# The Application of Mesenchymal Stem Cells on Wound Repair and Regeneration
## 10 In Vitro Models
### 11. In Vivo Models  
Human stem cells can provide substantial opportunities, contributing with differentiated and undifferentiated cells that can be used in drug discovery, gene therapy and regenerative medicine. Moreover, stem cells can also be transduced ex vivo and the manipulated cells reintroduced into the host, which means another therapeutic approach for some specific diseases [10]. However, the selection of the most appropriated stem cells, to obtain the required efficacy in wound healing, is a challenge. Embryonic stem cells possess a great capability of self-renewal, but there are some ethical issues regarding the use of these cells. Adult stem cells derived from the peripheral blood, umbilical cord, bone marrow or adipose tissue have limited capacity of self-renewal but their therapeutic application in human skin tissues are more acceptable [10]. In addition, endogenous stem cell populations can have a role in several steps of wound healing, such as neovascularization and tissue remodeling. Concerning embryonic stem cells, they are pluripotent and can differentiate into three primary germ layers (endoderm, mesoderm, ectoderm), as well as into keratinocytes (when some specific growth factors are present). Keratinocytes are fundamental since they are able to produce multilayered epidermis in culture. However, the ethical controversial (cells have to be harvested from live embryos) and the potential immune rejections redirect regenerative medicine into other types of cell [10; 69]. An alternative, is the use of induced pluripotent stem cells (iPSCs). These cells are engineered from adult somatic cells (keratinocytes and fibroblasts) applying transcription factors such as KLF4, Sox-2 and c-Myc. Several studies refer to significant development in the differentiation of the mouse. Ref. [PERSON] et al., demonstrated that rodent dermal papilla cells when removed from the hair follicles and transplanted intact into the recipient skin induce the development of the hair follicle and consequent hair growth [70]. These cells are a good choice in chronic wound treatment and in unresolved wounds because of their multipotent capacity and limited immunoreactivity [10]. iPSCs' ability to differentiate into different cell types and the unlimited growth was demonstrated in some in vivo models. Ref. [PERSON] et al., developed an in vivo transplantation model named as a clustering-dependent embryoid body transplantation method, which was used in nude mice and could be adequately connected to the nerve fibers, epidermis and arrector pili muscleswithout tumorigenesis [71]. In addition, Ref. [PERSON] et al., also used these cells to generate a human 3D skin from iPSCs-derived fibroblasts, iPSCs-derived keratinocytes and iPSCs-derived melanocytes [72]. MSCs are used in various chronic and acute injuries. For instance, Ref. [PERSON] et al., followed green fluorescent protein (GFP)-labeled bone marrow transplanted into non-GFP mice to understand how these cells react in cutaneous wounds and found evidence of improvement of the wounds. This study found GFP positive stem cells in the sebaceous gland, hair follicles and epidermis twenty one days after the transplantation of GFP bone marrow cells [10; 73]. In addition, the process of wound healing is considered different among species, although some phases like inflammation, cell proliferation, blood clotting and extracellular matrix remodeling remain equal. For example, some lower vertebrates like fish (zebrafish) and amphibians (axolotl) have the capacity to regenerate skin perfectly. On the other hand, for adult mammals like humans it is more difficult to achieve this total regeneration, consequently the formation of a scar prevents the complete recovery of the skin function. Sheep and rats are animal models commonly used to provide important information about the regeneration of missing structures [3; 21]. However, as previously mentioned, there are various in vivo studies verifying the use of stem cells as a therapeutic option for wound healing.