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C. M. LAPIERE AND J. GROSS 



ture. Successive extractions with solvents of increasing ionic strength 

 remove older collagen, which is more highly organized into fibrils. 



Acid-extractable collagen is tho 

 fibrils. The remaining collagen ca 



ght to be derived from still older 

 be removed onlv bv denaturation 



Fig. 3. Schematic representation of a hypothesis formulated to help explain 

 occurrence of the different extractable collagen fractions. The rodlike units repre- 

 sent tropocollagen particles. Cold physiological saline (0.14 m NaCl) extracts 

 the most recently formed collagen molecules (and perhaps also those resulting 

 from physiological degradation ) , which are completely dissociated or in the 

 loosest association; hypertonic salt solutions extract the same material plus older 

 collagen in a more ordered state of aggregation; acid citrate buffer extracts all 

 the above plus some of the older collagen in the typical fibrillar form. The in- 

 soluble fibrils were of a sufficient age so that the degree of cross linking has 

 prevented solubilization. (From Gross, 1959.) 



procedures and is most likely in a very stable, cross-linked form. 

 This hvpothesized scheme is represented diagrammaticallv in Fig. 3. 

 It should be noted that these collagen fractions are operational, and 

 not necessarily phvsiologically or metabolicallv homogeneous. For 

 example, it has been suggested bv Jackson ( 1957 ) that a portion of 



