Stem cells: types, properties, and history

Definition and lineage position of stem cells

Stem cells are undifferentiated or partially differentiated cells in multicellular organisms that can give rise to multiple cell types and proliferate indefinitely to produce more stem cells. They are the earliest cells in a cell lineage and are found in both embryonic and adult organisms, though their properties differ. Stem cells are distinguished from progenitor cells (which cannot divide indefinitely) and from precursor/blast cells (which are usually committed to differentiating into a specific cell type). In mammals, embryonic stem cells (ESCs) originate from the inner cell mass (ICM) of the blastocyst (about 50–150 cells at days 5–14). These ICM cells have stem-cell capability and, in vivo, differentiate into all body cell types through the formation of the three germ layers (ectoderm, mesoderm, and endoderm), making them pluripotent. When isolated and cultured in vitro, they can be maintained in the stem-cell state as ESCs. Adult stem cells reside in specific body niches (e.g., bone marrow or gonads) and are typically multipotent or unipotent, serving to replenish rapidly lost cell types. Adult stem cells are a small minority of cells compared with progenitors and terminally differentiated cells.

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Stem cells versus progenitor and precursor cells

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