The response of cells and tissues to severe acute and/or chronic injury is classically separated into two very broad processes, regeneration (perfect restoration of the original tissue) and tissue remodeling. The commonest type of tissue remodeling involves replacement of existing tissue with collagen and other extracellular matrix (ECM) proteins, otherwise known as scar tissue, a process more commonly known as fibrosis, which will be the only tissue remodeling process we will discuss today. If both processes fail, there is loss of tissue. In the real world, all three processes may occur at once.
Excess extracellular matrix interferes with organ function by mechanical obstruction of various critical functions (blood flow, gas exchange). The excess ECM changes the function of the organ due the different biologic properties of the altered amount and character of the ECM. Inadequate repair including fibrosis and tissue loss is a component of at least half of the fatal diseases in the developed world including atherosclerosis, and most chronic lung, liver and kidney diseases.
Historically, fibrosis was considered an inevitable consequence of severe or persistent tissue injury. More recent experimental evidence suggests that it is possible to directly target (reduce) fibrosis even in the setting of equivalent tissue injury. However, little current clinical therapy directly targets fibrosis. Instead therapy is most often directed either at the initial injury (e.g. hypertension in case of renal disease) or the "inflammatory component" of various fibrotic diseases thought to drive the process. Current approaches directly targeting fibrosis have had some limited clinical success to date.
Injury reactions can be divided into two basic categories:
The choice between these outcomes is determined by the balance between the severity of the injury (direct injury) and the ability of the tissue to regenerate (homeostatic pathway) and/or scar (pathologic outcome). Regeneration results from the proliferation and differentiation of native tissue cells including localized stem cells, which restores the tissue to the pre-injury state. Tissue remodeling for purposes of this session will be equated with tissue fibrosis, the deposition of extracellular matrix including collagen. Fibrotic tissue is not as functional as the original tissue but ensures some degree of structural stability after severe tissue injury. If neither process is sufficient, a cavity or tissue defect results. In cases, diffuse tissue loss occurs leading to widespread tissue destruction e.g. emphysema or osteoporosis.
Since regeneration requires tissue cells to proliferate to replace injured cells, only tissues with cells that have this property can regenerate. The epithelial component of tissues like skin and gut and the hematopoietic component of the bone marrow turn over normally and are referred to as labile. Other so called stabile organs, including liver and to some extent lung, do not normally turn over but can be induced to do so after the organ undergoes injury. Tissues that do not normally regenerate in the adult such as heart and brain are called permanent. Significant injuries to these last named tissues lead to fibrosis or, in the case of brain, gliosis and liquefaction. Even if the tissue can potentially regenerate, if the injury is severe enough to damage the underlying tissue framework e.g. in setting of extensive necrosis, there will be fibrosis.
The most dramatic acute form of fibrosis occurs when a large region of tissue undergoes necrosis such as after an infarction (heart, gut), infectious abscess, trauma or surgical intervention (surgical wound). Since there has been destruction of the underlying tissue framework, the necrotic tissue will be replaced by fibrosis or it will result in a cavity (e.g. in the lung) or defect (heart, gut, wound).
After necrosis there is extravasation of serum into the space, activation of the clotting system and acute inflammation. Platelets are exposed to extracellular matrix components, triggering aggregation, clot formation and hemostasis. Neutrophils and macrophages are recruited to digest necrotic material. This is followed by formation of so-called granulation tissue, which consists of a highly cellular mix of fibroblasts, myofibroblasts (cells with features of both smooth muscle and fibroblasts) and capillaries in a background of "provisional matrix", which is initially rich in fibrin and plasma fibronectin and then rich in hyaluronic acid (HA). At the end of the process, there is resorption of the provisional matrix, the fibroblasts and endothelial cells undergo apoptosis and there is laying down of mature extracellular matrix, especially collagen, leading to paucicellular scar tissue. The myofibroblasts promote wound contraction, which helps to bring disrupted epithelial surfaces close together. Finally, epithelial cells divide and migrate over the basal layers to regenerate the epithelium. The closing of the epithelium is required for the process to end. For skin wounds, a distinction is sometimes made between healing by first intention, in which case a single sharp cut initiates the process, which requires minimal gap filling, and healing by second intention in which case a much larger defect needs to be filled.
Disorders in acute wound repair can be divided into two basic groups: inadequate repair leading to failure of wound healing and excess wound repair, leading to exaggerated scar (e.g. keloids) and / or contraction deformities, especially on the extremities. Clinically relevant features that can prevent wound repair include superimposed infection (either an infected wound or necrotic infection), diabetes or other vascular disorder, nutritional status (including vitamin C levels), steroid therapy, mechanical factors (such as lack of mechanical stability of the wound (e.g. inadequate sutures), foreign bodies (e.g. bone fragments in trauma) and the location of the injury (tendons tend to be avascular and hence heal poorly).
