Stem cells

Stem cells are defined by two properties: self-renewal (division producing more stem cells, often by asymmetric division that yields one stem cell and one committed progenitor) and potency (the range of cell types they can produce). They usually reside in a protective microenvironment, the niche, which controls their quiescence and activation.

Totipotent cells — the zygote and the earliest blastomeres — can form an entire organism including the extra-embryonic tissues (trophoblast/placenta). Pluripotent cells — the inner cell mass of the blastocyst and the embryonic stem (ES) cells derived from it — can form every cell type of the three germ layers but not the trophoblast; induced pluripotent stem (iPS) cells are adult cells reprogrammed to this state (Oct4, Sox2, Klf4, c-Myc). Multipotent adult (tissue) stem cells produce the cell types of one tissue or lineage: haematopoietic stem cells in bone marrow (CD34⁺; all blood cells), mesenchymal stem/stromal cells (bone, cartilage, fat), intestinal crypt stem cells (Lgr5⁺; renew the gut lining every ≈3–5 days), basal cells of the epidermis and airway, and neural stem cells (subventricular zone, hippocampal dentate gyrus). Unipotent stem cells produce one cell type but still self-renew — e.g. skeletal muscle satellite cells and spermatogonial stem cells.

Adult stem cells sustain the labile tissues (skin, gut, blood) and allow repair. Their limited numbers, niche dependence and the accumulation of mutations in them with age link stem cells to both regeneration and cancer.