Once the wound is stabilized, it shifts into reconstruction mode. This proliferation phase does not occur at a specific time, but continues constantly in the background. Immediately after the inflammatory phase begins to organize within a network of collagen and elastin, produced by fibroblasts, a granulation tissue with new capillary formation (neovascularization or angiogenesis) begins to deliver in situ the oxygen, nutrients, and cells necessary for tissue repair. Around days 5 to 7, the platelet-derived growth factor attracts fibroblasts and, along with the transforming growth factor (TGF), enhances the division and multiplication of fibroblasts.
They infiltrate the wound site, proliferate, and differentiate into myofibroblasts to begin producing new collagen, hyaluronic acid, and fibronectin. These components form the core of the wound that will serve as a scaffold. Subsequently, re-epithelialization begins to occur with the migration of cells from the periphery of the wound and adjacent edges.
Initially, only a thin superficial layer of epithelial cells is deposited, but a thicker and more durable layer of cells will fill the wound over time. Subsequently, neovascularization occurs both through angiogenesis, forming new blood vessels from existing ones, and through vasculogenesis, which is the formation of new vessels from endothelial progenitor cells (EPCs). Once collagen fibers are deposited on the fibrin structure, the wound begins to mature. It also begins to contract and is facilitated by the continuous deposit of fibroblasts and myofibroblasts. At this stage, the skin appears red and raised.
At this stage, macrophages still play a crucial role by producing growth factors or cytokines capable of promoting fibroblastic proliferation and collagen synthesis. At this point, the scar is a young fibrosis containing numerous fibroblasts and a loose fibrillar framework on the periphery of the substance loss.
The fleshy bud is composed of fibroblasts, an inflammatory infiltrate (monocytes, lymphocytes, polymorphonuclear cells), fibrin on the surface, and new blood vessels within an edematous fibrillar framework. The contraction of the wound to bring its edges closer together is closely linked to the formation of granulation tissue and the transformation of certain fibroblasts into myofibroblasts capable of contracting and transmitting their contractile activity to the surrounding tissue through interaction between the proteins of the cytoskeleton and the extracellular matrix. This phase, very active from the 7th day, can last up to 3 weeks.
Following tissue repair, the wound contracts and is gradually covered by a new epithelium (epithelialization). The epidermal cells (keratinocytes) multiply and begin to cover the granulation tissue starting from the edges of the wound. In order to properly migrate, these keratinocytes require a healthy, moist, and level granulation tissue. Following the formation of this initial cellular layer, the epithelium is thickened by cell division and soon becomes more resilient. The wound is closed.