CONNECTIVE TISSUE

Connective tissue deposition or collagen remodeling during the maturation phase depends on continued collagen synthesis in the presence of collagen destruction.

Collagenases and matrix metalloproteinases in the wound assist in the removal of excess collagen, although synthesis of new collagen persists. Tissue inhibitors of metallo-proteinases limit these collagenolytic enzymes so that a balance exists between formation of new collagen and removal of old collagen.

During remodeling, collagen becomes more organized. Fibronectin gradually disappears, and hyaluronic acid and glycosaminoglycans are replaced by proteogly-cans. Type III collagen is replaced by type I collagen. Water is resorbed from the

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176 Nutrition and Wound Healing

scar. These events allow collagen fibers to lie closer together, facilitating collagen cross-linking and ultimately decreasing scar thickness. Intramolecular and intermo-lecular collagen cross-links result in increased wound bursting strength. Remodeling begins approximately 21 d after injury, when the net collagen content of the wound is stable and may continue indefinitely.

The tensile strength of a wound is a measurement of its load capacity per unit area. The bursting strength of a wound is the force required to break a wound regardless of its dimension. Bursting strength varies with skin thickness. Peak tensile strength of a wound occurs approximately 60 d after injury. A healed wound reaches only approximately 80% of the tensile strength of unwounded skin. The roles of topical manganese, calcium, zinc, and ascorbic acid dressings in the treatment of chronic wounds and positive steps in tensile strength remain to be elucidated.

The zinc- and calcium-dependent family of proteins called the matrix metallo-proteinases is collectively responsible for the degradation of the extracellular matrix.

Members of this family, such as the collagenases, stromelysins, and the gelatinases, are involved in the routine tissue remodeling processes, such as wound healing, embryonic growth, and angiogenesis. Imbalance between the active enzymes and their natural inhibitors leads to the accelerated destruction of connective tissue associated with the pathology of diseases such as rheumatoid arthritis and osteoar-thritis (Figure 10.1). Micronutrients, and particularly trace elements, again, are constituents of complex physiologic processes dealing with all of these stages.

Angiogenesis (via platelet inhibition) launches the collagen/extracellular matrix interaction that initiates the release of clotting factors, growth factors, and cytokines.

In short, following this period of hemostasis, neutrophils, and the business of phago-cytosis to remove foreign materials, bacteria and damaged tissue move to the forefront.

Macrophages help with the engulfment of foreign materials and release additional growth

FIGURE 10.1 Wound healing mechanisms. So-called wound healing stages associated with these wound healing mechanisms include hemostasis, inflammation, proliferation, and remodeling.

Contraction

Epithelialization

Connective tissue deposition Amputation

Abrasion

Laceration

Trace Elements and Wound Healing 177

factors to clean up the wounded area. If this inflammation with associated reactive oxygen species (ROS) and catabolic enzymes is not halted, healing will not occur.

Once inflammation is controlled, new extracellular matrix is then formed with fibroblasts that begin the so-called proliferative phase. Fibrotic lesions (often in association with increased densities of mast cells) are characterized by excessive matrix deposition and reduced remodeling.

The cellular components and associated mediators of these stages can be found in Table 10.1.

Interleukins (about 20 have been identified) are cytokines produced by lymphocytes, monocytes, and macrophages. They regulate the cell-mediated response of the immune system. For example, interleukin-1 (IL-1) is involved in the triggering of the immune response, starting acute inflammation and maintaining chronic inflam-mation. Interleukin-2 (IL-2) is produced by helper T cells and induces proliferation of immune cells, both T and B. Interleukin-3 (IL-3) promotes the differentiation and proliferation of stem cells of the leukocyte family. Interleukin-6 (IL-6) is produced by various cells, including tumor cells, and acts as a stimulant of plasma proteins and B and T cells. Interleukin-12 (IL-12), produced by a range of cells, activates T cells and natural killer cells and promotes the response to a range of pathogens, including the human immunodeficiency viruses (HIV) of IL-2. It appears to be one of the most promising interleukins for the control of viral, bacterial, and protozoal infections.

Zinc, for example, is a trace element that is essential for immune functions. It directly induces monokine secretion by monocytes. There is also a specific inhibition of IL-1 receptor-associated protein kinase (IRAK) by zinc ions. Therefore, in contrast to an indirect stimulation of T cells due to zinc-induced monokines, higher concen-trations of zinc directly inhibit T cell functions by means of specific inhibition of IRAK and subsequent signaling events, such as NFκB activation. The complex natures of these tiny nutrients are further illustrated by these divergent effects of zinc on different cell populations that depend on zinc concentration.^3,4

Interferons are also cytokines belonging to a family of antiviral proteins that occur naturally in the body. α-Interferon and β-interferon probably exert an overall suppressive effect on the immune system. γ-Interferon is produced by immune system cells and enhances T cell recognition of antigens.

