Wound healing represents one of the most complex and coordinated biological processes in human physiology, involving tightly regulated cellular and molecular events that restore tissue integrity following injury.
Under normal circumstances, wounds progress through a predictable sequence of overlapping phases to achieve complete repair.
However, when these orchestrated processes become dysregulated, chronic non-healing wounds develop, causing significant patient suffering and imposing substantial burdens on healthcare systems worldwide.
The current understanding of normal wound healing physiology and the pathophysiological mechanisms underlying chronic wounds reveals a multifaceted interplay between cellular dysfunction, molecular signaling disruption, and environmental factors that collectively impair the healing trajectory.
Here we examine the fundamental physiological processes of normal wound repair and elucidates the pathophysiological alterations that characterize chronic wound development.
The Physiological Sequence of Normal Wound Healing
Wound healing under normal physiological conditions follows a remarkably coordinated sequence of overlapping phases: hemostasis, inflammation, proliferation, and remodeling.
Each phase represents a distinct set of cellular and molecular events that must be precisely orchestrated to achieve efficient tissue repair.
These phases do not exist in isolation but rather demonstrate significant temporal overlap, with the culmination of one phase initiating the next in a carefully synchronized progression that ultimately restores tissue integrity14.
The intricate interplay between various cell types, growth factors, cytokines, and extracellular matrix components during these phases ensures that wounds heal in a timely and effective manner.
Understanding this normal sequence provides crucial insights into how disruptions at various points can lead to pathological wound healing outcomes, particularly the development of chronic, non-healing wounds that remain stubbornly fixed in specific phases of the healing process47.
Hemostasis: The Foundation of Wound Repair
Hemostasis represents the critical initial phase of wound healing, characterized by a series of events that occur immediately following tissue injury to control bleeding and establish a foundation for subsequent repair processes.
This phase begins with rapid vasoconstriction of damaged blood vessels, triggered by vasoconstrictors such as endothelin released from injured endothelium, along with circulating catecholamines, epinephrine, norepinephrine, and prostaglandins from damaged cells1.
Platelets produce platelet-derived growth factor (PDGF), which preferentially activates smooth muscle cells in vessel walls, causing contraction and temporarily reducing blood loss1.
However, since this reflexive contraction provides only temporary cessation of bleeding due to increasing hypoxia and acidosis, subsequent activation of the coagulation cascade becomes essential for long-term hemostatic resolution1.
The coagulation process involves both primary hemostasis, with platelet aggregation forming a platelet plug, and secondary hemostasis, where soluble fibrinogen converts to insoluble fibrin strands that create a mesh1.
The combined platelet plug and fibrin mesh form a thrombus that not only stops bleeding but also serves as a provisional scaffold for infiltrating cells necessary for wound healing while releasing complement proteins and growth factors that initiate the subsequent inflammatory response14.
Inflammatory Phase: Mobilizing Cellular Defenses
The inflammatory phase follows hemostasis and represents a critical period when immune cells are recruited to the wound site to combat potential infection and begin the process of debris clearance.
This phase is initiated through transcription-independent pathways that can be rapidly activated, including calcium waves, reactive oxygen species (ROS) gradients, and purigenic molecules1.
Within minutes of injury, intracellular calcium increases at wound edges and propagates to the center, while damage-associated molecular patterns (DAMPs), hydrogen peroxide, lipid mediators, and chemokines released by injured cells provide signals for recruiting inflammatory cells, particularly neutrophils1.
Neutrophils arrive first as a defensive measure against bacteria, followed by monocytes approximately 48-96 hours after injury, which transform into tissue-activated macrophages13. The recruitment of these cells is facilitated by chemokines – small proteins containing cysteines in their molecular structure – which bind to G protein-coupled receptors on cell surfaces and trigger directional cell movement1.
Simultaneously, components of the adaptive immune system, including Langerhans cells, dermal dendritic cells, and T cells, are activated to combat both self and foreign antigens, contributing to the complex immunological response that characterizes this phase13.
Proliferative Phase: Rebuilding Tissue Architecture
As the inflammatory phase gradually subsides, the proliferative phase of wound healing commences, characterized by multiple simultaneous processes aimed at rebuilding tissue architecture.
During this stage, growth factors produced by inflammatory cells and migrating epidermal and dermal cells act in autocrine, paracrine, and juxtacrine fashion to induce and maintain cellular proliferation while initiating cellular migration – events that are essential for granulation tissue formation and epithelialization2.
A robust angiogenic response becomes crucial during this phase, as new blood vessels must form to provide adequate nutrient delivery and gas exchange to support the metabolic demands of cells within the wound bed2. Granulation tissue, primarily formed by activated fibroblasts that synthesize new extracellular matrix (ECM) and help contract the wound, serves as a scaffold for other cellular components including newly synthesized ECM, blood vessels, and inflammatory cells1.
Concurrently, re-epithelialization occurs through the proliferation of both unipotent epidermal stem cells from the basement membrane and de-differentiation of terminally differentiated epidermal cells, while tissue-resident stem cells for skin appendages such as sebaceous glands, sweat glands, and hair follicles activate local appendage repair1.