Chronic injury can lead to fibrosis by two conceptually distinct processes. (Note that Robbins does not yet recognize these as two separate processes.) One involves direct chronic tissue injury such as might occur via ischemia or toxic injury e.g. alcohol in the liver. In this case, the tissue will release pro-fibrotic mediators directly (either from the cells or by fragments of extracellular matrix), will signal to local fibroblasts directly (via cell-cell contact) or will activate local tissue macrophages to release pro-fibrotic mediators. Alternatively, in the setting of chronic inflammation e.g. due to autoimmunity, extrinsic antigenic stimulation (e.g. asthma) or persistent innate immunity (e.g. atherosclerosis), macrophages may release pro-fibrotic mediators due to the inflammatory process itself. In this setting, these macrophages are commonly what are referred to as alternatively activated macrophages, which are polarized to produce pro-fibrotic (and to some degree anti-inflammatory) mediators as opposed to conventional macrophages, which produce pro-inflammatory mediators. Chronic inflammation can also directly damage tissue that then contributes to the direct pathway mentioned above. Direct tissue injury can also induce some degree of chronic inflammation via mechanisms described in a previous lecture. Due to the complexity of these varying pathways, the relative importance of each in any given disease or patient is therefore not always known.
Fibrotic tissue is less functional than healthy tissue not simply due to absence of normal parenchyma but due to aberrant behavior of cells when placed in a highly fibrotic environment. For example, fibrotic matrix is mechanically much more stiff than normal matrix, which leads to changes in cell behavior e.g. fibroblasts are more readily activated. Thus, fibrosis itself may also drive the fibrotic process.
The actual cells that produce collagen have been controversial over the years. Historically, it was thought that local fibroblasts were activated to become myofibroblasts, which then produced collagen. Over the years, other cell types have been invoked. Epithelial cells in culture and in embryology are known to undergo epithelial myoepithelial transition (EMT) in which due to epigenetic changes, cells change differentiation from epithelial to mesenchymal type of cell and produce collagen. The EMT molecular signature is commonly associated with fibrosis and experimentally interfering with the EMT process has been associated with reduced fibrosis. However, EMT appears to be quantitatively insufficient for the majority of the collagen produced suggesting instead a role in regulating fibrosis. Other proposed cell sources include pericytes (a specialized cell found around capillaries) and fibrocytes, a circulating cell type discovered at Yale, which is actually derived from bone marrow but also makes collagen.
As mentioned, granulation tissue is a major component of the wound response to necrosis. The vascularity of granulation tissue requires creation of new vascular channels followed by their regression, the process of which is well described in Robbins. The inability to create such channels, and / or the failure to have them regress, is a feature of unsuccessful tissue repair. Fibrotic tissue is typically poorly vascularized which also contributes to abnormal organ function.
A major soluble mediator that drives fibrosis is transforming growth factor (TGF) beta. It is hard to overestimate the importance of this molecule in tissue maintenance and repair. Changes in biologically active levels are principally due to activation of stores deposited in the extracellular matrix rather than from new biosynthesis. Biologically relevant levels of TGF beta are therefore hard to quantitate. Among many other functions, TGF beta leads to increased synthesis and reduced degradation of collagen. TGF beta has such a central role in maintenance of body function that direct inhibition is considered likely to be toxic. Other approaches e.g. inhibiting activation of TGF beta only in diseased tissue, have been undertaken and are in clinical trials. Other pro-fibrotic factors include platelet derived growth factor (PDGF) and interleukin-13 (IL-13), which are also subject to clinical trials.
It is important to know that while we have described fibrosis as a response to injury in such a way that makes it appear to be an unavoidable consequence of severe and/or chronic injury, there are a number of situations in which this is not true. Invertebrate and some non-mammalian adult vertebrates have the ability to undergo regeneration of tissues far beyond human adults. Human fetal tissue is much more capable of undergoing regeneration than adult tissue even under conditions that would otherwise lead to fibrosis in the adult. Numerous correlative differences are known between adult and fetal tissue that might explain this difference including reduced inflammation, altered amount and characteristics of both the newly produced extracellular matrix and growth factors, and different cell signaling responses to injury. Finally, experimental evidence in adult mammals (typically in mice) shows that it is possible to reduce scar and enhance regeneration after injury via blockade of any of a large number of pathways. These are actively being pursued as therapeutics for human disease in a wide variety of settings with some modest success to date.