348 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



Reduction in length becomes more striking when the tendons are 

 placed in hot water (60 to 70° C). They suddenly contract, while 

 swelling, and at the same moment the birefringence and X-ray dia- 

 gram vanish. This unusual reduction in length imparts rubber-like 

 elasticity to the tendon. After careful elongation the X-ray diagram 

 reappears and continued stretching will finally restore and establish 

 the inelastic collagen fibre. All this resembles the behaviour of rubber 

 which, when unstretched, produces no X-ray diagram, but gives a 

 pattern after it has been considerably stretched. Meyer and Mark 

 (1930) point out another interesting property common to both 

 materials. If the contracted tendons or unstretched rubber be frozen 

 in liquid air and the objects be then smashed, they crumble to a friable 

 mass, like sand; whereas under similar treatment native tendons or 

 elongated rubber will split up into a fibrillar mass. From this it may 

 be inferred that the polypeptide chains of the collagen fibres contract, 

 as in /9-keratin, and fold up. But whereas folding of the ^ -> a-keratin 

 type is limited, with collagen it is so violent that the straight protein 

 chains shrivel up completely. Evidently, the impulse of polypeptide 

 chains to shorten in the free state is very widespread and, if means 

 are found to make this process reversible and to regulate it, a model 

 will be provided for the contractile muscle fibres. 



Suhfnicroscopic striation of collagen fibres. While collagen fibrils are 

 perfectly smooth in the ordinary microscope, they appear to be 

 striated in the electron microscope. This striation was first reported 

 by ScHMiTT, Hall and Jakus (1942) and by Wolpers (1944). The 

 period of the cross-bands in collagen fibrils of the human tendons 

 and human skin is 640 A (Schmitt, Hall and Jakus, 1943; Gross, 

 1950). This corresponds to the macroperiod found in kangaroo 

 tendons by X-ray small angle diffraction (Bear, 1944). X-ray analysis 

 has also revealed the remarkable fact that the extended collagen chains 

 do not form a three-dimensional lattice, the direction of their side 

 chains changing arbitrarily in the chain lattice (Bolduan and Bear, 

 1950). 



Pratt and Wyckoff (1950) have shown that in the particularly 

 clean fibrils of collagen from dog heart, the segments are bordered 

 by pairs of cross striae (Fig. 174). Sometimes a third cross-band is 

 seen in each segment between the pairs. Then the fibril appears 

 continuously cross-striated with a period of ca. 640 A/3 = 210 A. 



