Types of stem cells :
There aremain types of stem cells , embryonic and adult , during embryogenesis , pluripotent embryonic stem cell is derived from the inner cell mass of blastocyst and has the ability to give rise to all three embryonic grem layers : ectoderm, endoderm and mesoderm (chambers & smith ,2004)
As the development proceeds , the need for organogenesis arises, and the embryo proper forms grem line stem cells (GSCs) for reproduction and somatic stem cells (SSCs) for organogenesis . they either are progressively restricted in the development , giving rise to multiple lineages ( including tissue-specific cells ), or are unipotent, giving rise ti single lineage cells destined for certain tissues. (chambers & smith ,2004)
After birth .adult stem cells, including both GSCs and SSCs, reside in special microenvironment termed the "niche," whish varies in natural and location depending on the tissue type the support ongoing tissue regeneration replacing cells lost due to natural cell death (apoptosis) or injury . (weissman, 2000; rossant2004)
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