AF also contains carbohydrates, proteins and peptides, lipids, lactate, pyruvate, electrolytes, enzymes, and hormones. The concentration of epidermal growth factor in amniotic fluid is fourfold greater than in maternal serum. AF also contains transforming growth factor (TGF)- , TGF-b1, and fibroblast growth factor (FGF). In a recent study, Moshiri and Oryan (12) demonstrated the effectiveness of FGF in restoring the morphologic and biomechanical properties of injured tendon in rabbits (13,14).
The innate immune system is the first line of defense against pathogens and includes anatomic and physiologic barriers, enzymes and antimicrobial peptides, and phagocytosis and release of pro-inflammatory mediators by neutrophils and macrophages. Many of the substances that constitute the innate immune system have been identified in AF and have been shown to have significant antimicrobial properties, including -defensins (human neutrophil defensins 1-3), lactoferrin, lysozyme, bactericidal/ permeability-increasing protein, calprotectin, secretory leukocyte protease inhibitor, psoriasin (S10 0A7), and a cathelicidin (15). These potent antimicrobials have shown broad-spectrum activity against bacteria, fungi, protozoa, and viruses. Perhaps the most important of these are the -defensins (human neutrophil defensins 1-3), which are found in significant concentrations in AF. Furthermore, lactoferrin is a glycoprotein with 2 binding sites for ferric ions. Lactoferrin is likely secreted into the AF by neutrophils and amniotic cells. Lacto-ferrin has both bacteriostatic activity, owing to the sequestration of iron, which is then unavailable for microbial growth, and bacteriocidal activity, by binding to bacterial outer membranes, triggering release of the lipopolysaccharide lactoferricin. Lactoferricin shows antimicrobial effects against viruses, protozoa, and fungi (16).
Human AF also contains factors that appear to minimize scarring, such as hyaluronic acid, which is found in high levels in AF and inhibits collagen synthesis. This hyaluronic acid-rich environment results from a relative lack of hyaluronidase in AF and the presence of hyaluronic acid-stimulating factor. In 1 study of the effect of AF on proteases important to wound healing, human AF was shown to enhance collagenase activity but to inhibit activation of hyaluronidase, elastase, and cathepsin (17,18).