Collagen fibrils length

From the viscous stress-strain curve using Equations (4.1), (4.2), and (8.2) we can calculate the collagen fibril length. The collagen fibril lengths in tendon range from about 20 pm for during tendon development to in excess... 

Table 8.2. Collagen fibril lengths based on mechanical measurements...

Recall that collagen is an extracellular matrix protein that serves as a major constituent of many connective tissues (see figs. 4.10 to 4.13). Collagen fibrils have a distinctive banded pattern with a periodicity of 680 A. Individual fibrils are composed of three polypeptide chains wound around one another in a right-handed helix with a total length of 3,000 A. Each of the polypeptide chains in the triple helix has a repetitious tripeptide sequence, Gly-X-Y, where X is frequently a proline and Y is frequently a hy-droxyproline. The latter amino acid is not one of the 20 that are specified genetically, so it must be formed posttransla-tionally by a modification of some of the prolines. 

Figure 7.6. Effective mechanical fibril length versus fibril segment length. Plot of effective fibril length in pm determined from viscous stress-strain curves for rat tail tendon and self-assembled collagen fibers versus fibril segment length. The correlation coefficient (R2) for the line shown is 0.944 (see Silver et al., 2003).

Figure 7.9. Relationship between mechanical properties and fibril length (L) for self-assembled collagen fibers. Plot of UTS (A) and elastic slope (B) versus L in im for self-assembled type I collagen fibers stretched in tension at strain rate of 50%/min. Points with fibril lengths less than 20 pm are for uncrosslinked self-assembled type I collagen fibers and the points above 20 pm are for crosslinked fibers. The correlation coefficient for the best fit line is given by R2.

Because collagen fibrils are thin long elements, their shape factors can be estimated from the equations used for prolate ellipsoids (see Figure 4.1). For prolate ellipsoids the shape factor is equal to a constant times the ratio of the major semiaxis length a, divided by the minor semiaxis length b, raised to power of 1.81 (Equation (8.5)). 

Collagen fibrils form various patterns in the tissues of animals. In tendons, for example, they are all oriented parallel to the length of the tendon. In cowhide (one source of leather), they form fibers that branch into all directions, forming a complex structure... 

It should be pointed out that Schmitt, et al. (1942) have shown, under certain conditions in the electron microscope, that collagen fibrils can apparently extend manyfold. This led Bear (1952) to suggest that a successful model should allow for such extensibility. However, under all other conditions collagen fibers have proved essentially inextensible beyond about 10% over rest-length, and in recent years this requirement for the collagen structure seems to have been generally abandoned. 

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