Collagens colloidal particles

Under all normal conditions the sign of the first kind of double refraction in collagen is positive relative to an optic axis paralleling the fiber axis. According to optical theory of colloidal systems (79, 188) this means that the macroscopic collagen fiber contains submicroscopic particles of anisodiametric shape with long dimensions aligned with the fiber axis. 

The intrinsic double refraction expresses the essentially anisotropic or internally crystalloidal character of the aligned particles themselves. Since formalin-treated fibers show positive intrinsic double refraction relative to the fiber axis (curve A of Fig. 29) this is probably the normal condition. Because of colloidal variabiUty of native collagen fibers the analysis of form and intrinsic components by means of immersion methods requires prior fixation. 

In general, a colloid is a system consisting of one substance (the dispersed phase a solid, liquid, or gas) finely divided and distributed evenly (relatively speaking) throughout a second substance (the dispersion medium or continuous phase a solid, liquid, or gas). Commonly encountered examples of colloids are milk (liquid fat dispersed as fine drops in an aqueous phase), smoke (solid particles dispersed in air), fog (small liquid droplets dispersed in air), paints (small solid particles dispersed in liquid), gels (polymer molecules which, when dissolved in solvent, impart a semisolid structure to the solution), and bone (small particles of calcium phosphate dispersed in a sohd matrix of collagen). 

Another way to account for the formation of polysilidc acid and colloidal silica from quartz particles is that the particles contain an unusually soluble fraction when freshly formed. Accordingly, Holt proposed an extended solubility theory, which postulated that silica dust particles can initially produce a supersaturated solution of silica which then polymerizes. Both Holt (331) and Soffge (306) emphasized the dissolution of silica followed by polymerization. The silica particles are collected and concentrated by phagocytes, creating higher local concentrations. Collagen is adsorbed and cross-linked by the silicic acid in a manner similar to silica tanning. 

---