Porth Chapter 4

wound healing in elderly

A number of structural and functional changes have been reported to occur in aging skin, including a decrease in dermal thickness, a decline in collagen content, and a loss of elasticity.29,30 The observed changes in skin that occur with aging are complicated by the effects of sun exposure. Since the effects of sun exposure are cumulative, older persons show more changes in skin structure. Wound healing is thought to be progressively impaired with aging. The elderly have alterations in wound-healing phases including hemostasis and inflammation, cell proliferation, and resolution.30 Keratinocytes, fibroblasts, and vascular endothelial cells display a reduced rate of proliferation. There is also a reported decrease in angiogenesis and collagen synthesis, impaired wound contraction, and slower reepithelialization of open wounds. Although wound healing may be delayed, most wounds heal, even in the debilitated elderly patient undergoing major surgical procedures. The elderly are more vulnerable to chronic wounds, chiefly pressure, diabetic, and ischemic ulcers, than younger persons, and these wounds heal more slowly. However, these wounds are more likely due to other disorders such as immobility, diabetes mellitus, or vascular disease, rather than aging.

Stem cells

Another type of tissue cell, called a stem cell, remains incompletely differentiated throughout life.1,2,5 In most continuously dividing tissues, the mature cells are terminally differentiated and short lived. As mature cells die the tissue is replenished by the differentiation of cells generated from stem cells. Stem cells are reserve cells that remain quiescent until there is a need for cell replenishment, in which case they divide, producing other stem cells and cells that can carry out the functions of the differentiated cell. When a stem cell divides, one daughter cell retains the stem cell characteristics, and the other daughter cell becomes a progenitor cell that undergoes a process that leads to terminal differentiation. Stem cells are characterized by three important properties: self-renewal, asymmetric replication, and differential potential.1,2 Self-renewal means that the stem cells can undergo numerous mitotic divisions while maintaining an undifferentiated state. Asymmetric replication means that after each cell division, some progeny of the stem cell enter a differentiation pathway, while others remain undifferentiated, retaining their self-renewal capacity. The progeny of each progenitor cell follows a more restricted genetic program, with the differentiating cells undergoing multiple mitotic divisions in the process of becoming a more mature cell type, and with each generation of cells becoming more specialized. In this way, a single stem cell can give rise to the many cells needed for normal tissue repair or blood cell production. The term potency is used to define the differentiation potential of stem cells. Totipotent stem cells are those produced by a fertilized ovum. The first few cells produced after fertilization are totipotent and can differentiate into embryonic and extraembryonic cells. Totipotent stem cells give rise to pluripotent stem cells that can differentiate into the three germ layers of the embryo. Multipotent stem cells are cells such as hematopoietic stem cells that give rise to a family of cells, including the red blood cells and all the various types of leukocytes. Finally, unipotent stem cells produce only one cell type but retain the property of self-renewal.

Influence of growth factors

Cell proliferation can be triggered by chemical mediators including growth factors, hormones, and cytokines.1,2,8-11 The term growth factor is generally applied to small proteins that increase cell size and cell division.2 In addition to cell proliferation, most growth factors have other effects. They assist in regulating the inflammatory process; serve as chemoattractants for neutrophils, monocytes (macrophages), fibroblasts, keratinocytes, and epithelial cells; stimulate angiogenesis; and contribute to the generation of the ECM. Some growth factors stimulate the proliferation of some cells and inhibit the cycling of other cells. In fact, a growth factor can have opposite effects on the same cell depending on its changing concentration during the healing process. Many of the growth factors are produced by leukocytes recruited to the site of injury or activated at the site by the inflammatory process. Other growth factors are produced by parenchymal cells or stromal cells in response to injury or loss. Growth factors are named for their tissue of origin (e.g., platelet-derived growth factor [PDGF], fibroblast growth factor [FGF]), their biologic activity (e.g., transforming growth factor [TGF]), or the cells on which they act (e.g., vascular endothelial growth factor [VEGF]). The sources and functions of selected growth factors are described in Table 4-1. The signaling pathways for the growth factors are similar to those of other cellular receptors that recognize extracellular ligands. The binding of a growth factor to its receptor triggers a series of events by which extracellular signals are transmitted into the cell, leading to the stimulation or inhibition of gene expression. These genes typically have several functions—they relieve blocks on cell cycle progression (thus promoting cell proliferation), prevent apoptosis, and enhance synthesis of cellular proteins in preparation for mitosis. Signaling may occur in the cell producing the growth factor (autocrine signaling), in cells in the immediate vicinity of the cell releasing the growth factor (paracrine signaling), or in distant target cells through growth factors that are released into the bloodstream (endocrine signaling). There is continued interest in developing growth factors as a means of increasing cell proliferation and enhancing wound healing as well as developing strategies to block growth factor signaling pathways that could be used to inhibit malignant cell proliferation in cancer.

