Stem cells: on the front line.

I am pleased to write this introduction to the new Journal of Cell Science Article Series on Stem Cells. Stem cell research is now regarded as one of the most exciting fields of biomedical research. However, its origins lie in several distinct research disciplines, including developmental biology, pathology and immunology. Developmental biologists have long been interested in how different cell types arise in the embryo, whereas a desire to understand how tissues are maintained in adulthood and how those processes fail in disease falls within the remit of pathologists. A key feature of adult tissues is cellular heterogeneity – the origin of diverse types of blood cell from a less-differentiated cell led immunologists to make major contributions to the field of stem cell research. Stem cells can be defined as cells with extensive renewal capacity and the ability to generate daughter cells that undergo further differentiation. They have a number of different origins and properties. Adult stem cells generate the differentiated lineages that are appropriate for the tissue in which they reside and are unipotent or multipotent, depending on whether they give rise to one or multiple differentiated cell types. Stem cells cultured from pre-implantation mouse and human embryos are known as embryonic stem (ES) cells and are pluripotent: they have the ability to generate all the differentiated cells of the adult organism. Pluripotent stem cells can also be derived from the post-implantation epiblast of mouse embryos (epiSCs) and from primordial germ cells (EG cells), progenitors of adult gametes. Adult cells can be reprogrammed to a pluripotent state by the transfer of an adult nucleus into the cytoplasm of an oocyte, by fusion with a pluripotent cell or by conversion to induced pluripotent stem cells (iPS cells). The first iPS cells were generated by retrovirus-mediated transduction of mouse fibroblasts with Oct4, Sox2, KLF4 and Myc. iPS cells can also be generated from adult human cells, from a range of tissues and from cells of patients with specific diseases. The reason why stem cell research is such an exciting field is that it not only offers the opportunity to understand fundamental mechanisms of self-renewal, differentiation and disease, but also because that knowledge can be applied to develop new treatments for a variety of diseases. Stem cell behaviour is governed both by intrinsic mechanisms, such as specific transcription factors, and by extrinsic signals from the local environment (i.e. microenvironment or niche). These …