All multicellular organisms must respond to injury. However, the outcome of such responses can either generate fibrotic tissue, simply close a gap in the organ without replacement, or proceed to regenerate the missed tissue in fully functional form [25
]. While the differences are often striking as one proceeds along the phylogenetic tree, even in humans the outcomes diverge [27
]. For example, fetal healing is typically characterized as a process that is rapid and results in regeneration of physiological tissue in a process referred to as scarless healing [27
]. In contrast, a much slower process that results in scarring is seen in adult healing [31
]. A number of underlying diseases or genetic predispositions accentuate dysregulated healing or even activate the scarring process in the absence of insult. This leads to pathological conditions as seen in a variety of fibrotic diseases such as single organ fibrosis of the lung, heart, liver or more widely distributed diseases like scleroderma or connective tissue diseases [33
]. Even though fibrosis may not be the primary triggering event in these diseases, it is the main element of the morbidity and mortality [36
]. These diseases are thus characterized by ongoing fibrotic generation in the absence of external injury or insult. Understanding the fibrotic aspect and the mechanism of such will serve as a key in developing therapeutic approaches.
For all organs, the normal adult response to injury occurs in four, overlapping but inter-related, time-dependent phases: hemostasis, inflammation, tissue formation/regeneration, and tissue remodeling/resolution [1
]. All these events require extensive communication between diverse cell types and input from the prior phase [38
]. Hemostasis initiates with the platelet plug and other coagulation events, such as serum protein cross-linking and vascular constriction [7
]. The aim is to prevent exsanguination and maintain adequate volume and pressure for uninjured organs and tissue. In addition, the serum proteins and platelet lysate provide a provisional matrix to support the wave of cells that follow. Inflammation is concomitant with this initial phase as numerous elements of the innate immunity are trapped within the initial platelet plug [41
]. In addition to further infiltration of the hemostatic plug secondary to chemoattractants released by platelets, these cells, mainly neutrophils and macrophages, provide for anti-microbial defense and remove dead cells and tissue [42
]. Importantly for our thesis, these cells produce ECM-modifying enzymes and, the macrophages in particular, matrix components that, together with the deposited fibrin, influence the entire subsequent reparative process. It is these deposits that hold damaged tissue together and provide a provisional matrix for the initial recruitment of inflammatory cells and later the migration of other resident cells including vessels and nerves [1
Fig. 1 Healing by secondary intention involves the closure of a large open defect, and in skin wound healing proceeds through stages that effect repair approaching the original tissue via a wave of signaling factors that orchestrate the cells to generate a paucicellular (more ...)
Neutrophils are the most abundant cells in the early inflammatory stages of healing. Neutrophils have diverse roles being part of innate immunity and signaling to other immune and adherent cells. As a critical phagocyte, neutrophils destroy and remove bacteria, foreign particles and damaged tissue. As neutrophils die, macrophages predominate, accumulating at the wound site following recruitment from circulation and from the resident population [48
]. The two initial waves of macrophages, over the first few days post-wounding, involve mainly a M1 phenotype that if not present leads to impaired healing with limited wound maturation [48
]. These macrophages act in unison with neutrophils to phagocytose debris and invading pathogenic microorganisms. A second role for these cells is as a source of chemoattractants and growth factors such as TGF-α
, HB-EGF, PDGF, and TGF-β
that act on the formed elements of the wound to promote immigration, proliferation and survival of these cells [44
]. These factors are also angiogenic [52
]. The invading cells, along with the macrophages, change the matrix from a loose fibrin lattice into the provisional matrix rich in fibronectin, hyaluronic acid, tenascin, and collagen and laminin types representative of an immature matrix [48
This invasion by formed elements—stromal cells, epithelial cells, vasculature, and nerves—marks the tissue formation/regeneration phase. This is characterized by the presence of relatively undifferentiated cells that are both proliferative and migratory, but do not fully function as their mature counterparts in the unwounded tissue. For the epithelial cells, this may be considered a transient epithelial mesenchymal transition (EMT), in which the mesenchymal keratinocytes cells present a less mature phenotype that not only provides for enhanced migration but also produces matrix components [56
]. Of note, the matrix at this phase resembles that during development and even tumor invasion. It has been shown that such a matrix is not just a product of an active unformed tissue but a stimulator of such. Tumor-associated matrix can drive a transient EMT in normal epithelial cells [7
]. The high levels of tenascin-C, fibronectin, and entactin coupled with very low levels of the small leucine-rich proteoglycans (SLRP), such as decorin and lumican, trigger cell proliferation and migration [22
]. At the same time, the cells are maintained in a less differentiated state, approaching EMT for the epithelial elements.
Similar to development but in contrast to tumor progression, this metaplastic stage is short-lived as the wound enters the remodeling/resolving phase [37
]. During this phase, the excessive cellularity of the regenerative phase is reversed with active apoptosis of stromal and epithelial cells, and dissolution of immature vessels. The ongoing cell proliferation is suppressed, with the balance shifting dramatically towards loss of cells. In skin wound healing, over 90% of the vessels and other formed elements may be lost during this period. This shift in cell concentration is an active process, in which a maturing matrix plays an active role [23
]. The remodeling phase involves the rapid degradation of components of the immature matrix and equally rapid synthesis of mature connective tissue proteins. The total amount of collagen increases as the stimulation of collagen I synthesis exceeds that of collagen III and other forms [62
]. Furthermore, the markers of matrix immaturity, fibronectin and tenascin-C, disappear and the SLRP content, such as decorin, lumican, and biglycan, increases dramatically [22
]. High concentrations of collagen I fibrils, particularly if crosslinked, are highly adhesive, a property that limits cell migration, while the SLRP molecules downregulate the signaling from a variety of growth factor receptors [22
]. Not only does this matrix reverse the stimulatory environment of the preceeding phase but these proteins couple with soluble chemokines to drive matrix contraction by the stromal cells (fibroblasts and myofibroblasts) to form a scar [64
The healing process in adults concludes with the resolution of the scar over the ensuing months. Collagen levels returns to normal tissue levels, as the tissue approaches, but does not reach, pre-injury homeostasis.
Unfortunately, this process does not always go as planned. In many instances, the scar will continue to remodel, grow, and expand resulting in clinically dysfunctional and debilitating scar. Here, we present evidence to suggest that, when matrix maturation is disrupted, chronic inflammation ensues even in the absence of continuing external stimuli or insults.