Pluripotency refers to the ability of cells to self-renew and differentiate into all 3 germ layers (ectoderm, endoderm and mesoderm). Human spermatogonial stem cells can be purified by antibodies against cell surface markers CD9, CD49f and GPR125 ( Conrad et al., 2008). They are unipotent in nature and continuously generate differentiating daughter cells for subsequent production of spermatozoa ( Fagoonee et al., 2011). Testicular spermatogonial stem cells are the germ-line cells for spermatogenesis, an ongoing process throughout the lifespan of the male animals. Therefore, human periodontal ligament-derived stem cell populations have been characterized not only by mesenchymal stem cell markers, but also by neural crest cell markers, such as p75, nestin, Slug and SOX10 (Huang et al., 2009 Mrozik et al., 2010). Periodontal ligament-derived stem cells are heterogeneous, composed of mesenchymal stem cells and putative neural crest cells. The periodontal ligament contains stem cell populations that can differentiate into cementum-forming cells or bone-forming cells ( Seo et al., 2004). Periodontal ligament, derived from the cranial neural crest, is a soft connective tissue embedded between the tooth root and the alveolar bone socket, supporting the teeth in situ and preserving tissue homeostasis. Human mesenchymal stem cells can be characterized by the positive expression of CD29, CD44, CD73, CD90, CD105, CD146 and STRO-1, and the negative expression of CD31, CD34, CD45, CD49f and CD133 ( Mödder et al., 2012 Tárnok et al., 2010). Mesenchymal stem cells differentiate into osteocytes, chondrocytes and adipocytes ( Arita et al., 2011 Pittenger et al., 1999). Mesenchymal stem cells were originally identified in the bone marrow, but have since been found in other systems such as adipose tissue, umbilical cord and menstrual blood ( Ding et al., 2011). Mesenchymal stem cells, also called marrow stromal cells, are another well-studied adult stem cell population. Hematopoietic stem cells can be characterized by positive selection of CD34, CD45, and CD133 markers and negative selection of CD31, CD105 and CD146 markers ( Tárnok et al., 2010). Adult bone marrow, umbilical cord blood and mobilized peripheral blood are sources of hematopoietic stem cells for transplantation in many blood-related diseases. They function to generate all cell lineages found in mature blood (erythroid, myeloid and lymphoid) and to sustain blood production during the entire life of an animal ( Kondo et al., 2003). Hematopoietic stem cells are the most characterized adult stem cell population. This chapter focuses on four adult stem cell populations: hematopoietic, mesenchymal, periodontal ligament-derived, and spermatogonial ( Table 1). The existence of adult stem cells has been reported in multiple organs these include: brain, heart, skin, intestine, testis, muscle and blood, among others.
The transiently-amplifying cells will undergo a limited number of cell divisions before terminal differentiation into mature functional tissue cells. Adult stem cells are presumed quiescent within adult tissues, but divide infrequently to generate a stem cell clone and a transiently-amplifying cell. The function of adult stem cells is the maintenance of adult tissue specificity by homeostatic cell replacement and tissue regeneration ( Wagers and Weissman, 2004). Stem cells found in fully developed tissues are defined as adult stem cells. Stem cells are undifferentiated cells defined by their abilities to self-renew and differentiate into mature cells.
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