Adult Stem Cell Sources

Such cells are often classified on the basis of their original source as either embryonic or adult stem cells. As the name suggests, embryonic stem cells are derived from the early embryo, whereas adult stem cells are present in various tissues of the adult species. Much of the earlier work on embryonic stem cells was conducted using mouse embryos. Human embryonic stem cells were first isolated and cultured in the laboratory in 1998. Research on adult stem cells spans some four decades, with the discovery during the 1960s of haematopoietic stem cells in the bone marrow (Chapter 10). However, the exact distribution profile, role and ability to manipulate adult stem cells (particularly those outside of the bone marrow) are subjects of intense current research, and for which more questions remain than are answered. 

Stem cells. Embryonic and adult stem cells are distinguished. Embryonic stem cells are taken from an early stage of the embryo, such as from blastocytes. They are undifferentiated and totipotent. Their potential to differentiate and to form different cell lines is unlimited. Adult stemcells are taken from the blood forming bone marrow, from epithelial cells from the skin and other sources. They are pluripotent. Both, embryonic totipotent and adult pluripotent stem cells can replace functionally differentiated cells and tissues in the body. Stem cells can divide. After division, they may form again a stem cell or proceed to a final, fully differentiated state. 

Stem cells normally are classified, based on their origin and differentiation capacity, as either embryonic or adult stem cells (1). Embryonic stem cells (ESCs) are derived from the inner cell mass of the blastocyst. ESCs can self-renew indefinitely and are pluripotent—(the ability to differentiate into all cell types in the embryo proper). Adult stem cells are undifferentiated (unspecialized) cells that are found in differentiated, or specialized, tissue. They have limited self-renewal capability and generally can only differentiate into the specialized cell types of the tissue in which they reside. These cells function as the reservoir for cell/tissue renewal during normal homeostasis or tissue regeneration. Sources of adult stem cells have been found in most tissues, including bone marrow, blood stream, cornea... 

Most of the aforementioned donor cells are derived from different types of stem cells. AU stem cells - whether from adult or embryonic sources - share a number of properties [24]. First, they are capable of self-renewal, which means that they can generate stem cells with similar properties. Second, the stem cells are clonogenic, which means that each cell can form a colony in which all the cells are derived from this single cell and have identical genetic constitution. Third, they are capable of differentiation into one or more mature cell types. The different stem cells can be categorized anatomically, functionally, or by cell surface markers, transcription factors, and the proteins they express. One clear division of the stem cell family is between those in adult somatic tissue (known as adult stem cells) and those isolated from the embryo (known as embryonic stem cells) (see Part I, Chapter 11). 

General Classification of Stem Cells Because adult somatic stem cells consist of a different population of the cells that share some common characteristics, it is difficult to rigorously divide the cells into some classifications. A new theory has currently proposed that stem cells are generated from a single cell source [21-24]. Thus, we review here adult stem cells according to the commonest classification as follows ... 

One of the early sources of adult stem cells to be investigated was bone marrow. A subset of multipotent progenitor cells was found to reside in the bone marrow niche and could be induced to differentiate down specific mesenchymal lineages [94], The bone marrow aspirate is collected from the iliac crest, followed by selection for plastic-adherent cells. 

Induced pluripotent stem cells are the adult cells that have been transformed into pluripotent stem cells through programming. Among adult stem cells, Mesenchymal stem cell is widely used as a multipotent source. Mesenchymal stem cell is derived from bone marrow stroma and can differentiate into a variety of cell types in vitro. Other sources of adult stem cells include the amniotic fluid and placental derived stem cells [29, 30]. 

Success for forming vessel networks in vivo has also been demonstrated using EPCs from human peripheral blood (Fuchs et al., 2009) particularly to address ischemia (Kawamoto et al, 2001, Kalka et al., 2000). Another source of adult stem cells that has similar proliferative potential but requires less invasive harvesting methods are adipose-derived stem cells from adipose stroma. These cells have demonstrated promising preclinical potential by differentiating into ECs and contributing to neovascularization upon implantation for the treatment of ischemia (Gao et al, 2005b, Miranville et al., 2004). 

Stem cells from a number of sources have been explored for regenerating the myocardium. Both multi-potent adult stem cells and pluripotent embryonic stem (ES) cells have been used to generate vascular cells and cardiac myocytes for therapy (Figure 34.3). A number of clinical studies have explored adult stem cells for their therapeutic potential in treating ischemic heart disease. While ES cells have only been investigated to date in preclinical models, a new clinical study is expected to assess safety of ES cell transplantation (Alper, 2009). In addition to stem cells, adult skeletal muscle cells have also been explored for clinical cardiac cell therapy. Each cell type is briefly reviewed in the context of myocardium regeneration. 

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