The formation of new blood vessels within this phase involves multiple cell types, with endothelial cells proliferating, migrating, and branching to form new vasculature, while pericytes provide structural integrity to these nascent vessels12.
Remodeling Phase: Restoring Functional Integrity
The final stage of wound healing, the remodeling phase, involves the maturation of the wound through contraction and matrix reorganization to restore functional integrity.
Wound contraction is primarily achieved by differentiated fibroblasts or myofibroblasts that, in response to transforming growth factor-α (TGF-α), tissue tension, and specific matrix proteins such as ED-A fibronectin and tenascin C, acquire smooth muscle actin-containing stress fibers2.
These myofibroblast-induced contractile forces are transmitted to the ECM via cytoskeleton-associated and ECM receptor-dependent mechanocoupling focal adhesion complexes, particularly integrin receptors, effectively drawing wound margins together12.
Another mechanism contributing to wound contraction involves fibroblast motility with consequent matrix reorganization, forming a dynamic and reciprocal process of slow ECM synthesis and degradation cycles that occur in a fibroblast-dependent manner2.
During this phase, granulation tissue is gradually replaced by normal connective tissue as the wound matures, with macrophages regaining their phagocytic phenotype and acquiring a “fibrolytic” profile that enables them to release proteases and phagocytize excessive cells and matrix components no longer required for wound closure1.
This delicate balance between synthesis and degradation ultimately determines the quality of the final scar tissue, with aberrations in this phase potentially leading to excessive scarring or fibrosis17.
Pathophysiological Mechanisms of Chronic Wounds
Chronic wounds represent a significant departure from the normal wound healing trajectory, characterized by a failure to progress through the established phases of repair in a timely and orderly manner.
A chronic wound is formally defined as one that does not advance through the standard healing stages—hemostasis, inflammation, proliferation, and remodeling—in a predictable timeframe, typically remaining unhealed after three months4.
These wounds become trapped in a self-perpetuating cycle of pathological inflammation, with multiple cellular and molecular abnormalities converging to prevent progression to subsequent healing phases23.
Unlike acute wounds that generally heal without significant intervention, chronic wounds present major challenges to both patients and healthcare providers, causing substantial physical and emotional distress while placing considerable financial burdens on healthcare systems worldwide4.
The pathophysiology underlying chronic wound development involves complex interactions between local tissue factors, systemic conditions, cellular dysfunction, and molecular dysregulation that collectively create an environment hostile to normal repair processes235.
Chronic Inflammation: The Central Pathological Feature
Chronic inflammation represents the hallmark feature of non-healing wounds, creating a self-perpetuating cycle that prevents progression to subsequent healing phases. In these wounds, excessive recruitment of inflammatory cells to the wound bed, often triggered by infection and facilitated by disproportionate expression of vascular cell adhesion molecule 1 and interstitial cell adhesion molecule 1 by resident endothelial cells, establishes a persistent inflammatory state23.
Inflammatory cells accumulated within the chronic wound produce various reactive oxygen species (ROS) that damage structural elements of the extracellular matrix (ECM) and cell membranes, leading to premature cell senescence and tissue destruction2.
These ROS, together with proinflammatory cytokines, induce excessive production of serine proteinases and matrix metalloproteinases (MMPs) that degrade and inactivate components of the ECM and growth factors necessary for normal cell function, while the simultaneous proteolytic degradation of proteinase inhibitors further amplifies this destructive process23.
The dysregulated immune response features prolonged expression of proinflammatory cytokines at the wound site, which alters the local microenvironment and contributes to excessive recruitment of pro-inflammatory myeloid cells3.
A critical factor in sustaining this chronic inflammation is the failure of macrophages to polarize from a pro-inflammatory M1 phenotype toward a reparative M2 phenotype, resulting in overexpression of inflammatory mediators such as interleukin-17 (IL-17), tumor necrosis factor-alpha (TNF-α), inducible nitric oxide synthase (iNOS), and ROS, which collectively create a hostile wound environment that impedes healing35.
Cellular Dysfunction in Chronic Wounds
Cellular dysfunction represents a fundamental aspect of chronic wound pathophysiology, with multiple cell types exhibiting abnormal phenotypes that impair their normal contributions to the healing process. Fibroblasts isolated from chronic wounds, including those from patients with diabetes, non-diabetic chronic wounds, or venous insufficiency, demonstrate lower mitogenic responses to growth factors such as platelet-derived growth factor-AB (PDGF-AB), insulin-like growth factor (IGF), basic fibroblast growth factor (bFGF), and epidermal growth factor, likely due to decreased receptor density on their surfaces2.
These fibroblasts also exhibit reduced motility compared to normal fibroblasts, further impeding granulation tissue formation and ECM deposition2.
Keratinocytes from chronic wounds display a paradoxical combination of hyperproliferation, evidenced by overexpression of the proliferation marker Ki67 and upregulation of cell cycle-associated genes such as CDC2 and cyclin B1, alongside impaired migratory potential linked to decreased production of laminin 332—an important epithelial ECM component and substrate for injury-induced keratinocyte migration2.
