Experimental work of the last two decades has revealed the general steps of the wound healing process. This complex network has been organized in three sequential and overlapping steps. The first step of the inflammatory phase is an immediate response to injury; primary sensory neurons sense injury and send danger signals to the brain, to stop bleeding and start inflammation. The following target of the inflammatory phase, led by the peripheral blood mononuclear cells, is to eliminate the pathogens and clean the wound. Once this is completed, the inflammatory phase is resolved and homeostasis is restored. The aim of the proliferative phase, the second phase, is to repair wound damage and begin tissue remodeling. Fibroplasia, reepithelialization, angiogenesis, and peripheral nerve repair are the central actions of this phase. Lastly, the objective of the final phase is to complete tissue remodeling and restore skin integrity. This review provides present day information regarding the status of the participant cells, extracellular matrix, cytokines, chemokines, and growth factors, as well as their interactions with the microenvironment during the wound healing process.
The skin provides a life-protective barrier between the body and the external environment against physical damage, pathogens, fluid loss, and has immune-neuroendocrine functions that contribute to the maintenance of body homeostasis [
Skin innervation consists of a dense network of sensory and autonomic fibers that form tight junctions with keratinocytes and transmit sensations of pain, temperature, pressure, vibration, and itch [
After injury, skin integrity must be promptly restored in order to maintain its functions. In this process, peripheral blood mononuclear cells, resident skin cells, extracellular matrix, cytokines, chemokines, growth factors, and regulatory molecules participate in the wound healing process. The intricate skin repair process has been organized in three sequential and overlapping steps: the inflammatory phase, the proliferative phase, and the remodelling phase. The inflammatory phase includes cutaneous neurogenic inflammation and hemostasis; these early events start in the first seconds after injury and last approximately 1 hour. Followed by the fast recruitment of neutrophils to the injured tissue during the first 24 hours and its posterior decline during the subsequent week. The progressive infiltration of inflammatory monocytes-macrophages to the wound starts the second day after injury and continues to increase, reaching its maximum during the proliferative phase, starting its decline during the following two weeks, becoming the dominant mononuclear cell in the tissue repair process. Circulating lymphocytes migrate to the skin early after injury reaching a plateau by day 4 and their presence continues for two more weeks before declining. The last phase starts in the second week after injury and includes remodeling the tissue previously formed in the proliferation phase and the organization of a scar in order to restore the skin integrity. This last stage could last for months. This review provides present day information regarding the central role of the resident and peripheral immune cells as well as the microenvironment and their interactions during the wound healing process.
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In healthy human adults, neutrophils constitute 50-70% of all leukocytes. Neutrophils circulate in the blood as quiescent cells with a lifespan of 8-12 hours and 1-2 days in tissues. In the final stages of their lifespan, they are cleared from the circulation in the liver, spleen, and bone marrow [
Circulating human monocytes originate from a monocyte-dendritic progenitor (hMDP) that gives origin to monocytes and a dendritic cell precursor (hCDP) in the bone marrow. Both of these cells are released to the blood and further differentiate in the peripheral tissues as macrophages or dendritic cells [
After injury, the presence of DAMPs and PAMPs is sensed by tissue-resident macrophages that in turn activate patrolling monocytes to migrate into the wound [
The skin immune system maintains and protects body integrity. Innate immune system, including neutrophils and monocyte-macrophages, provides a non-specific immediate response to pathogens and toxins. Innate cells collaborate with T and B cells of the adaptive immune system that retain specific memory for a long time to fight specifically intracellular and extracellular pathogens.
Innate lymphoid cells (ILCs) consist of three family subsets with different cell lineage markers compared to T, B, and natural killer (NK) cells. Group 1 contains NK cells, releases interferon gamma (INF
After injury, DAMPs and PAMPs released from damaged cells and pathogens are sensed by a diversity of immune and nonimmune cells present in the skin through a system of pattern recognition receptors that include transmembrane Toll-like receptors and C-type lectin receptors and cytoplasmic proteins, retinoic acid-inducible gene-I-like receptors, and NOD-like receptors (NLRs) [
Skin homeostasis and peripheral tolerance to commensals and self-antigens are controlled by skin immunosuppressive CD4+Foxp3+ regulatory T cells (
CD4+ helper T cells include several subsets: Th1, Th2, Th17, Th22, and Th9 providing host defense by releasing diverse cytokines that in turn promote the release of INF
B cells are part of the humoral branch of the immune system. They differentiate into antibody production plasma cells, present antigens to T cells, and regulate local immune responses by releasing growth factors and proinflammatory and anti-inflammatory cytokines [
Neutrophil accumulation in the wound increases during the initial inflammatory phase and starts declining 4 days later until the end of the week [
The proliferative phase is identified by (a) fibroplasia, including fibroblast proliferation and differentiation into myofibroblasts, extracellular matrix deposition, and wound contraction, (b) reepithelialization and epithelial-mesenchymal interaction between keratinocytes and fibroblasts, (c) angiogenesis, including endothelial cell proliferation and new vessel formation, and (d) peripheral nerve repair, consisting in collateral reinnervation and nerve regeneration. Macrophages are the dominant inflammatory cells orchestrating the proliferative phase of skin wound repair [
Fibroblasts are an ill-defined heterogeneous group of cells with great plasticity and different roles in distinct dermal layers [
Reepithelialization starts 16-24 hours after injury and continues until the remodeling phase of wound repair [
During the proliferation phase, the macrophage anti-inflammatory phenotype (M2) emerges as the dominant cellular population, orchestrating the interaction with endothelial cells, fibroblasts, keratinocytes, extracellular matrix (ECM), and peripheral nerves [
