (Jaffe, 1933; 

 Cameron, 1961). Others have suggested that histocytes are capable 

 of resorption ( Muhlethaler, 1953; Goldhaber, 1961) or can serve 

 as precursors of osteoclasts (Hancox, 1956; Jee and Nolan, 1962). 

 Conceivably, these cells (and others) represent mesenchymal spe- 

 cializations which have retained the capacity for resorption, and are 

 capable of activating it under suitable circumstances. 



A number of stimuli are known which appear capable of evoking 

 the resorptive potential in bone cells. For example, resorption often 

 occurs when bone is relieved of its normal, functional stresses ( Geiser 

 and Trueta, 1958), or when its component cells have been injured 

 or killed. Pressure on bone generally leads to its resorption ( Reitan, 

 1951; Storey, 1955; Young, 1959). Parathyroid hormone, cortisone 

 (Storey, 1957; Urist and Deutsch, 1960), changing levels of sex 

 hormones (in birds: Bloom et al., 1958), and possiblv vitamin A 

 (Barnicot and Datta, 1956) all accelerate resorption. Diploic re- 

 sorption may occur in response to proliferation of associated hemo- 

 poietic tissue (Ascenzi and Marinozzi, 1958). In tissue culture, 

 resorption is stimulated by high levels of oxygen (Goldhaber, 1961). 



Evidently, a wide variety of stimuli is capable of provoking a 

 similar response from the cells of bone. This is a strong indication 

 that the complexity lies in the reacting system, the cells themselves. 

 The cellular response involves an integrated mobilization of meta- 

 bolic machinery, which apparentlv precludes the simultaneous utili- 

 zation of these resources for other specialized functions (such as 

 bone formation ) . 



Elicitation of the resorptive response probablv involves the co- 

 ordinated activation of a group of interrelated genetic loci, which 

 code information required for the synthesis of the specific proteins 

 (including enzymes) involved in resorption. Conceivablv, the vari- 



