488 R. W. YOUNG 



matrices of bone and calcified cartilage. In no case was there evi- 

 dence that any of these features was effective either in determining 

 regions in which resorption occurred, or in modifying rates of re- 

 sorption within these regions. The possible resistance of osteoid to 

 resorption could not be studied, since in normal animals uncalcified 

 matrix was uniformly present in sites of osteogenesis, and in 

 PTE-treated animals no osteoid was detected at the intervals ex- 

 amined. 



The abnormally pronounced resorption occasioned by administra- 

 tion of parathyroid extract appeared to represent an exaggeration 

 of normal resorption already in progress (cf. Engfeldt and Zetter- 

 strom, 1954) and a spreading of this process into adjacent regions. 

 This conclusion is consistent with experiments which have demon- 

 strated that there is a lag between the time of uptake of bone-seek- 

 ing isotopes and the time at which they are released from bone 

 following parathyroid stimulation. At the time of administration, 

 these osseous precursors are incorporated mainly in sites of osteo- 

 genesis (e.g., in the metaphyseal zone of bone apposition). Later, 

 some of the labeled sites are reached by resorptive processes (e.g., 

 in the metaphyseal zone of trabecular resorption ) . The lag between 

 initial uptake and subsequent normal resorption is influenced by 

 the age (rate of growth) of the animal. In 6-day-old rats, for ex- 

 ample, regions of apposition in the metaphysis (labeled by matrix 

 precursors such as glycine-H'^ ) mav be found undergoing resorption 

 as early as 4 to 6 hours later (Young, unpublished material). In 

 older animals, this interval is progressively and appreciably length- 

 ened. The metaphyseal zone of long bones represents a major site 

 of resorption following parathyroid stimulation. Consequently, raised 

 parathvroid hormone levels will accelerate the release of bone-seek- 

 ing isotopes within hours after their administration in young animals, 

 but will not stimulate their removal until later in older animals. 



Thus, findings derived from experiments of this nature using young 

 animals (interpreted as showing an effect on new bone) and those 

 from old animals (interpreted as showing an effect on old bone) 

 may be brought into harmony under the explanation oflFered above, 

 that the hormone accelerates resorption in areas already character- 

 ized by resorptive processes. An acceleration of normal resorption 



