BONE CELLS AND BONE RESORPTION 475 



were prepared using NTB2 liquid emulsion (Eastman Kodak). 

 Following 1 week's exposure, the preparations were developed in 

 Dektol for 2 minutes at 17°C. Some sections were stained with PAS 

 prior to autoradiography. All sections were stained with hematoxylin 

 following development of the autoradiograms. 



FiNDINOS 



Differences in Degree of Calcification and in Organic Mass 



In soft x-ra\' microradiograms of undecalcified sections, less heav- 

 ily calcified regions were distinguished from those of greater miner- 

 alization bv the greater exposure (blackening) of the emulsion in 

 the former, due to the increased permeabilit\' of the matrix to the 

 x-irradiation (Figs. 10, 13, and 20). In the immature bones and 

 calcified cartilage of these young rats, such differences are not promi- 

 nent. Uncalcified matrices of cartilage or of bone (osteoid) do not 

 perceptibly absorb the soft x-rays, and thus were not distinguishable 

 in the microradiograms. Thev could be detected by comparison of 

 the microradiograms with the corresponding ground sections (Figs. 

 13 and 14). In ultrasoft microradiograms of dehydrated, decalcified 

 sections, differences in dr)- (organic) mass content (Greulich, 1961) 

 were reflected in the greater blackening of the underlying emulsion 

 in regions of decreased mass (Figs. 5 and 7). 



Distinction beticeen Old and New Bone 



Differences in the relative ages of different regions of bone may 

 often be deduced from the established sequence of events in normal 

 bone growth. Such differences were objectively demonstrated in 

 autoradiograms of bones from glycine-HMnjected animals ( Figs. 8, 

 9, and 22 ) . Bone matrix present prior to the time of injection ( "old" 

 bone) remained unlabeled, although the walls of osteocyte lacunae 

 within these regions became labeled if they were being remodeled 

 at the time of injection. Bone matrix formed within 4 hours after 

 injection was heavilv labeled, and appeared as a prominent reaction 

 band. Matrix formed for several days thereafter was diffusely and 

 progressively weakly labeled (cf. Young, 1962b). 



