Collagen fiber structure

Collagen fibers are long and thick in the middle dermis but become increasingly fine toward the outer epidermis as well as toward the inner hypodermis. Apart from this structural characteristic, which is common to all animals, the skin of each species has a different and unique morphology that significantly affects its properties (Calnan and Haines 1991). 

The collagen fibers leave small compartments where apatite nanocrystals are deposited during a controlled biomineralization process [20]. The collagen acts as a structural framework in which plate-like nanocrystals of carbonated hydroxyapatite (CHA) are embedded to strengthen the bone. The chemical formula of biological CHA can be represented as follows ... 

As another extracellular component in the cornea, the Bowman s layer is an acellular and amorphous band between the corneal epithelium and stroma. The layer is about 8-12 [im thick and consists of randomly arranged collagen fibers (types I and III) and proteoglycans. The physiological function of Bowman s layer is not yet completely understood, since not all animal species exhibit this membrane in the corneal structures, but an important role in the maintenance of the corneal epithelial structure is expected or probable, since a damaged Bowman s membrane usually results in scarring during wound repair [16],... 

Defects in collagen synthesis, structure, or assembly Into fibers are the principal basis for a group of connective tissue disorders called Ehlers-Danlos syndrome (EDS). 

Figure 5.125 Structure of collagen fibers based on collagen molecule, microfibrils and fibrillar aggregation. Reprinted, by permission, from I. V. Yannas, in Biomaterials Science, B. D. Ratner, A. S. Hoffman, F. J. Schoen, and J. E. Lemons, eds., p. 86. Copyright 1996 by Academic Press.

In contrast to soft biologies, whose mechanical properties primarily depend upon the orientation of collagen fibers, the mechanical properties of mineralized tissues, or hard biologies, are more complicated. Factors such as density, mineral content, fat content, water content, and sample preservation and preparation play important roles in mechanical property determination. Specimen orientation also plays a key role, since most hard biologies such as bone are composite structures. For the most part, we will concentrate on the average properties of these materials and will relate these values to those of important, man-made replacement materials. 

However, it is well known that with increasing age microfibrosis is observed which in turn will seperate the fibers from each other and thereby enhance the degree of nonuniformity as discussed in the first chapter of this book. This is accompanied by a reduction in side-to-side connections [Spach and Dolber, 1986]. Thus, with increasing age the intercellular communication can be expected to be reduced probably due to structural changes in the tissue with deposition of collagenous fibers. Concomitant changes in the gap junction distribution are probably secondary to cardiac diseases, although at present an effect of age per se cannot be excluded. 

The sclera and the cornea are the toughest and outermost layers of the eye and resist the normal internal pressure of 13 to 19 mmHg. This intraocular pressure (IOP) gives the eye its shape and maintains its dimensions that are necessary for sharp vision. The sclera covers 5/6 of the eye s surface and the cornea the remaining 1/6. Although the principal structural element of both tissues comprises of type 1 collagen fibers, differences in size and orientation of the fibers, degree of hydration, and presence of mucopolysaccharides are responsible for differences in transparency. The avascular cornea receives nourishment from the tear film, the aqueous humor, and the limbal vessels. In contrast, the sclera is vascularized and is supplied by several blood vessels, particularly in the uppermost layers (episclera). 

The vitreous humor volume is about 4 ml in an adult. Its viscosity is 2-4 times that of water and is dependent on the concentration of sodium hyaluronate. Although it has the outward appearance of a transparent, viscoelastic gel, it contains fine diameter type II collagen fibers (8-12 nm diameter) that entrap the large coiled hyaluronic acid molecules. These fibers give the gel a spherical structure with a dent in the anterior surface (the hyaloid fossa). The main constituent of the vitreous is water (98%) with a refractive index of 1.33, but the gel is... 

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