Stem cells are a class of undifferentiated cells that are able to differentiate into specialized tissue cell types. There are two types of stem cells: embryonic stem cells (ESC) and adult stem cells (ASC). The ESC is originated from the inner cell of blastocysts, and the ASC is located in specific tissues, such as bone marrow, adipose tissue and blood.
In ESC, BMP/TGF-β signaling pathway plays a key role in maintaining pluripotency and self-renewal. It signals through Smad proteins, and the FGF signaling pathway, which activates the MAPK and Akt pathways. The Wnt signaling pathway also promotes pluripotency. OCT-4, SOX2, and NANOG are three main transcription factors that are expressed and activated by these pathways. Induced pluripotent stem cells (iPSC) are pluripotent cells that can be generated from differentiated cells with forced expression of specific reprogramming factors. Both ESC and iPSC can be induced to develop into distinct cell types that associated with three primary germ layers: ectoderm, mesoderm and endoderm. Signaling pathways that control the development of these cell lineages, including BMP/TGF-β, Notch, Wnt/β-catenin, Hedgehog and Hippo pathways, which regulate cell division, growth and differentiation. Defects in stem cell signaling are related to developmental disorders and cancer.