Cell proliferation

Cell proliferation refers to the process of increasing cell numbers by mitotic division. Cell differentiation is the process whereby a cell becomes more specialized in terms of structure and function.1,2 Stem cells are undifferentiated cells that have the capacity to generate multiple cell types (to be discussed). In normal tissue the size of the cell population is determined by a balance of cell proliferation, death by apoptosis (see Chapter 2), and emergence of newly differentiated cells from stem cells2 (Fig. 4-1). Several cell types proliferate during tissue repair including remnants of injured parenchymal tissue cells, vascular endothelial cells, and fibroblasts. The proliferation of these cell types is driven by proteins called growth factors. The production of growth factors and the ability of these cells to respond and expand in sufficient numbers are important determinants of the repair process. All of the different cell types in the body originate from a single cell—the fertilized ovum. As the embryonic cells increase in number, they differentiate, facilitating the development of all the different cells and organs of the body. The process of differentiation is regulated by a combination of internal processes involving the expression of specific genes and external stimuli provided by neighboring cells, the ECM, and a variety of growth factors. The process occurs in orderly steps, with each progressive step being exchanged for a loss of ability to develop different cell characteristics. As a cell becomes more highly specialized, the stimuli that are able to induce mitosis become more limited. Neurons, which are highly specialized cells, lose their ability to proliferate once development of the nervous system is complete. In other, less-specialized tissues, such as the skin and mucosal lining of the gastrointestinal tract, a high degree of cell renewal continues throughout life. Even in these continuously renewing cell populations, the more specialized cells are unable to divide. Many of these cell populations rely on progenitor or parent cells of the same lineage. Progenitor cells are sufficiently differentiated so that their daughter cells are limited to the same cell line, but they have not reached the point of differentiation that precludes the potential for active proliferation. Some cell populations have self-renewing multipotent stem cells, such as the epithelial stem cells, that can differentiate into the different cell types throughout life.

Healing by Primary and Secondary Intention

Depending on the extent of tissue loss, wound closure and healing occur by primary or secondary intention (Fig. 4-6). A sutured surgical incision is an example of healing by primary intention. Larger wounds (e.g., burns and large surface wounds) that have a greater loss of tissue and contamination heal by secondary intention. Healing by secondary intention is slower than healing by primary intention and results in the formation of larger amounts of scar tissue. A wound that might otherwise have healed by primary intention may become infected and heal by secondary intention.

Angiogenesis and Ingrowth of Granulation Tissue

Granulation tissue is a glistening red, moist connective tissue that fills the injured area while necrotic debris is removed1,2 (Fig. 4-5). It is composed of newly formed capillaries, proliferating fibroblasts, and residual inflammatory cells. The development of granulation tissue involves the growth of new capillaries (angiogenesis). Angiogenesis is a tightly regulated process that includes migration of endothelial cells to the site of tissue injury, formation of capillary buds, and proliferation of endothelial cells, followed by fusion and remodeling of the endothelial cells into capillary tubes. Several growth factors induce angiogenesis, but the most important are VEGF and basic FGF-2. In angiogenesis, VEGF stimulates both proliferation and motility of endothelial cells, thus initiating the process of capillary sprouting. FGF-2 participates in angiogenesis mainly by stimulating the proliferation of endothelial cells. During angiogenesis, new blood vessels are leaky because of incompletely formed interendothelial cell junctions and because VEGF increases vascular permeability. This leakiness explains the edematous appearance of granulation tissue and accounts in part for the swelling that may persist in healing wounds long after the acute inflammation has subsided.

