The resident population of diverse progenitors is possibly recruited and selected for their developmental potential to meet a specific function. Where there is a cyclic demand for somatic cell renewal, short lived, intermediate transit amplifying (TA) cells proliferate extensively before differentiation into adult cells. Alternatively, an asymmetric cell division produces a reserve stem cell and a cytoplasmic partitioned progenitor cell committed to a specific pathway. Activated stem cells divide symmetrically to produce identical cells for self-renewal. SSCs are often multipotent and their lineage leads to uni-potent progenitors for terminal differentiated cells. The surrounding niche cells regulate stem and progenitor cells and serve both as a specific topographical and functional site. The stem cell population is often a mixture of quiescent stem cells (or active stem cells) and progenitor cells in various levels of differentiation. The niche histological composition varies extensively in different tissues but often includes stromal cells, extracellular matrix, blood vessels, neurons and tissue related precursor differentiated cells. Niches are tissue specific sites in vivo consisting of differentiated cells that modulate stem cells. During differentiation SSCs are established in unique cell/ECM niches. They are retained from organogenesis throughout life for cell maintenance, repair and regeneration. If the stem-progenitor cell population is normally depleted or destroyed by trauma, differentiated cells from the niche microenvironment can restore the specific stem potency which suggests the process of dedifferentiation.Īdult somatic stem cells (SSCs) are self-renewing groups of cells in tissues and organs that can produce specific lineages of precursor cells leading to differentiated cell progeny. Certain quiescent stem cells also serve as a potential cell reservoir for trauma induced cell regeneration through adaptive changes in differentiation of stem cells, progenitor cells and differentiated cells. A possible mechanism may be alteration in the differentiation capacity of the resident or introduced cells. Regulation of the progenitors is the result of signals from the stem cell niche that can cause adaptive changes in the behavior or function of the stem -progenitor cell lineage. When variations in the functional need of the tissue or organ occurs, the progenitor cells exhibit flexibility in their differentiation capacity. The activation of precursors appears to be stochastic and results in a population of heterogeneous progenitor cells. Nominally, multipotent stem cells in one or more niches follow specific lineages of differentiation that can be followed by diverse markers of differentiation. Some of their progeny are sequestered into separate cell niches of tissues as adult somatic stem cells at various times during organ development and differentiation These are diverse cell populations of stem and progenitor cells that respond to homeostatic needs for cell and tissue maintenance and the cycling of differentiated cells for physiological/ endocrinological changes. Cell and tissue specific somatic stem cells develop as dynamic populations of precursor cells to discrete tissue and organ differentiation during embryonic and fetal stages and their potential evolves with development.
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