**2.1 Inflammatory phase**

The initial healing begins with the formation of a fibrin clot at the site of the wound that serves as a temporary extra-cellular matrix (ECM) and provides the necessary stimulus to summon inflammatory cells [11].

Characterised by haemostasis and inflammation, collagen exposure during injury leads to the activation of the intrinsic and extrinsic pathways of the clotting cascade, thereby initiating the inflammatory phase. Vasoconstrictors like thromboxane A2 and prostaglandin 2-α are immediately released, leading to the formation of a clot made of collagen, platelets, thrombin and fibronectin. The release of cytokines and growth factors initiates the inflammatory response and the fibrin clot serves as ECM for the arriving cascade of inflammatory cells (including neutrophils, monocytes, fibroblasts and endothelial cells) with the neutrophils being the first cells to arrive (**Figure 1A** and **B**). As these inflammatory regulators arrive, the signal released changes from

#### *Scarless Wound Healing DOI: http://dx.doi.org/10.5772/intechopen.105618*

vasoconstrictors (that aided in initial clot formation) to vasodilators to allow for the increased cellular traffic [10].

The presence of Interleukin (IL)-1, Tumour Necrosis factor (TNF)- α, Transforming Growth factor (TGF)-β, Platelet Factor-4 (PF-4) attract monocytes from nearby tissue and blood that convert into macrophages which in turn are essential for transitioning the wound from its initial inflammatory phase to the proliferative phase*.* Macrophage activation generally occurs between 2 and 4 days post injury and leads to angiogenesis (the formation of new blood vessels) and fibroplasia (growth of fibrous tissue)*.* It also leads to the synthesis of nitric oxide that plays multiple roles including providing pain relief by serving as a partial agonist at opioid receptors*,* encouraging vasodilation and having anti-inflammatory effects [10]. Interestingly, nitric oxide is considered as a pro-inflammatory mediator that induces inflammation due to its overproduction in pathophysiological situations [10, 12].

### **2.2 Proliferation phase**

The proliferation phase is another complex stage in wound healing that involve multiple simultaneous processes occurring at the site of the injury including (but not limited to) epithelialisation, angiogenesis, fibroplasia, granular tissue formation and collagen deposition (**Figure 1C**) [10, 11].

Epithelialisation*,* one of the early steps in wound repair occurs in one of two ways based on the severity of the wound:

**Basement membrane intact—**If the wound is shallow, leaving the basement membrane intact; epithelial cells migrate upwards in a normal pattern as epithelial progenitor cells remain undamaged. This type of injury allows the epidermis to be restored within 2–3 days.

**Basement membrane damage—**In case of deeper wounds where the basement membrane has been affected, the epithelial cells at the edges begin proliferation and sending out projections to aid in clot formation and re-establishment of protective barrier. This is followed by angiogenesis, which leads to endothelial cell migration and capillary formation, which allows granulation and tissue deposition due to the nutrient supply. The protective barrier formed by the epithelial cell projections from the edge of the wound plays crucial role in providing a protective role by preventing bacterial invasion and fluid loss.

Epithelialisation, while initially stimulated by inflammatory cytokines from the inflammatory phase of wound healing, leads to upregulation of keratinocyte growth factor (KGF) by the stimulation of IL-1 and TNF-*α*. KGF, in turn stimulates keratinocytes from wound-adjacent regions to migrate to the site of injury and proliferate as well as differentiate in the epidermis [10].

Granulation tissue formation is the final stage in the proliferation phase, characterised by activation of fibroblasts which in turn initialise collagen synthesis and turn into myofibroblasts that aid in wound contraction. Wound contraction, induced by the TGF-β1 that is secreted by macrophages, is primarily carried out by the fibroblasts present at the wound site (wound fibroblasts) that have less proliferative potential as compared to those at the periphery. Platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) are the primary signals that drive the attraction of fibroblasts and their activation [10, 11].

This in turn leads to the synthesis of a provisional matrix that is made up of collagen type-III, glycosaminoglycans and fibronectin [9–11].

#### **2.3 Maturation phase**

The maturation phase is often deemed as the most important phase from a clinical perspective as it is characterised by collagen deposition (**Figure 1D**). Any issue with collagen deposition and deviation in the orderly networked fashion it is meant to take can lead to compromised wound strength. Conversely, excessive collagen deposition leads to the development of a hypertrophic or keloid scar [8, 9].

As the wound matures, the collagen that is initially thinner than that produced over uninjured skin changes to become thicker and organised along the wound such that they are organised around the regions that are under greater stress than their surroundings. It must be noted that the collagen in these granulation tissues that develop in wound regions has greater hydroxylation and glycosylation of lysine residues, making it different from those that are found in uninjured areas. While the tissue strength rarely returns to its pre-injury state, wound strength gradually increases over time with the region needing a minimum of 90 days to regain 80% of its original strength (this may be compared with its strength during the first week of wound healing which is only 3%) [10, 11].
