However, the discovery of nuclear reprogramming methods such as somatic cell nuclear transfer method and use of transcriptional factors to induce pluripotency in any cell type are demonstrated as powerful tools to reverse this hierarchy ( Gurdon, 1962 Kato and Tsunoda, 1993 Campbell et al., 1995, 1996 Wilmut et al., 1997 Kato et al., 1998 Wakayama et al., 1998 Wakayama and Yanagimachi, 1999 Takahashi and Yamanaka, 2006 Takahashi et al., 2007). Both the self-renewal capacity and differential potential are reduced during their journey from totipotent to mature cell state. Thus, the traditional developmental dogma follows the differentiation of totipotent stem cells to PSCs, PSCs to multipotent stem cells, multipotent stem cells to unipotent stem cells and finally mature cells. The developmental potential of unipotent stem cells is further restricted and they remain able to give rise to only a single cell type (for example, blast forming unit-erythroid (BFU-E) may be differentiated into erythrocytes). The multipotent stem cells do not differentiate into cell types of different tissue origin under normal physiological circumstances. Another important class of stem cells is lineage specific multipotent stem cells which have limited differentiation potential and develop only in their tissue/cell types. These cells are termed as PSCs and have various functional properties depending on their culture conditions. Both human and mouse derived Embryonic Stem Cells (ESCs) are demonstrated to generate all types of cells, but lack potential to contribute to the extra-embryonic cells such as placenta. It is important to notice that cells derived from fertilized egg (zygote/blastomeres) have the potential to generate all the embryonic and extra-embryonic cells potential referred to as totipotency and thus can give rise to the whole organism, however their developmental potential remains undefined in vitro ( Kelly, 1977). Whereas, multipotent and unipotent stem cells are defined to bear limited self-renewing capacity and these cells, differentiate into a particular tissue type or cell lineage. The most primitive stem cells have the potential to generate all the cell types in any organism and termed as pluripotent stem cells (PSCs) ( Hanna J. Stem cells are unique in their capabilities to either self-renew or differentiate into various cell lineages. The careful evaluation of all these methods could help in generating standard identifying procedures/markers for them. Many reports are defining the particular set of different functional assays, and molecular marker used to demonstrate the developmental states and functional capacities of stem cells. These molecular markers include diagnosis of transcriptional, epigenetic, and metabolic states of stem cells. The potency of these stem cells can be defined by using a number of functional assays along with the evaluation of various molecular markers. While pluripotent stem cells may give rise to all types of cells in an organism, Multipotent and Unipotent stem cells remain restricted to the particular tissue or lineages. They are classified as pluripotent, multipotent, and unipotent as demonstrated through their potential to generate the variety of cell lineages. Stem cells are defined by their capabilities to self-renew and give rise to various types of differentiated cells depending on their potency.
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