## 3 Undifferentiated Cells (Stem Cells)
### Fetal Stem Cells
#### Applications in BTE  
In order to evaluate the osteogenic differentiation potential of HAM-derived cells, [PERSON] et al. examined the presence of the bone-related markers in the cultured cells [138]. Their results revealed that both cell populations have similar surface expression profiles of mesenchymal progenitors (CD90, CD44, CD73, CD166, CD105, and CD29). However, the differentiation potential of hAMSCs was much greater than hAECs based on the results of Alizarin Red staining and qRT-PCR (osteopontin (OP) and ALP). Other studies also confirmed the osteogenic potential of these cells. For example, [PERSON] et al. showed the in vitro and in vivo potential of hAECs regarding bone regeneration [139]. They assessed an in vitro osteogenic differentiation process of the cells via biochemical staining, qRT-PCR, and immunofluorescence staining. The in vivo osteogenesis activity of hAECs was evaluated by implantation of the cells alone or in combination with \(\beta\)-tricalcium phosphate (\(\beta\)-TCP) scaffolds. The in vitro results showed up-regulation of bone-related genes (RUNX2, osterix (Oss), ALP, collagen I, and OP) with positive biochemical staining for osteoblasts. Moreover, the up-regulation of TGF-\(\beta\) was observed, which is related to the epithelial-mesenchymal transformation process involved in the osteogenic differentiation of hAECs. The in vivo implantation of the hAECs-seeded scaffolds led to enhanced bone reconstruction as well as reduced the early host immune response to the implanted scaffolds.  
To show the osteogenic potential hAMSCs, [PERSON] et al. in 2013 tried to differentiate hAMSCs into osteoblastic-like cells by using co-incubation with bovine bone matrix Nukbone(NKB) [140]. The authors prepared the NKB disks and placed them in cell culture plates in three different conditions including (1) positive control (AM-hMSC seeded on polystyrene disk with H-DMEM supplemented with ascorbic acid, dexamethasone, and \(\beta\)-glycerophosphate), (2) negative control (AM-hMSC cultured on polystyrene disk in H-DMEM), and (3) AM-hMSC with H-DMEM seeded on the NKB disk as experimental group. The obtained results showed more visible signs (over-expression of bone-related genes) of osteogenic differentiation of the cells cultured on the NKB disk when compared to other groups.  
In a comparative study, [PERSON] et al. investigated the osteogenic potential of three different types of stem cells including hAFMSCs, hAECs, and hBMSCs both in vitro and in vivo [141]. The obtained results revealed that there were significant differences among the cells in terms of cell morphology and proliferation and the immuno-phenotypical profile. The authors found that, although hAECs exhibited a confirmed osteoblastic differentiation capacity, hAFMSCs and hBMSCs showed a higher osteoblastic phenotype. All the cells were seeded onto \(\beta\)-TCP scaffolds and were subcutaneously implanted into the nude mice to further investigate the osteogenic capacity. The immuno-histochemical analysis of the samples confirmed the up-regulation of the osteogenic markers (OP and BGLAP) in all three groups (the \(\beta\)-TCP scaffolds seeded with hAECs, hAFMSCs, and hBMSCs) as compared to the controls, which indicates the in vivo osteogenic potential of all three cell sources. However, there was a different expression profile of genes involved in ossification among the cell types since hAFMSCs and hBMSCs show a higher osteoblastic phenotype as compared to hAECs.