Blood Flow and Oxygen Delivery

Impaired healing due to poor blood flow and hypoxia may occur as a result of wound conditions (e.g., swelling) or preexisting health problems.24,25 Arterial disease and venous pathology are well-documented causes of impaired wound healing. In situations of trauma, a decrease in blood volume may cause a reduction in blood flow to injured tissues. For healing to occur, wounds must have adequate blood flow to supply the necessary nutrients and to remove the resulting waste, local toxins, bacteria, and other debris. Molecular oxygen is required for collagen synthesis and killing of bacteria by phagocytic white blood cells. It has been shown that even a temporary lack of oxygen can result in the formation of less-stable collagen.24 Wounds in ischemic tissue become infected more frequently than wounds in well-vascularized tissue. Neutrophils and macrophages require oxygen for destruction of microorganisms that have invaded the area. Although these cells can accomplish phagocytosis in a relatively anoxic environment, they cannot digest bacteria. Oxygen also contributes to signaling systems that support wound healing. Recent research suggests that almost all cells in the wound environment are fitted with specialized enzymes to convert oxygen to reactive oxygen species (ROS).24 These ROS function as cellular messengers that support wound healing, stimulating cytokine action, angiogenesis, cell motility, and extracellular matrix formation. The availability of respired oxygen to wound tissues depends on vascular supply, vasomotor tone, the partial pressure of oxygen (PO2) in arterial blood, and the diffusion distance for oxygen (see Chapter 21). The central area of a wound has the lower oxygen level, with dermal wounds ranging from a PO2 of 0 to 10 mm Hg centrally to 60 mm Hg in the periphery, while the PO2 of arterial blood is approximately 100 mm Hg.24 Transcutaneous oxygen sensors are available for use in measuring wound oxygenation. From a therapeutic standpoint oxygen can be given systemically or administered locally using a topical device. Although topical oxygen therapy is not likely to diffuse into the deeper tissues, it does have the advantage of oxygenating superficial areas of the wound not supported by intact vasculature. Hyperbaric oxygen therapy delivers 100% oxygen at two to three times the normal atmospheric pressure at sea level.26 The goal of hyperbaric oxygen therapy is to increase oxygen delivery to tissues by increasing the partial pressure of oxygen dissolved in the plasma. Hyperbaric oxygen is currently reserved for the treatment of problem wounds in which hypoxia and infection interfere with healing.

Cell cycle

In order to understand cell proliferation, whether physiologic (as in tissue regeneration and repair) or pathologic (as in cancer), it is important to learn about the cell cycle, an orderly sequence of events in which a cell duplicates its genetic contents and divides. During the cell cycle, the duplicated chromosomes are appropriately aligned for distribution between two genetically identical daughter cells. The cell cycle is divided into four distinct phases referred to as G1, S, G2, and M. Gap 1 (G1) is the postmitotic phase during which deoxyribonucleic acid (DNA) synthesis ceases while ribonucleic acid (RNA) and protein synthesis and cell growth take place (see Understanding the Cell Cycle).1-4 During the S phase, DNA synthesis occurs, giving rise to two separate sets of chromosomes, one for each daughter cell. G2 is the premitotic phase and is similar to G1 in that DNA synthesis ceases while RNA and protein synthesis continue. Collectively, G1, S, and G2 are referred to as interphase. The M phase is the phase of nuclear division and cytokinesis. Continually dividing cells, such as the stratified squamous epithelium of the skin, continue to cycle from one mitotic division to the next. When environmental conditions are adverse, such as nutrient or growth factor unavailability, or cells become terminally differentiated (i.e., highly specialized), cells may exit the cell cycle, becoming mitotically quiescent and reside in a special resting state known as G0. Cells in G0 may reenter the cell cycle in response to extracellular nutrients, growth factors, hormones, and other signals such as blood loss or tissue injury that trigger cell renewal. Highly specialized and terminally differentiated cells, such as neurons, may permanently stay in G0. Within the cell cycle are checkpoints where pauses or arrests can be made if the specific events in the phases of the cell cycle have not been completed. There are also opportunities for ensuring the accuracy of DNA replication. These DNA damage checkpoints allow for any defects to be edited and repaired, thereby ensuring that each daughter cell receives a full complement of genetic information, identical to that of the parent cell.1-3 The cyclins are a family of proteins that control the entry and progression of cells through the cell cycle.1-4 Cyclins bind to (thereby activating) proteins called cyclin-dependent kinases (CDKs). Kinases are enzymes that phosphorylate proteins. The CDKs phosphorylate specific target proteins and are expressed continuously during the cell cycle but in an inactive form, whereas the cyclins are synthesized during specific phases of the cell cycle and then degraded once their task is completed. Different arrangements of cyclins and CDKs are associated with each stage of the cell cycle. For example, cyclin B and CDK1 control the transition from G2 to M. As the cell moves into G2, cyclin B is synthesized and binds to CDK1. The cyclin B-CDK1 complex then directs the events leading to mitosis, including DNA replication and assembly of the mitotic spindle. Although each phase of the cell cycle is monitored carefully, the transition from G2 to M is believed to be one of the most important checkpoints in the cell cycle. In addition to the synthesis and degradation of the cyclins, the cyclin-CDK complexes are regulated by the binding of CDK inhibitors. The CDK inhibitors are particularly important in regulating cell cycle checkpoints during which mistakes in DNA replication are repaired. The finding that the cell cycle can be reactivated by removing CDK inhibitors in quiescent and nonproliferating cells has potential implications for tissue repair and cell replacement therapy.4

