Stem Cells - The Source of Life
Stem cells are the cells that have a capacity for self-renewal (producing more of the same cell type) and differentiation into other cell types. Two major stem cell types are present in mammals, embryonic stem cells and adult stem cells.
Embryonic Stem Cells
Embryonic stem cells are found early in the development in the inner mass of the blastocyst and possess properties of both self-renewal and potency. The embryonic stem cells are pluripotent cells, meaning they can give rise to an unlimited number of other cell types.
Pluripotent embryonic stem cells are identified by the expression of several pluripotency markers, including OCT4, SOX2, and NANOG. These transcription factors suppress the genes that lead to differentiation and thus maintain the pluripotency of the cells.
During embryonal development, the embryonic stem cells differentiate and give rise to the three germ layers, including ectoderm, endoderm, and mesoderm, from which all the tissues and organs are ultimately formed. For instance, the dorsal part of the ectoderm will specialize into neuroectoderm, which later will undergo neurulation and encephalization and will finally develop into the central nervous system.
Adult Stem Cells
Adult (or somatic) stem cells are found in various tissues of the organism after birth. Adult stem cells are lineage-restricted (multipotent) and produce only the cells representing the tissue of origin. Together with progenitor cells, they contribute to the normal turnover of cells in adult tissues or may act as a repair system.