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Stem cells are undifferentiated cells with extensive self-renewal properties that help them maintain their population during the fetal and adult stages of life. They can specialize in all cell types of the human body. However, their differential potential may vary and can be classified into five types. Stem cells can be (1) Totipotent, (2) Pluripotent, (3) Multipotent, (4) Oligopotent, and (5) Unipotent. Each stem cell has a specific origin; the fertilized egg or zygote is a totipotent cell and can develop into the embryonic and extraembryonic tissues. As development progresses, the zygote undergoes repeated rounds of cell divisions and eventually loses its totipotency to become a 64- cell mass called a blastocyst. The blastocyst consists of the rapidly proliferating inner cell mass (ICM) and the trophectoderm (TE) outer layer. Both ICM and TE are sources of pluripotent stem cells. ICM can differentiate into any embryonic tissue, including blood cells, muscle cells, neurons, skin, and intestinal cells, but not the extraembryonic tissues. In contrast, TE cells can only form the placenta, amnion, and chorion. ICM is also called embryonic stem cells or ES cells. ES cells are routinely used in regenerative medicine to treat stroke and neurodegenerative diseases such as Parkinson's or Alzheimer's.

Multipotent stem cells include adult stem cells or ASCs. They reside in the specific tissue type and produce cells of that particular tissue lineage only. For example, ASCs of the bone marrow are called hematopoietic stem cells or HSCs, which can differentiate into all blood cells but not gut cells or muscle cells. Oligopotent stem cells have further restricted tissue lineage and can differentiate to only fewer cell types of the tissue. The bronchoalveolar stem cells (BASCs) of the lungs are oligopotent and can differentiate into the bronchiolar epithelium or alveolar epithelium. Unipotent cells can only generate one kind of cell, such as muscle stem cells. They mature into only muscle cells in the body.

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