Impaired Inflammatory and Immune Responses

Inflammatory and immune mechanisms function in wound healing. Inflammation is essential to the first phase of wound healing, and immune mechanisms prevent infections that impair wound healing. Among the conditions that impair inflammation and immune function are disorders of phagocytic function, diabetes mellitus, and therapeutic administration of corticosteroid drugs. Phagocytic disorders may be divided into extrinsic and intrinsic defects. Extrinsic disorders are those that impair attraction of phagocytic cells to the wound site, prevent engulfment of bacteria and foreign agents by the phagocytic cells (i.e., opsonization), or cause suppression of the total number of phagocytic cells (e.g., immunosuppressive agents). Intrinsic phagocytic disorders are the result of enzymatic deficiencies in the metabolic pathway for destroying the ingested bacteria by the phagocytic cell. The intrinsic phagocytic disorders include chronic granulomatous disease, an X-linked inherited disease in which there is a deficiency of myeloperoxidase and nicotinamide adenine dinucleotide peroxidase (NADPH)-dependent oxidase enzyme. Deficiencies of these compounds prevent generation of hydrogen superoxide and hydrogen peroxide needed for killing bacteria. Wound healing is a problem in persons with diabetes mellitus, particularly those who have poorly controlled blood glucose levels.27 Studies have shown delayed wound healing, poor collagen formation, and poor tensile strength in diabetic animals. Of particular importance is the effect of hyperglycemia on phagocytic function. Neutrophils, for example, have diminished chemotactic and phagocytic function, including engulfment and intracellular killing of bacteria, when exposed to altered glucose levels. Small blood vessel disease is also common among persons with diabetes, impairing the delivery of inflammatory cells, oxygen, and nutrients to the wound site. The therapeutic administration of corticosteroid drugs decreases the inflammatory process and may delay the healing process. These hormones decrease capillary permeability during the early stages of inflammation, impair the phagocytic property of the leukocytes, and inhibit fibroblast proliferation and function.

phases of repair

Repair by connective tissue deposition can be divided into three phases: (1) hemostasis, angiogenesis, and ingrowth of granulation tissue; (2) emigration of fibroblasts and deposition of extracellular matrix; and (3) maturation and reorganization of the fibrous tissue (remodeling).1,2,10-16 It usually begins within 24 hours of injury and is evidenced by the migration of fibroblasts and the induction of fibroblast and endothelial cell proliferation.2 By 3 to 5 days, a special type of tissue called granulation tissue is apparent.2 The granulation tissue then progressively accumulates connective tissue, eventually resulting in the formation of a scar, which is then remodeled.

