What do we mean by stem cells?
If a differentiated cell divides by mitosis, all its descendants will be identical to it and to one another. A stem cell can do more, producing stem cells or differentiating into specialised cells. They’re also self-renewing, which means that they can go on and on dividing to create more stem cells, maintaining the stem cell pool.
Stem cells vary in their potency (how much they can differentiate). A newly fertilised egg, or zygote, and the products of its first few divisions are made of totipotent cells. These cells can give rise to any cell type in the body, and the placenta, and so can produce a whole organism. Embryonic stem cells, a few steps beyond the egg in development, are pluripotent – they can become many types of specialised cell, but not placental cells.
Multipotent cells, which can give rise to just a few types of cell, include those in bone marrow that can generate red or white blood cells. Most adult stem cells (those found in differentiated tissues and organs) are multipotent. Unipotent cells, such as those in the skin, make just one fully differentiated cell type, usually where lots of new cells are needed regularly.
A lot of research focuses on the transitions between these states. One basic question is whether pluripotent cells maintain pluripotency ‘by default’ or need a continuing signal to keep them differentiating.
Recent research suggests that pluripotency, in embryonic stem cells, is mediated by a network of epigenetics (proteins that influence whether or not a gene is read) by activating genes involved in sustaining pluripotency and repressing genes that are involved in differentiation and development. Furthermore, it has been suggested that epigeneticmultipotent regulation (factors that affect how easily DNA can be read) works alongside transcription factors to maintain pluripotency by preventing development-related genes being read. It has also been suggested that this allows these genes to be inactive but still ready to be used if the cell needs to undergo rapid differentiation.Lead image:
Wellcome Library, London CC BY NC