## 3 Undifferentiated Cells (Stem Cells)
### Adult Stem Cells
#### 3.4.3. Umbilical Cord Mesenchymal Stem Cells (UC-MSCs)  
##### 3.4.3.1. Umbilical Cord Wharton's Jelly-Derived Mesenchymal Stem Cells (WJMSCs)  
General Information  
In mammals, the umbilical cord is comprised of umbilical arteries and veins surrounded by Wharton's jelly (WJ). The term WJ was initially proposed by an English physician and anatomist in 1656 [210]. In humans, the main components of the WJ consist of glycosaminoglycans (hyaluronic acid and chondro sulfate) and collagen fibers. The cellular components of this gelatinous tissue include different cell types such as fibroblasts, myofibroblasts, smooth muscle cells, and a subset of stromal cells [211]. The presence of MSCs in the WJ was not determined until 2004 when fibroblast-like cells of this tissue were proven to be MSCs [212]. The WJ-MSCs can be isolated from the umbilical cord matrix, which consists of two distinct regions known as inter-vascular and sub-amnion regions. The number and nature of these cells are different from other regions of UC including peri-vascular and vascular  
Figure 7: Schematic illustration of various stimulating factors affecting the differentiation ASCs towards osteoblasts. With permission from Reference [206]. endothelium zones [213]. Similar to the procedure of human WJ collection, the isolation of WJ-MSCs is noncontroversial and without ethical issues [214]. These cells have also been isolated from other mammals including bovine [215], horse [216], and cat [217]. Like other MSCs, WJ-MSCs possess plasticity characteristics and, thereby, may differentiate into cells with origins of ectoderm (e.g., neurons) [218], mesoderm (e.g., osteoblasts) [219], and endoderm (e.g., hepatocytes) [220]. However, a number of advantages have been proposed for these cells in comparison with other MSCs including faster proliferation rate, greater expansion capability, immunosuppressive properties, and better efficiency in terms of therapeutic characteristics [212]. Two of the main methods for isolation of the WJ-MSCs include the density gradient centrifugation approach and enzymatic digestion [221]. It has been shown that the enzymatic degradation approach is the most efficient for harvesting the WJ-MSCs. Due to the large size of the UC, the number of isolated cells is usually high (approximately \(\sim\)1.5 \(\times\) 10\({}^{6}\) cells/cm of the umbilical cord) [222]. The WJ-MSCs with appropriate features such as immunosuppressive characteristics may be ideal candidates for TE and cell-based therapy strategies [223]. In addition, these cells may support the expansion of other stem cells to excrete various cell growth factors, chemokines, and cytokines [224; 225]. Applications in BTE  
WJ-MSCs are being currently considered as an alternative to BM-MSCs for bone repair and regeneration strategies. It has been previously reported that this cell type is able to give rise to osteogenic lineages after culturing in the osteogenic medium [226]. In addition to the ability to osteogenic differentiation, WJ-MSCs are ideal candidates for BTE with respect to their abundant resources, no damage procurement, and low immunogenicity. Moreover, [PERSON] et al. showed WJ-MSCs do not undergo a malignant transformation during long-term culturing (up to 15 passages) in serum-free medium [227]. However, it has been shown that the osteogenic potential of WJ-MSCs is lower than BM-MSCs in vitro and in vivo conditions [228; 229]. In order to address this issue, the genetic engineering of these cells has been proposed as an appropriate approach. On this matter, [PERSON] et al. inserted the gene encoding Ost into WJ-MSCs to improve their osteogenic capacity [230]. They showed that transfection of Osx into the cells results in enhanced ALP activity without any adverse effects on the cell proliferation rate. The implantation of Osx-overexpressing WJ-MSCs into the subcutaneous pocket of nude mice resulted in a higher ectopic bone formation in comparison to the non-transfected WJ-MSCs after four weeks.