Naturally Occurring Stem Cells

As introduced in Chapter 1, stem cells are critical for tissue development and healing in the bodies of animals. They can be derived from prenatal (embryos, fetuses) or post-natal (newborns through adults) tissues. Stem cells have the ability to transform into more defined cell types (e.g., lung, skin, brain, blood), under particular growth conditions. This transformation is called differentiation. Factors that influence differentiation include signaling molecules (diffusible growth factors; nondiffusible extracellular matrix molecules), and the features of the structural surroundings (e.g., connective tissue and other biological/nonbiological scaffolds). These factors work by turning on or off the expression of differentiation- associated genes. The varied expression of genes among the different cell types is what defines them. Levels of differentiation can be described in terms of potency. Potency is dependent on stem cell origin. For example, embryonic stem cells are more potent than those isolated from adult tissues. The most undifferentiated stem cells are called totipotent. These cells can turn into any type of cell within the adult body. Pluripotent cells have the potential to differentiate into many cell types, but not all. Next comes multipotent, oligopotent, and finally unipotent. Pluripotent cells can be artificially created from differentiated cells.

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