Stem cells by potency, and plasticity

Stem cells are also grouped by potency: how many kinds of cell they can become. From most to least:

• Totipotent: can form the whole organism, including the embryo and the extra-embryonic membranes (such as the placenta). The zygote, and all the cells of the embryo up to the eight-cell stage, are totipotent.
• Pluripotent: can form almost all cell types, from all three germ layers, but not the placenta or yolk sac. The inner cell mass of the blastocyst is pluripotent (the outer layer, the trophectoderm, forms the placenta), as are ESCs.
• Multipotent: can form a closely related family of cells. Haematopoietic stem cells give red blood cells, white blood cells and platelets; neural stem cells give neurons and the two main support cells of the nervous system, oligodendrocytes and astrocytes.
• Unipotent: can make only their own type of cell: for example, epidermal (skin), muscle and endothelial stem cells.

So totipotent and pluripotent cells belong to embryonic life, while multipotent and unipotent cells are found in adult life (as adult or somatic stem cells).

Plasticity. Some adult stem cells, given the ability (competence) and the right surroundings, can become cells of a different tissue. Several explanations have been proposed:

• trans-determination: a stem cell switches to another lineage;
• trans-differentiation: a specialised cell takes on the character of another specialised cell type;
• dedifferentiation: a specialised cell goes back to a less specialised state, then specialises again (common in lower vertebrates, rare in mammals);
• heterogeneity: a tissue may contain several kinds of stem cells;
• cell fusion: a stem cell fuses with a cell of another tissue.