Table 1.
Key Therapeutic Targets Involved in Reparative Wound Healing and Known Differences in Regenerative Healing
| Reparative healing | Regenerative healing | |
|---|---|---|
| Hemostasis phase | ||
| Time pointa | Starting immediately after wounding and lasting a few hours to 2 days27,200 | Does not occur in regenerative healing; reepithelialization starts immediately after wounding (murine and rat models)33,40 |
| Cells | Platelets are activated and drive clot formation, which prevents excessive blood loss and protects the wound from infection25,35–37 | Not well characterized in the published literature |
| Signaling molecules | VEGF is released by platelets201 | Not well characterized in the published literature |
| ECM | Cross-linked fibrin and fibronectin contribute to clot formation and provide an initial structure for cell movement25,35,38 | A fibronectin clot forms39 Tenascin is present in the tissue surrounding the wound and helps with rapid reepithelialization33,62 There are high levels of hyaluronic acid30 |
| Inflammatory phase | ||
| Time point | Starts on day 1 during hemostasis and can last up to day 827,200 | Although it is known that this phase is attenuated in fetal wounds, the timing of the appearance of cells and cytokines associated with inflammation has not been characterized in human or other large mammalian fetuses61,62 |
| Cells | Toll-like receptors on damaged cells trigger the innate immune response43,44 Leukocytes protect the wound from infection46 Neutrophils secrete signaling molecules to debride the wound, degrade the clot, attract additional inflammatory cells, and contribute to angiogenesis46,49,54,55,133,202–204 M1 macrophages clear debris from the wound71,133,205 Mast cells reduce blood coagulation and increase fluid accumulation52,53 Natural killer cells and plasmacytoid dendritic cells contribute to antimicrobial activity, angiogenesis, and tissue repair56–59 |
Few inflammatory cells are present; larger or more severe wounds may elicit a stronger inflammatory response40,41,62,63 Macrophages are present, but are not responsive to the wound63 Mast cells may be present, but are not activated64 |
| Signaling molecules | Inflammatory cytokines and chemokines (e.g., TNF-α, TGF-β1, IL-1, IL-6, and IL-8) promote the migration of immune cells to the site of inflammation49,51,133,203,206,207 Proteases debride the wound and eliminate toxins from damaged tissue46,47 Growth factors (e.g., HGF, VEGF, and FGF) promote angiogenesis49,54,55 Histamine and heparin reduce blood coagulation and increase fluid accumulation52,53 Type I interferons contribute to wound healing and antimicrobial activity56,57 |
Expression of inflammatory cytokines and chemokines, including IL-6 and IL-8, is reduced or absent65–67 Anti-inflammatory IL-10 expression is increased68,69 |
| ECM | New blood vessels start to form49,54,55 | Angiogenesis does not increase, and does not contribute to, inflammation40,62 |
| Proliferation phase | ||
| Time point | Starts 3–10 days after wounding and can last until day 2527,200 | Reepithelialization starts immediately and wound closure is achieved 2–3 days after wounding (murine, rat, and lamb models)33,40,119 |
| Cells | M2 macrophages secrete signaling molecules to attract fibroblasts and keratinocytes to the wound49,71,205 Fibroblasts migrate and proliferate to deposit the ECM for granulation tissue72,74,76 Stem cells or mesenchymal progenitor cells from hair follicles, injured nerves, and the bone marrow, and dedifferentiated cells from underlying fat contribute to tissue generation79–82 Endothelial cells and endothelial progenitor cells form new blood vessels77,78 Activated mast cells contribute to angiogenesis208,209 Fibroblasts differentiate into myofibroblasts rich in α-SMA fibers, which contract to narrow the wound opening and increase vascularization74,83–86 Keratinocytes from the surrounding tissue and stem cells from the interfollicular epidermis and hair follicles reepithelialize the wound81,87–91 |
Endothelial progenitor cells originate from the bone marrow and contribute to angiogenesis and increased blood circulation99 Increased migration of fibroblasts increases hyaluronic acid content of the ECM95,98 Fibroblasts and keratinocytes produce an organized ECM33,95 Fibroblasts are resistant to TGF-β1–induced differentiation into α-SMA–positive myofibroblasts210,211 Fibroblasts contract and contribute to wound closure104,107,108 |
| Signaling molecules | Cytokines (including IL-1 and IL-6), chemokines, and growth factors (including VEGFs and TGF-β) attract fibroblasts and keratinocytes to the wound49,71 PlGF helps stimulate angiogenesis of the granulation tissue212 TGF-β1 induces fibroblasts to differentiate into myofibroblasts83,84 |
High levels of IL-10: upregulate hyaluronic acid96,97; increase migration and invasion of fibroblasts95,98; and help regulate the formation of ECM and fibroblast differentiation66,95,211 Low levels of VEGF decrease angiogenesis102 |
| ECM | Collagen types I and III, fibronectin, hyaluronic acid, and proteoglycans form the ECM of the granulation tissue72,74,76 | An organized ECM of fibronectin, tenascin, chondroitin sulfate, and hyaluronic acid is produced by fibroblasts and keratinocytes33,39,62,95 An actin cable surrounding the wound brings the wound edges closer together107,109 Angiogenesis and blood circulation increase, although not to the same degree as in reparative healing62,99–102 |
| Remodeling phase | ||
| Time point | Starts around days 21–23 and can last for up to 2 years27,200,213 | Starts 3 days after wounding and is complete by 14 days (murine model with human skin transplant, lamb model)119,214,215 |
| Cells | Fibroblasts, keratinocytes, and inflammatory cells secrete MMPs111–113 | Fibroblasts lay down collagen in a pattern similar to the surrounding skin114,118,119,216 |
| Signaling molecules | MMPs break down the granulation tissue and remodel the ECM into a more permanent structure111–113 |
Higher levels of antifibrotic TGF-β3 than profibrotic TGF-β1 and TGF-β230,103,105,106 IL-10 downregulates the expression of collagen type I105,106,116,117 |
| ECM | Collagen is laid down in parallel bundles to form the more permanent ECM and scar tissue30,114,115 The ratio of fibrillar collagen types I to III increases and shifts to that of normal skin; fibril size increases to that of a healthy dermis over time83 The number of blood vessels in the granulation tissue regresses to the density of unwounded skin86,110 |
Collagen is laid down in a basket-weave pattern similar to that of uninjured skin114,118,119 Lower ratio of collagen type I to III105,106,116,117 Blood vessel density is also reduced to levels similar to the surrounding tissue101 |
a
Most of the research into regenerative healing has been done in nonhuman fetuses, which have different gestational lengths in comparison to humans; thus, the chronology of fetal wound healing in humans is not well established. The animal models used for the time points given are provided.
α-SMA, alpha-smooth muscle actin; ECM, extracellular matrix; FGF, fibroblast growth factor; HGF, hepatocyte growth factor; IL, interleukin; MMP, matrix metalloproteinase; PlGF, placental growth factor; TGF, transforming growth factor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor.