Abstract
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. 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. Nominally, multipotent stem cells in one or more niches follow specific lineages of differentiation that can be followed by diverse markers of differentiation. The activation of precursors appears to be stochastic and results in a population of heterogeneous progenitor cells. When variations in the functional need of the tissue or organ occurs, the progenitor cells exhibit flexibility in their differentiation capacity. 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. A possible mechanism may be alteration in the differentiation capacity of the resident or introduced cells. 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. 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.