Emigration of Fibroblasts and Deposition of Extracellular Matrix

Scar formation builds on the granulation tissue framework of new vessels and loose ECM. The process occurs in two phases: emigration and proliferation of fibroblasts into the site of injury, and deposition of extracellular matrix by these cells. The recruitment and proliferation of fibroblasts is mediated by a number of growth factors including FGF-2 and TGF-β. These growth factors are released from endothelial cells and from inflammatory cells that are present at the site of injury. As healing progresses, the number of proliferating fibroblasts and formation of new vessels decrease and there is increased synthesis and deposition of collagen. Collagen synthesis is important to the development of strength in the healing wound site. Ultimately, the granulation tissue scaffolding evolves into a scar composed of largely inactive spindle-shaped fibroblasts, dense collagen fibers, fragments of elastic tissue, and other ECM components. As the scar matures, vascular degeneration eventually transforms the highly vascular granulation tissue into a pale, largely avascular scar.

Proliferative capacity of tissues

The capacity for regeneration varies with the tissue and cell type. Body tissues are divided into three types depending on the ability of their cells to undergo regeneration: (1) continuously dividing, (2) stable, and (3) permanent tissues.1,2 Continuously dividing or labile tissues are those in which the cells continue to divide and replicate throughout life, replacing cells that are continually being destroyed. They include the surface epithelial cells of the skin, oral cavity, vagina, and cervix; the columnar epithelium of the gastrointestinal tract, uterus, and fallopian tubes; the transitional epithelium of the urinary tract; and bone marrow cells. These tissues can readily regenerate after injury as long as a pool of stem cells is preserved. Bleeding, for example, stimulates the rapid proliferation of replacement cells by the blood-forming progenitor cells of the bone marrow. Stable tissues contain cells that normally stop dividing when growth ceases. Cells in these tissues remain quiescent in the G0 stage of the cell cycle. However, these cells are capable of undergoing regeneration when confronted with an appropriate stimulus; thus, they are capable of reconstituting the tissue of origin. Stable cells constitute the parenchyma of solid organs such as the liver and kidney. They also include smooth muscle cells, vascular endothelial cells, and fibroblasts, the proliferation of which is particularly important to wound healing. The cells in permanent tissues do not proliferate. The cells in these tissues are considered to be terminally differentiated and do not undergo mitotic division in postnatal life. The permanent cells include nerve cells, skeletal muscle cells, and cardiac muscle cells. These cells do not normally regenerate; once destroyed, they are replaced with fibrous scar tissue that lacks the functional characteristics of the destroyed tissue. Stem Cells

fibosis

The formation of fibrous connective tissue, as in the repair or replacement of parenchymatous elements.

inflammatory phase

The inflammatory phase of wound healing begins at the time of injury and is a critical period because it prepares the wound environment for healing.18 It includes hemostasis (see Chapter 12) and the vascular and cellular phases of inflammation. Hemostatic processes are activated immediately at the time of injury. There is constriction of injured blood vessels and initiation of blood clotting by way of platelet activation and aggregation and deposition of fibrin. After a brief period of constriction, these same vessels dilate and capillaries increase their permeability, allowing plasma and blood components to leak into the injured area. In small surface wounds, the clot loses fluid and becomes a hard, desiccated scab that protects the area. The cellular phase of inflammation follows and is evidenced by the migration of phagocytic white blood cells that digest and remove invading organisms, fibrin, extracellular debris, and other foreign matter. The neutrophils arrive within minutes and are usually gone by day 3 or 4. They ingest bacteria and cellular debris. Within 24 to 48 hours, macrophages, which are larger phagocytic cells, enter the wound area and remain for an extended period. These cells, arising from blood monocytes, are essential to the healing process. Their functions include phagocytosis and release of growth factors that stimulate epithelial cell growth, angiogenesis, and attraction of fibroblasts. When a large wound occurs in deeper tissues, neutrophils and macrophages are required to remove the debris and facilitate closure. Although a wound may heal in the absence of neutrophils, it cannot heal in the absence of macrophages.

