**2. The skin and the wound healing process**

The skin is the main barrier that protects the body from the external environment, maintaining the homeostasis and with self-healing capacity. It is a complex organ, with different layers (epidermis, dermis, and hypodermis), that upon the loss of integrity, whether due to a disease or a lesion, needs to re-establish its function [1].

The wound healing process is a complex cascade of events that must occur in sequence and at the adequate time, in order to be successful. It has four overlapping phases: hemostasis, inflammation, proliferation, and remodeling. After a lesion, the first reaction of the body is to prevent blood loss, using the platelets to form a blood clot (hemostasis). Then, the inflammatory phase starts by recruiting inflammatory cells into the lesion site that will produce growth factors, cytokines and enzymes, increasing the temperature, redness, swelling, and local pain. If this phase extends in time, there will be a chronic inflammation that will harm the wound healing. The next phase is the proliferative, which consists in covering and filling the void space created by the lesion. For this, wound contraction occurs by local fibroblasts that differentiate into myofibroblasts. In addition, endothelial cells proliferate and migrate to form

*Application of Cell-Based Therapies in Veterinary Dermatology DOI: http://dx.doi.org/10.5772/intechopen.111553*

**Figure 1.** *Wound healing cascade and scar formation.*

new blood vessels at the lesion site. It starts in the 4th day post-injury and can last for 2 weeks. Then, the fourth phase (remodeling) occurs and extracellular matrix deposits, promoting the re-epithelization and neovascularization, as the collagen fibers change from type III to type I, helping the tissue to remodel and regain its flexibility and tensile strength. This phase begins 2–3 weeks post-lesion and can last for several years [1, 8, 9].

The wound healing process can sometimes fail and, although the process is not fully understood, the prolonged chronic local inflammation is associated with an abnormal regeneration, as it supports the formation of scars, as demonstrated in **Figure 1**. There are also several factors that increase the risk of inappropriate wound healing, such as smoking, malnutrition, infections, age, metabolic diseases, medications, and even radiation [1, 8]. Tissue engineering, using stem cell-based therapies is being explored in various research fields obtaining good outcomes. Stem cells have gained a lot of attention because of some of their capacities, such as differentiation and the ability to aid tissue regeneration [10].
