Chapter 1: Stem Cell Maintenance and Self-renewal
In order to maintain the pluripotency of embryonic stem cells (ESCs), they are commonly cultured in the presence of feeder cells, serum and growth factors. Growth factors related to crucial signaling pathways involved in the maintenance of pluripotency have been identified and include Leukemia inhibitory factor (LIF) and bone morphogenic protein (BMP) for mouse ESCs (mESCs) and fibroblast growth factor 2 (FGF-2, basic FGF) and TGF-β for human ESCs (hESCs). Wnt signaling was also determined to be important for the maintenance of both mESCs and hESCs.
The presence of undefined components, such as the feeder cells and serum, can contribute to experimental variability and pathogenic contaminations. To alleviate this issue, feeder and serum-free culture protocols were developed. Small molecules are valuable components in these feeder-free and serum-free conditions, because they are chemically defined, of guaranteed high purity and lot to lot consistency, and have biological effects that are usually rapid, reversible and dose dependent.
Small molecules can maintain pluripotency by either inhibiting pathways that induce differentiation, such as the MEK-ERK and Ras-GAP signaling, or activating self-renewal promoting pathways, such as Wnt signaling.
While regulating a single pathway can benefit cell self-renewal, using small molecules with synergistic properties can prove to be more effective. Some small molecules can also target a component that is involved in several pathways, such as the inhibitors of the glycogen synthase kinase-3β (GSK-3β), which regulates Wnt/β-catenin and Hedgehog signaling, among others.
Another important issue that should be considered when culturing hESCs is the fact that they are much more sensitive than mESCs to enzymatic dissociation during the passages involved in the cultivation process. The addition of small molecules alone, or in combination with other reagents, was found to lessen this type of cell death.
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