Proliferative Phase

The proliferative phase of healing usually begins within 2 to 3 days of injury and may last as long as 3 weeks in wounds healing by primary intention. The primary processes during this time focus on the building of new tissue to fill the wound space. The key cell during this phase is the fibroblast. The fibroblast is a connective tissue cell that synthesizes and secretes collagen and other intercellular elements needed for wound healing. Fibroblasts also produce numerous growth factors that induce angiogenesis and endothelial cell proliferation and migration. As early as 24 to 48 hours after injury, fibroblasts and vascular endothelial cells begin proliferating to form the granulation tissue that serves as the foundation for scar tissue development. This tissue is fragile and bleeds easily because of the numerous, newly developed capillary buds. Wounds that heal by secondary intention have more necrotic debris and exudate that must be removed, and they involve larger amounts of granulation tissue. The newly formed blood vessels are leaky and allow plasma proteins and white blood cells to leak into the tissues. The final component of the proliferative phase is epithelialization, which is the migration, proliferation, and differentiation of the epithelial cells at the wound edges to form a new surface layer that is similar to that destroyed by the injury. In wounds that heal by primary intention, these epithelial cells proliferate and seal the wound within 24 to 48 hours. Because epithelial cell migration requires a moist vascular wound surface and is impeded by a dry or necrotic wound surface, epithelialization is delayed in open wounds until a bed of granulation tissue has formed. When a scab has formed on the wound, the epithelial cells migrate between it and the underlying viable tissue; when a significant portion of the wound has been covered with epithelial tissue, the scab lifts off. At times, excessive granulation tissue, sometimes referred to as proud flesh, may form and extend above the edges of the wound, preventing reepithelialization from taking place. Surgical removal or chemical cauterization of the defect allows healing to proceed. As the proliferative phase progresses, there is continued accumulation of collagen and proliferation of fibroblasts. Collagen synthesis reaches a peak within 5 to 7 days and continues for several weeks, depending on wound size. By the second week, the white blood cells have largely left the area, the edema has diminished, and the wound begins to blanch as the small blood vessels become thrombosed and degenerate.

Remodeling Phase

The third phase of wound healing, the remodeling process, begins approximately 3 weeks after injury and can continue for 6 months or longer, depending on the extent of the wound. As the term implies, there is continued remodeling of scar tissue by simultaneous synthesis of collagen by fibroblasts and lysis by collagenase enzymes. As a result of these two processes, the architecture of the scar becomes reoriented to increase the tensile strength of the wound. Most wounds do not regain the full tensile strength of unwounded skin after healing is completed. Carefully sutured wounds immediately after surgery have approximately 70% of the strength of unwounded skin, largely because of the placement of the sutures. This allows persons to move about freely after surgery without fear of wound separation. When the sutures are removed, usually at the end of the 1st week, wound strength is approximately 10%. It increases rapidly over the next 4 weeks and then slows, reaching a plateau of approximately 70% to 80% of the tensile strength of unwounded skin at the end of 3 months.2 An injury that heals by secondary intention undergoes wound contraction during the proliferative and remodeling phases. As a result, the scar that forms is considerably smaller than the original wound. Cosmetically, this may be desirable because it reduces the size of the visible defect. However, contraction of scar tissue over joints and other body structures tends to limit movement and cause deformities. As a result of loss of elasticity, scar tissue that is stretched fails to return to its original length. An abnormality in healing by scar tissue repair is keloid formation.20 Keloids are benign tumorlike masses caused by excess production of scar tissue (Fig. 4-7). They tend to develop in genetically predisposed individuals and are more common in African Americans and other dark skinned people.1,2,19 The majority of keloids lead to considerable cosmetic defects, but can grow to sufficient size to become symptomatic by causing deformity or limiting joint mobility. Thus far the majority of keloid research has focused on growth factors and signaling pathways, but unfortunately, reliable preventative or treatment measures have yet to be established.

Maturation and Remodeling of the Fibrous Tissue

The transition from granulation to scar tissue involves shifts in the modification and remodeling of the ECM. The outcome of the repair process is, in part, a balance between previously discussed ECM synthesis and degradation. The rate of collagen synthesis diminishes until it reaches equilibrium with collagen degradation. The degradation of collagen and other ECM proteins is achieved through a family of metalloproteinases, which require zinc for their activity. The metalloproteinases are produced by a variety of cell types (fibroblasts, macrophages, synovial cells, and some epithelial cells), and their synthesis and secretion are regulated by growth factors, cytokines, and other agents.10,11 Their synthesis may be suppressed pharmacologically by corticosteroids. Metalloproteinases are typically released as inactive precursors that require activation by enzymes, such as proteases, that are present at sites of injury.