After injury, severed nerves affect the homeostatic function of the skin. The restoration of neurological functions after traumatic peripheral nerve injury involves two processes: collateral reinnervation and nerve regeneration. Skin denervation stimulates collateral sprouting of nociceptive skin afferents from close undamaged axons to reinnervate the skin [
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After acute skin injury, fibrinogen, fibronectin, proteoglycan, and platelets from plasma come into contact with collagen of the extracellular matrix (ECM), forming a fibrin rich early provisional matrix cross-linked with fibronectin (EPM) [
After injury, basal keratinocytes at the wound edge start the endothelial mesenchymal transition by losing their desmosome connection to each other and the hemidesmosome bond to the basal membrane. The cytoskeleton is then reorganized, losing its cuboidal shape and adopting a flattened morphology with lamellipodia, expressing K6 and K16 that allow them to begin migration into the provisional matrix to fill the gap. Simultaneously, the keratinocytes that remain behind the edge begin to proliferate [
Wound healing angiogenesis is thought to be an ongoing process in two phases: the proliferation of new blood vessels and the pruning and remodeling phase. Hypoxic conditions after injury stimulate the synthesis of hypoxia inducible factor-1(HIF-1) in vascular endothelial cells, fibroblasts, keratinocytes, and macrophages, followed by the release of angiogenic factors VGEF, FGF, PDGF, and TGF-
The transection of the peripheral nerve after injury is followed by retraction of the stumps. The poorly vascularized bridge between the stumps becomes hypoxic. Macrophages sense hypoxia and release VEGF, promoting angiogenesis along the original tubes of the bridge formed of basement membrane. Meanwhile, distal stump degenerated by Wallerian degeneration; Schwann cells are detached from the degenerating axons, releasing their myelin, and dedifferentiate into a progenitor-like state. These dedifferentiated SC recruit more macrophages, and together, clean myelin and axon debris. Macrophages promote the vascularization of the bridge between the two stumps, preparing the site for axonal regrowth. Simultaneously, dedifferentiated Schwann cells migrate along the recently formed vasculature, forming the bands of Büngner and guiding the regrowing axons to their original target. Once the axons reinnervate, their original targets, Schwann cells, redifferentiate and remyelinate axons, leading to the termination of the inflammatory response [
In this last phase of wound healing, the granulation tissue undergoes a gradual diminishing process. The epidermis, dermal vasculature, nerves, and myofibers of the skeletal muscle are remodeled, forming a functional tissue. Vascular components of fibroblasts and myofibroblast of the granulation tissue are decreased and PBMC cells undergo apoptosis or leave the wound. Similarly, the amounts of proteoglycans and glycosaminoglycans that provided structural and hydration role are diminished. Collagen metalloproteinases released by fibroblasts and macrophages degrade collagen Type III of the granulation tissue and replace it with collagen Type I, which is further reorganized into paralleled fibrils, forming a low cellularity scar [
Two common complications are associated with alterations of the normal skin acute wound healing process: fibrosis and chronic skin wounds. These alterations affect millions of people around the world, representing a major health challenge and healthcare expenditure for patients and countries globally. Some challenging problems will be briefly addressed in the following paragraph.
Fibrosis is characterized by excessive production of extracellular matrix. In human skin, fibrosis is recognized as hypertrophic scars and keloids. Hypertrophic scars grow after surgery, trauma, or burns causing deformity and contractures across the joints. Keloids develop as profuse scarring that extends beyond the limits of the original injury causing deformity, pruritus, and hyperesthesia [
The second complication is chronic nonhealing wounds, clinically known as venous and arterial leg ulcers, pressure sores, and diabetic foot ulcers. We will refer briefly to the most important pathophysiological challenges of diabetic foot ulcers.
Diabetic foot ulcer is a serious and expensive complication of diabetes associated with peripheral vascular disease and neuropathy in the lower limbs that frequently end in amputation [
Experimental work of the last two decades has revealed the general steps of the wound healing process. All cells, tissues, cytokines, chemokines, and growth factors of the skin participate in the wound healing process, revealing redundant and pleiotropic functions and interactions in many of the cellular and extracellular participants in wound repair. Further understanding of this complex network will elucidate how skin cell interaction with the changing tissue microenvironment defines their phenotype in every stage of tissue repair. Present knowledge has revealed that when cells are healthy, the inflammatory phase is well orchestrated, lasting only a few days, and the following stages of tissue repair: reepithelialization of the wound, granulation tissue formation, wound contraction, and scar formation, proceed normally. However, when cells are dysfunctional, as in diabetes, the inflammatory process is extended, the integrity of the skin is not restored, and ulcer or pathological fibrosis occurs. Macrophages are the dominant cells present in all phases of tissue repair. They have an essential regulatory role and are therefore seen as important therapeutic targets to control the wound healing process in the future.
Angiopoietin 1
Angiopoietin receptor
Macrophage colony stimulating factor-1
Epidermal growth factor
Fibroblast growth factor-2
Fibroblast growth factor-7
Fibroblast growth factor-10
Granulocyte-macrophage colony-stimulating factor
Hypoxia inducible factor
Hepatocyte growth factor
Interleukin-1
Interleukin-2
Interleukin-4
Interleukin-5
Interleukin-6
Interleukin-10
Interleukin-17
Interleukin-22
Insulin-like growth factor-1
Monocyte chemoattractant protein
Platelet derived growth factor
Platelet derived growth factor subunit B
Transforming growth factor beta
Tumor necrosis factor alpha
Vascular endothelial growth factor.
The author has no conflicts of interest to declare.
We would like to thank Edda Schiutto and Mario Cruz for valuable comments to the manuscript and to Mariana Ceballos Cañedo for her outstanding contribution to the design and conception of the figures present in this article.