ABSTRACT
Human embryonic stem cells (hESCs) are derived from the inner cell mass (ICM) of the blastocyst produced by in-vitro fertilization (IVF) techniques. They share with ICM the ability to self-renew and the pluripotent capacity.1 In addition, their morphological characteristics are similar to ICM cells:2 large nuclei with a high nucleus-to-cytoplasm ratio, characteristic for the short cell cycle; prominent nucleoli and very little heterochromatin, indicative of open chromatin structure, a feature of pluripotent cells; expression of markers of pluripotency. We can anticipate that in-vitro manipulations to maintain their unlimited self-renewal and the artificially created microenvironment (stem cell niche) induce epigenetic changes to adjust to cell culture conditions. This is the reason why ESCs can be considered as culture ‘artifacts’ in the sense that their properties and fate are to some extent different from those of the ICM cells of the blastocyst. In spite of these limitations, hESCs can be used to model early human development that is, for ethical reasons, impossible to study directly. The other application of hESCs is to ‘tailor’ them to meet our needs by acquiring sometimes therapeutically useful, unphysiological properties that are not exhibited in a blastocyst. As the most important characteristic of hESCs is their pluripotency, the ability to generate every cell type of a body, it is important to ensure that hESCs remain capable of generating functionally normal cells. We hypothesize
that to retain this ability the in-vitro conditions for hESC derivation and propagation in the undifferentiated state should mirror to some extent the in-vivo environment. What do we know about the transitional microenvironment of the ICM cells? Can we use some of the available information to produce optimal culture systems that will provide the environment for the most accurate gene replication, activation and epigenetic modifications? Here, we are going to address the role of oxygen tension and present evidence to support derivation and propagation of hESCs in low oxygen.
