Stem Cells

Stem cells can renew themselves through mitotic cell division and have the capability to differentiate into more specialized cells. Based on the differentiation capability, stem cells are classified as follows:

Totipotent stem cells: They can differentiate into all types of cells for the development of an organism. Examples: the zygote (fertilized egg) and the cells produced by its first few divisions.

Pluripotent stem cells: They can differentiate into many types of cells. Examples: cells in the inner cell mass of a blastocyst.

Multipotent stem cells: They can differentiate into several kinds of cells. Examples: (1) Hematopoietic stem cells that give rise to red blood cells, white blood cells and platelets. (2) Skin stem cells that give rise to various types of skin cells.

At the molecular level, the major difference between totipotent stem cells and more specialized cells lies in the pattern of DNA methylation. Methylation at a specific region (e.g., enhancer or promoter) of a gene can suppress its expression while demethylation allows the gene to be expressed. Two transcription factors, Oct-4 and Nanog, are known to play a critical role in maintaining the pluripotency of stem cells. Oct-4 prevents the differentiationof the inner cell mass into trophoblast cells,while Nanog blocks the differentiation into primitive endoderm. The two genes are suppressed in differentiated cells. Their expression and suppression have been demonstrated to be controlled by DNA methylation (reference).

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