Comparison between Culture in Hypoxic and Ambient EnvironmentsSince 1963, when the isolation and self-renewing properties of mouse bone marrow cells were first reported [78, 79], until now most of the research efforts have been focused on the identification of molecular markers [4, 80, 81]. This has allowed sellectchem the isolation of different types of tissue-specific stem or progenitor cells [82�C85] and has also assisted to define the differentiation of stem or progenitor cells into a particular cell type [86, 87]. Moreover, the development of specific methods for functional stem cell isolation and identification is highly important, in order to study the molecular mechanisms behind the multipotentiality and self-renewable capacity of stem cells and also for the establishment of stem cell-based regenerative therapeutics.

This trend has overshadowed the importance of O2 concentration, a key environmental factor that might play a vital role on stem cell fate and function [40]. Unfortunately till now in most laboratories, stem cells are typically cultured under the ambient O2 concentration without paying attention to the metabolic milieu of the niche in which they grow or normally reside [88]. However, in recent years, scientists have started to manipulate the O2 concentration in cell cultures by maintaining a niche-like hypoxic environment. Though the effect of hypoxic culture conditions on the proliferation and differentiation potential of MSCs has been reviewed by few researchers [77, 89], the effect of hypoxia on the genetic stability, early senescence, and site-specific migration of MSCs has not been reviewed in depth.

Thus, on the basis of recent research outcomes, the effect of different O2 concentration on MSCs biology is further discussed.3.1. Proliferation of MSCsCapability for self-renewal is a key feature of stem cells. An increased proliferation rate is necessary for more efficient use of stem cells in regenerative therapies. Fehrer et al. (2007) demonstrated that bone marrow-derived MSCs (BM-MSCs) cultured in 3% O2 concentration showed significantly increased in vitro proliferative lifespan, with approximately 10 additional population doublings (PDs) (28.5 �� 3.8PD in 20% O2 and 37.5 �� 3.4PD in 3% O2) before reaching senescence compared to cells cultured in the ambient O2 environment [38].

In addition, early passaged MSCs cultured in hypoxic conditions also exhibit increased proliferative lifespan along with significant difference in population doubling [37]. Furthermore, it is possible to harvest more than 1 �� 109 MSCs from the first five passages cultured in 3% O2, whereas in ambient condition only 2 �� 107 cells can be obtained [37]. Higher in vitro expansion rate in hypoxic conditions has also been reported Dacomitinib by several other researchers [90�C93].