8.7 collagen: medical applications
collagen is a major structural protein, forming molecular cables that strengthen the tendons and vast, resilient sheets that support the skin and internal organs. Bones and teeth are made by adding mineral crystals to collagen. Collagen provides structure to our bodies, protecting and supporting the softer tissues and connecting them with the skeleton. It is composed of three chains, wound together in a tight triple helix, each chain being over 1400 amino acids long. A repeated sequence of three amino acids forms this sturdy hydroxyproline structure. Every third amino acid is glycine that fits perfectly inside yhe helix. The special amino acids sequence make the tight collagen triple helix particularly stable. Every third amino acid is a glycine, and many of the remaining amino acids are proline or hydroxyproline.
The collagen molecule is a triple helix assembled from three individual protein chains. The triple helix is further assembled into larger structures known as fiber. The collagen fibers play an important role in binding platelets under conditions of blood flow. These 'type I' collagen molecules associate side-by-side, like fibers in a rope, to form tough fibrils. These fibrils crisscross the space between nearly every one of our cells. these form a basement membrane(collagen-2), which forms a tough surface that supports the skin and many organs. A different collagen ('type-IV') forms the structural basis of this membrane.
Collagen has found widespread medical uses. Urology, dermatology, orthopaedics, vascular and general surgery utilize collagen in various forms ranging from injectable solution to sponge-like materials. In addition, collagens extracted from animal species, primarily bovine, are used in the preparation of a wind variety of commercial products including:
-biological dressings
-tissue culture applications
-dermal injectables
Collagen is an ideal biomaterial for the development of medical and other commercial products because it is highly biocompatible. is readily available at high purity and can be manufactured in such diverse forms as pastes, gel, films, sponges, and felt-like sheets using a variety of process methods. In cosmetic treatments it is able to:
-smooth facial lines and wrinkles
-add definition to lip line borders
-smooth smile lines
-improve 'marionette' lines
-decrease frown lines
-improve vertical lip lines
-fill shallow acne scars
The role of collagen in blood clotting is complex and multi-factorial. Platelet- collagen interactions have received considerable attention because collagen is considered to the most thrombogenic consitituent of the vessel wall. After injury, platelets exposed to collagen in the sub-endothelial layer adhere rapidly to the exposed collagen fibrils. Platelet binding to collagen can occur through a direct platelet-collagen interaction or can be mediated via von Willebrand factor forming a bridge between collagen and platelets. Platelets bind to collagen aggregate, adsorb, and concentrate clotting factors.
Platelet-bound fibrinogen is converted to fibrin which forms a cross-linked network. The fibrin network which forms reinforce the otherwise friable platelet plug. The bound, activated platelets are completely degranulated,releasing ADP, thromboxane, and other secretory products which facililate clotting.Since the discovery of the role of collagen in blood clotting, collagen obtained from animal skin and tendon has been processed into loose fibrillar forms and felt-like sheets or collagen fleece and used to stop bleeding in an increasing number of procedures including spleen repair, laparoscopy, oral surgery, and general surgery.
Collagen also plays an essential role in the wound healing process. Acting as a tissue scaffold, it is used as a carrier vehicle for cells in tissue engineered products for dermal wound repair and as a carrier vehicle for growth factors in bond repair. Collagen fibers are one of the best scaffolds for cell migration and proliferation. Collagen interact with fibronectin and other adhesion protiens to promote cellular in-growth which speeds up wound healing. Type I collagen has been shown to attract fibroblasts in cell culture and appears to cause directed migration of cells. Drugs, cells, and growth factors use collagen as both the delivery vehicle and structural support for tissue development and in-growth.