# Hypoxia Modulates Regenerative Potential of Fetal Stem Cells
## 4 Discussion
Furthermore, we found that both NPC normoxia and hypoxia expanded cell samples showed significantly higher expression levels of condensation marker genes _CDH2_ and _FN1_ than SDSC normoxia and hypoxia expanded cell samples. Given that cell condensation is an integral component of chondrogenesis [23; 24], as indicated by the higher _CDH2_ and chondrogenic marker gene expression relative to corresponding SDSC pelletsdespite a smaller pellet size, NPCs likely form denser chondrogenic pellets than SDSCs. This characteristic is expected as NPCs are more closely related to chondrocytes in the undifferentiated state. However, even though NPCs share a common phenotype with articular cartilage cells along with similar expression of COL2A1, ACAN, and SOX9, NPCs display morphologic and physiologic differences and can be distinguished using gene expression. FOXF1 has previously been identified as a potential human NPC marker as it displays the highest expression levels in NPCs in comparison to other cell sources [22]. In contrast, FBLN1 is an articular cartilage marker and a negative NP marker [31]. Accordingly, our data indicated that D0 SDSC pellets had higher _FBLN1_ expression than their NPC counterparts, which is reasonable since SDSCs are more closely related to articular cartilage. Later on, during chondrogenic induction, NPC pellets displayed a large increase in _FBLN1_ levels, suggesting characteristic changes toward an articular cartilage-like phenotype. This finding was also confirmed by _FOXF1_ levels, which were initially more highly expressed in NPCs but lowered in expression during differentiation. Interestingly, while our data show that NPCs demonstrate greater chondrogenic potential than SDSCs, SDSCs showed greater adipogenic and osteogenic potentials. These results are mirrored by a previous study comparing fetal spine cells and adult BMSCs which found fetal spine cells displayed lower adipogenic and osteogenic potential [32]. In BMSCs, hypoxia has been shown to enhance MSC proliferation and osteogenesis, likely through growth factor production [33]. However, as an intervertebral disc cell, NPCs have been clinically observed to hinder disc calcification, likely due to NPC secretion of bone morphogenetic protein antagonists, such as chordin, gremlin 1, and noggin, that inhibit osteogenic differentiation [34]. In our study, we found that hypoxic expansion and differentiation increased adipogenic capacity. The finding aligns with reports that hypoxic exposure during expansion enhances adipose-derived MSC adipogenic potentials [35]. Certain studies have found that hypoxia (1-2%) can trigger reactive oxygen species generation, thus inducing adipogenesis [36; 37]. This increase in adipogenesis is further observed in BMSCs under extreme hypoxia (0.2%) [38]. It should be noted that in this study, we utilized human fetal stem cells. Unlike human adult stem cells, human fetal stem cells exist in an inherently different environment in which, while the overall fetal physiological environment is slightly more hypoxic than the adult physiological environment, fetal immature cartilage typically has blood vessels. Compared to adult MSCs, fetal MSCs have shown greater self-renewal capacity and multilineage potential, which make them good prospective candidates for cell-based therapies. Furthermore, fetal cells have low immunogenicity as they express lower levels of human leukocyte antigen (HLA) class I and do not express HLA class II [39; 40], thus making them more suitable candidates for therapeutic transplantation. Compared to embryonic stem cells, they are also less ethically controversial. A limitation of this study is that the culture system is not perfect. We used a standard incubator of 20% oxygen and 5% carbon dioxide to represent \"normoxia\"; however, we should note that 20% oxygen is higher than most cell physiological environments [41].