# Hypoxia Modulates Regenerative Potential of Fetal Stem Cells
## 1 Introduction  
Adult mesenchymal stem cells (MSCs) are a potential solution for cell-based tissue engineering and regeneration due to their self-regenerative capacity and potential to differentiate into a multitude of cell types [1]. However, many challenges exist. Given the fact that most patients are elderly, their stem cells are inherently senescent. Moreover, a large quantity of autologous adult stem cells is required for tissue engineering--a process that necessitates long-term ex vivo expansion, which can cause cell senescence, resulting in lowered plasticity and limited proliferation capacity [2]. These challenges restrict the clinical application of adult MSCs despite their well-characterized properties [3].  
In comparison to adult MSCs, fetal MSCs have greater differentiation potential, lower immunogenicity, longer telomeres, and increased intrinsic homing and engraftment ability [4, 5]. Despite their immense potential for regenerative medicine, given ethical concerns [6], there exists limited knowledge regarding the regenerative potential of fetal MSCs and their responses to external environment stimulation, which have been well-characterized in adult MSCs, such as decellularized extracellular matrix (dECM) [7], hypoxia, and soluble factors such as basic fibroblast growth factor [8, 9]. Our previous report found that, as an alternative to adult MSCs, which experience replicative senescence, human fetal synovium-derived stem cells (SDSCs) from passage 9 exhibited higher chondrogenic potential than those from passage 2 [10], indicating that fetal MSCs might have different regenerative potentials compared to adult MSCs. We also found that expansion on dECM could modulate the chondrogenic potential of human fetal SDSCs [10, 11], suggesting that the external environment can adjust fetal cell biological properties for regenerative purposes. There are few articles investigating the influence of hypoxia on the- regenerative potential of fetal stem cells despite the importance of hypoxia in adult stem cell differentiation [12].  
In this study, two types of fetal cells with chondrogenic potential [13], SDSCs (a tissue-specific stem cell for chondrogenesis) [14] and nucleus pulposus cells (NPCs, a chondrogenic progenitor cell) [15], were used to characterize the influence of hypoxia in both the cell expansion (differentiation potential) and chondrogenic induction phases (degree of differentiation). Hypoxia is critical for chondrogenesis [16], given that articular cartilage is an avascular tissue with oxygen levels of about 1-5% [17]. Furthermore, increasing evidence indicates that hypoxia plays a role in the adipogenesis and osteogenesis of adult MSCs [18] and the inherently hypoxic stem cell niche facilitates signal transduction [19]; however, we still lack knowledge regarding the influence of hypoxia on the adipogenic and osteogenic differentiation of fetal cells. In addition to clarifying the complex interplay of hypoxia and fetal stem cells in an in vitro cell culture environment, the findings of this study will also facilitate our understanding of the crosstalk between chondrogenesis and adipogenesis [20], as well as osteogenesis and adipogenesis [21].