TABLE 10.1

Wound Healing Stages

Stage Cellular Components Cellular Mediators and Activity Hemostasis Endothelial cells, platelets Fibronectin, platelet-derived growth factor Inflammation Neutrophils, macrophages,

lymphocytes

Cytokines, interleukins, proteases

Proliferation Endothelial cells, fibroblasts, cytokines, epithelial cells

Collagen synthesis

Remodeling Fibroblasts, epithelial cells Collagenases, gelatinases, degradation, metalloproteinases, stromelysins 1731_C010.fm Page 177 Thursday, September 21, 2006 4:22 PM

178 Nutrition and Wound Healing

Growth factors (GFs), unlike steroids that penetrate the cell, attach to the cell membrane, and their actions are primarily autocrine (action is on the cell that produced the GF), juxtacrine (action is on the adjacent cell), paracrine (action is on the local environment), and, of least importance, endocrine (action is on a distant cell).

Fibroblast GF and GF-α and GF-β have shown to be of little benefit in wound healing; however, platelet-derived GF (AA, AB, and B) and particularly recombinant platelet-derived GF (becaptermin) have been shown to be of value in diabetic foot ulcer healing.

Platelet-rich plasmas (PRPs) are used in a variety of clinical applications, based on the premise that higher growth factor content should promote better healing. PRP supernatants promote strong osteoblast and endothelial cell divisions, supporting the concept that PRPs may be beneficial in wound healing.^5–7 There are approximately 6 kg of protein in a 70-kg human. Losing one third of this compartment almost always leads to death. Weight loss and impaired nutritional status are associated with increased complications following injury or surgery. There is a significant reduction in the plasma C-reactive protein response in malnourished patients but no difference from controls in the responses of α-1-antitrypsin, α-1-acid glycoprotein, or in the trace elements iron or zinc, which reflect induction of ferritin and metallothionein, respectively.

There is an early increase in IL-6, soluble receptors of tumor necrosis factor (TNF), and in IL-1 receptor antagonist in both groups, but there is no detectable increase in plasma TNF or IL-1. There is no difference between the well-nourished and malnourished group for any of these markers of cytokine network activation.

Weight loss is therefore associated with a reduction in aspects of the acute phase response, but this is due to impaired effectiveness rather than to reduced magnitude of the cytokine response.⁸

To sum up, in addition to this complex array of wound mediators, appropriate nutrition support, meticulous debridement, and vigilant infection control play impor-tant roles in the arena of wound healing. The extent of nutritional repletion and its impact on wound healing continues to deserve aggressive study.

Savvy clinicians would intuitively agree that good nutritional status is key for patient wellbeing and in turn must play an essential role in the wound healing process. Ignoring nutritional status may compromise the patient’s ability to heal and subsequently prolong the stages of wound healing.

TABLE 10.2

Growth Factors and Cytokines

Colony-stimulating factor (CSF) Epidermal growth factor (EGF) Fibroblast growth factor (FGF) Insulin-like growth factor (IGF) Interleukins (ILs)

Interferon-α (INF-α) and INF-α 2b Platelet-derived growth factor (PDGF) Transforming growth factor-β (TGF-β)

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PROTEIN

Protein deficiency contributes to poor healing rates through reduced collagen for-mation and wound dehiscence. High exudate loss can result in a deficit of as much as 100 g of protein in 1 d. There is a correlation between low serum albumin and body mass index (BMI) and the development of pressure ulcers. Also, low serum albumin and high Waterlow score have a positive association. The body automatically renews tissue while we are asleep, but this does not mean that protein synthesis does not take place during our wakeful hours. Glucose provides the body with its power source for wound healing, and this gives energy for angiogenesis and the deposition of new tissue. Therefore, it is vital that the body receives adequate amounts of glucose to provide additional energy for wound healing. Fatty acids are essential for cell structure and play an important role in the inflammatory process. Vitamins, particularly those associated with skin integrity (vitamins A, D, and C), are also important in wound healing.⁹

Protein synthesis at the wound site must be increased for collagen deposition and healing to occur. Patients with protein–calorie malnutrition have diminished hydoxyproline accumulation (an index of collagen deposition) in subcutaneously implanted polytetrafluoroethylene catheters compared with normally nourished controls.¹⁰