The accumulation of senescent cells, particularly in the dermal layer, represents another critical cellular abnormality in chronic wounds, as these cells resist apoptosis and secrete a senescence-associated secretory phenotype (SASP) comprising inflammatory cytokines, chemokines, proteases, and growth factors that collectively disrupt the wound microenvironment5.
Endothelial cells in chronic wounds demonstrate impaired proliferation and migration, leading to insufficient angiogenesis and neovascularization, which results in inadequate oxygen and nutrient supply for resident cells and further compromises the healing process25.
Molecular Imbalances Driving Chronicity
At the molecular level, chronic wounds are characterized by significant imbalances in key signaling pathways, growth factors, cytokines, and proteolytic enzymes that collectively disrupt the normal progression of healing.
A critical molecular disturbance involves the proteolytic balance, where excessive levels of matrix metalloproteinases (MMPs) and other proteases significantly outweigh their inhibitors, leading to uncontrolled degradation of extracellular matrix components and growth factors23.
This imbalance is perpetuated by pro-inflammatory cytokines such as interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), which not only increase MMP production but also reduce the tissue inhibitors of MMPs (TIMPs), creating a self-sustaining cycle of matrix destruction3.
Despite often increased production of growth factors in chronic wounds, their bioavailability is substantially decreased due to proteolytic degradation, rendering them unable to effectively stimulate cellular proliferation, migration, and matrix synthesis23.
Molecular pathways associated with cellular senescence, including the p53 and p21 pathways, demonstrate increased activation in chronic wounds, contributing to the accumulation of senescent cells that secrete inflammatory mediators and perpetuate the inflammatory state5.
In diabetic wounds specifically, hyperglycemia-induced oxidative stress activates the p38 MAPK pathway in senescent cells, contributing to the secretion of pro-inflammatory cytokines and MMPs that impair healing, while excessive accumulation of reactive oxygen species resulting from increased serum glucose and advanced glycation end products further drives cellular senescence and chronic inflammation5.
Contributing Factors to Wound Chronicity
Multiple factors contribute to the development and persistence of chronic wounds, creating complex, multifactorial conditions that resist conventional treatment approaches.
Underlying systemic diseases, particularly diabetes mellitus, significantly impair wound healing through various mechanisms including hyperglycemia-induced oxidative stress, impaired angiogenesis, neuropathy, and immune dysfunction45.
Ischemia represents another major contributing factor, as inadequate blood supply to the wound area results in hypoxia, nutrient deficiency, and impaired removal of metabolic waste products, creating an environment unfavorable for cellular function and proliferation4.
The presence of necrotic tissue within the wound bed serves as a physical barrier to healing while also providing a substrate for bacterial growth and prolonging the inflammatory response4.
Improper moisture balance, whether excessive or insufficient, disrupts the optimal microenvironment required for cellular migration and proliferation, with excessively moist wounds at risk for maceration and bacterial overgrowth, while overly dry wounds impede cellular migration across the wound bed4.
Bacterial infection, particularly when it progresses to biofilm formation, presents a formidable challenge to healing, as biofilms protect bacteria from antibiotic treatment and host immune responses while stimulating a persistent inflammatory state through the continuous release of endotoxins34.
Poor vascular conditions, common in patients with peripheral arterial disease, venous insufficiency, or diabetes, further compromise healing by limiting the delivery of oxygen, nutrients, and immune cells to the wound site while also impairing the removal of metabolic waste products and inflammatory mediators34.
Conclusion
The complex physiology of normal wound healing and the multifaceted pathophysiology of chronic wounds highlight the intricate balance required for successful tissue repair. Normal wound healing proceeds through carefully orchestrated phases of hemostasis, inflammation, proliferation, and remodeling, each characterized by specific cellular and molecular events that together achieve tissue restoration.
When this delicate balance is disrupted, chronic wounds develop, characterized by persistent inflammation, cellular dysfunction, molecular imbalances, and the influence of various contributing factors that collectively prevent progression through the normal healing sequence.
Understanding these physiological and pathophysiological mechanisms provides crucial insights for developing more effective therapeutic strategies for chronic wound management. Future advances in wound care will likely focus on targeting specific aspects of this pathophysiology, such as modulating the inflammatory response, addressing cellular senescence, restoring proteolytic balance, and enhancing growth factor bioavailability.
Additionally, personalized approaches that consider individual patient factors, including underlying diseases, vascular status, and local wound characteristics, will be essential for optimizing treatment outcomes in this challenging clinical area.
References
- https://journals.physiology.org/doi/full/10.1152/physrev.00067.2017
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3428147/
- https://pmc.ncbi.nlm.nih.gov/articles/PMC9104327/
- https://en.wikipedia.org/wiki/Chronic_wound
- https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2024.1400462/full
- https://www.nature.com/articles/s41580-024-00715-1
- https://royalsocietypublishing.org/doi/10.1098/rsob.200223