There are many adult stem cells, perhaps one or more for each tissue of the body. The unexpected and simplifying conclusion is that all adult stem cells have a similar profound paracrine activity that is essential for the normal turnover and regeneration especially active and discernable at sites of injury. The important distinction of this focus is the long and interesting list of cellular features that all adult stem cells have in common, not on the complex spectrum of distinctive phenotypes that the stem cells can become. Below, the focus of this treatise is only on adult human stem cells and their embryonic forefathers are left to others to discuss. This is even more complicated because there are both an array of tissue progenitors at various progressive stages of embryonic development and tissue-specific adult stem cells, which do different things and contribute to the changing biology of the organism in intricately different ways. Clearly, the array of stem cells and their differences seem almost too complicated to grasp which may require simplifying groupings or new logics. There are now a large number of human stem cells that are being studied and reported in both the scientific and lay literature.
Moreover, each stem cell establishes a unique habitat next to or in contact with blood vessels. Thus, the HSC and MSC do the same injury function although they are uniquely different. The HSC and MSC function at tissue sites of injury to cause the tissue to regenerate. The mesenchymal stem cell (MSC) can produce fat or bone. For example, the hematopoietic stem cell (HSC) produces all the cells in blood. However, stem cells from different tissues have a common core of properties that are similar in function. These stem cells can divide and differentiate into a specialized tissue like muscle or liver. These stem cells provide the basis for the maintenance, repair, and regeneration of these tissues. The adult body has many stem cells, at least one for every major tissue. These similarities, especially their unique interactions and association with vasculature, distinguish these adult cells from their embryonic progenitors and by virtue of these similarities define these unique adult stem cells. They are all dominant cells, multipotent, have a well-defined niche on or in contact with blood vessels, they all actively sense and respond to their local environment, they are all paracrine secretors, they are all immune-modulatory and trophic, and they all have a profound effect on site-specific regeneration following injury. These striking differences aside, MSCs, NSCs, and hematopoietic stem cells (HSC) have a very striking number of similarities. Mesenchymal stem cells (MSCs) are quite different in comparison with neural stem cells (NSCs) when focusing on the range of different differentiated cell types that can develop from their progeny. In addition, there exists a class of adult stem cells that are multipotent and/or have injury-specific functions. Every adult tissue has its own resident committed progenitor or stem cell.
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