Collagenous Substances

Collagenous Substances, Fibers. Collagenous fibers or fibrils (synonyms: collagen fibers, connective tissue fibers, German: Bindegewebsfasern, French: fibres conjonctifs) are one type of fiber in connective tissue; they are named so because they contain collagen and yield glue (glutin) when boiled. Collagenous fibers appear as pale, thin (0.6-0.9 μ), straight, and very long threads, which unlike elastic fibers never branch. Attempts by some authors to find an axial thread and sheath in the composition of collagenous fibers have not been confirmed. With rare exceptions, collagenous fibers are not found in isolation but are gathered into bundles, cylindrical or ribbon-like, of varying thickness; the bundles may branch. In loose connective tissue, the bundles are arranged in various directions and undulate in waves; in dense tissue (tendons, membranes) they run straight and gather into bundles of higher orders or form plates. When forming part of a bundle, the fibers are glued together by a special protein or mucilaginous substance and give the bundle a light longitudinal striation. The cementing substance dissolves in various reagents that cause the bundle to break down into separate fibrils. These include: lime water with prolonged treatment (6-8 days), barium water (4-6 hours), potassium permanganate solution, saturated picric acid, Müller's fluid. From 1% acetic acid and other acids, the bundles swell considerably; subsequent washing with water, 10% NaCl or strong acids again shrink them, and the fibrils become visible. When swelling, bundles of loose connective tissue expand unevenly and acquire a net-like shape; at constrictions, shiny circular fibers running across the bundle, obliquely or spirally, are noticeable. Their nature remains unexplained: they have been considered actual fibers of collagenous, elastic, or reticular nature, processes of stable cells covering the bundle, and finally (most often) remnants of a special sheath of the bundle that bursts and twists into cords when swelling. Under the action of heavy metal salts, chromic acid, and tanning substances, collagenous fibers become densified and insoluble, which is the basis for tanning leather. Collagenous fibers are stained by picric carmine (pink color, unlike elastic fibers which stain yellow) and acid dyes, but in some places also by basic dyes, especially in pathological cases (Cajal). Many stains for collagenous fibers have been proposed (van Gieson, M. Heidenhain, Mallory, Hansen, etc.), but they cannot be considered specific. In physical properties, collagenous fibers show double refraction (positive, axis along the length), are very slightly extensible (rupture at 5% elongation) and possess great elastic resistance (modulus of elasticity E = 2,600-8,800 kg/cm²); they bend easily. These physical properties form the basis of the physiological significance of collagenous fibers (transmission of movement by tendons, strength and easy bendability of membranes, etc.). Collagenous fibers develop from special cells—fibroblasts (see Connective tissue, development). Pathology of collagenous fibers—see Connective tissue. Collagenous substances. Besides various types of fibrous connective tissue, cartilage (cartilage glue, chondrin) and bone (bone glue, ossein) also yield glue when boiled, as they contain collagenous substances in their basis. All these types of glue differ from each other by impurities of other substances contained in the mentioned formations (chondromucoid and chondroitin sulfuric acid in cartilage, osseomucoid in bone).

V. Karpov. Collagen, collagenous substance (or collagenous substance) belongs to the group of albuminoids; it is widely distributed in the body of vertebrate animals, being the main component of the fibers of loose connective tissue, tendons, ligaments, fascia, entering into the composition of bone tissue in the form of ossein, and also forming the basis of cartilages, cornea, and fish scales in mixture with other substances. The elementary composition of collagen: C-50.75%, H-6.47%, N-17.86%, S+O-24.92%. Collagen of various tissues has somewhat different composition. Collagen is insoluble in water, in salt solutions, diluted alkalis and acids. In the latter, collagen swells. A characteristic feature of collagen is its ability, when heated with water, especially in the presence of small amounts of acid, to pass into glutin (see), which when heated to 130° again passes into a collagen-like substance. The cleavage products of collagen and glutin are the same. When collagen is hydrolyzed, glycine is formed in the greatest amount (about 20%), then glutamic acid, arginine, leucine, proline, hydroxyproline, lysine, alanine, aspartic acid. Attention is drawn to the absence of tryptophan, tyrosine, and cystine in the hydrolysis products, which determines the incompleteness of collagen as a food protein. Collagen contains a large amount of nitrogenous bases, the nitrogen of which, according to Hausmann, constitutes 35.85% of the total amount of nitrogen. The aromatic group in collagen is represented by phenylalanine. Due to the absence of tyrosine and tryptophan, collagen does not give the Millon and Lieberman reactions for proteins. Collagen dissolves in gastric and pancreatic juice; however, in the latter only on condition of preliminary swelling in a weak acid solution or heating with water to 70°. Collagen can be obtained from bones by treating them with HCl, which dissolves the mineral components of the bone, followed by thorough washing with diluted alkali and then water.