Extracellular Matrix and Cell-Matrix Interactions

The understanding of tissue regeneration and repair has expanded over the past several decades to encompass the complex environment of the ECM. There are two basic forms of ECM: (1) the basement membrane, which surrounds epithelial, endothelial, and smooth muscle cells; and (2) the interstitial matrix, which is present in the spaces between cells in connective tissue and between the epithelium and supporting cells of blood vessels. The ECM is secreted locally and assembles into a network of spaces surrounding tissue cells (see Chapter 1). There are three basic components of the ECM: fibrous structural proteins (e.g., collagen and elastin fibers), water-hydrated gels (e.g., proteoglycans and hyaluronic acid) that permit resilience and lubrication, and adhesive glycoproteins (e.g., fibronectin, laminin) that connect the matrix elements to one another and to cells1-3,9 (Fig. 4-3). Integrins are a family of transmembrane glycoproteins that are the main cellular receptors for ECM components such as fibronectin and laminin. They bind to many ECM components, initiating signaling cascades that affect cell proliferation and differentiation. Fibroblasts, which reside in close proximity to collagen fibers, are responsible for the synthesis of collagen, elastic, and reticular fibers, and complex carbohydrates in the ground substance. The ECM provides turgor to soft tissue and rigidity to bone; it supplies the substratum for cell adhesion; it is involved in the regulation of growth, movement, and differentiation of the cells surrounding it; and it provides for the storage and presentation of regulatory molecules that control the repair process. The ECM also provides the scaffolding for tissue renewal. Although the cells in many tissues are capable of regeneration, injury does not always result in restoration of normal structure unless the ECM is intact. The integrity of the underlying basement membrane, in particular, is critical to the regeneration of tissue. When the basement membrane is disrupted, cells proliferate in a haphazard way, resulting in disorganized and nonfunctional tissues. Critical to the process of wound healing are transitions in the composition of the ECM. In the transitional process, the ECM components are degraded by proteases (enzymes) that are secreted locally by a variety of cells (fibroblasts, macrophages, neutrophils, synovial cells, and epithelial cells). Some of the proteases, such as the collagenases, are highly specific, cleaving particular proteins at a small number of sites.10,11 This allows for the structural integrity of the ECM to be retained while healing occurs. Because of their potential to produce havoc in tissues, the actions of the proteases are tightly controlled. They are typically produced in an inactive form that must first be activated by certain chemicals likely to be present at the site of injury, and they are rapidly inactivated by tissue inhibitors. Recent research has focused on the unregulated action of the proteases in disorders such as cartilage matrix breakdown in arthritis and neuroinflammation in multiple sclerosis.11

Infection, Wound Separation, and Foreign Bodies

Wound contamination, wound separation, and foreign bodies delay wound healing. Infection impairs all dimensions of wound healing.28 It prolongs the inflammatory phase, impairs the formation of granulation tissue, and inhibits proliferation of fibroblasts and deposition of collagen fibers. All wounds are contaminated at the time of injury. Although body defenses can handle the invasion of microorganisms at the time of wounding, badly contaminated wounds can overwhelm host defenses. Trauma and existing impairment of host defenses also can contribute to the development of wound infections. Approximation of the wound edges (i.e., suturing of an incision type of wound) greatly enhances healing and prevents infection. Epithelialization of a wound with closely approximated edges occurs within 1 to 2 days. Large, gaping wounds tend to heal more slowly because it is often impossible to effect wound closure with this type of wound. Mechanical factors such as increased local pressure or torsion can cause wounds to pull apart, or dehisce. Foreign bodies tend to invite bacterial contamination and delay healing. Fragments of wood, steel, glass, and other compounds may have entered the wound at the site of injury and can be difficult to locate when the wound is treated. Sutures are also foreign bodies, and although needed for the closure of surgical wounds, they are an impediment to healing. This is why sutures are removed as soon as possible after surgery. Wound infections are of special concern in persons with implantation of foreign bodies such as orthopedic devices (e.g., pins, stabilization devices), cardiac pacemakers, and shunt catheters. These infections are difficult to treat and may require removal of the device.