Prolifera tive phase
The proliferative phase can be subdivided into three major processes: reepithelialization (regeneration of the epidermis), the granulation tissue formation (dermal repair), and angiogenesis (revascularization of the granulation tissue) (Gurtner et al., 2008). In a normal situation, these processes will typically lead to the formation of scar tissue, depending on the size and depth of the original injury. In order for appropriate completion of these cellular processes, communication must be coordinated between multiple different cell populations, including immune cells, myofibroblasts, endothelial cells, and keratinocytes (Eming et al., 2014). This can make assessing gene expression particularly challenging and difficult to interpret. These cells, as well as specific gene expression markers that allow identification of their presence in the wound, are shown in Fig. 6.6.
Neutrophils and macrophages produce trophic factors that will activate the resident cells of the wound during the inflammatory phase. Among those, the surrounding keratinocytes will facilitate reepithelialization by proliferating and migrating across the damaged area to reestablish barrier function. Two days after wounding, keratino- cytes will upregulate the expression of keratin 10, 1, and 2 (K10, K1, and K2), as well as laminin 332 (Fuchs and Weber, 1994; Laplante et al., 2001). Concurrently, stromal cells are activated and upregulate secrete periostin, a protein that mediates stromal cell differentiation into myofibroblasts (Elliott et al., 2012). Myofibroblasts migrate into the wound bed to secrete a temporary granulation tissue made out of type I/III collagens and fibronectin in order to fill the space left empty by the injury. Other resident cell populations, including endothelial cells and pericytes, also invade the granulation tissue to revascularize the healing tissue (Morikawa and Ezaki, 2011).
Toward the end of the proliferative phase, there is a shift in the role of the myofibroblast as they start to turn over the extracellular matrix (ECM) through the secretion of
Figure 6.6 Main cell types contributing to the wound-healing response. a-SMA, a-smooth muscle actin; G-CSF, granulocyte-colony stimulating factor; iNOS, inducible nitric oxide synthases; K10, K1, and K2, keratin 10, 1, and 2; NG2, neural/glial antigen 2; Tie-2, tyrosine kinase with immunoglobulin and EGF homology domains 2.
matrix metalloproteinases (MMPs), combined with the synthesis of new ECM components, including collagen type I (Gurtner et al., 2008). The remodeling phase is also characterized by the regression of the overdeveloped capillary network that was formed during the proliferative phase, with endothelial cells undergoing apoptosis, reducing the number of capillaries.