and Valentin Häcker conducted experiments on Ascaris and Cyclops, respectively,
and postulated that the cells between the fertilized egg and committed germ cells can
be considered as stem cells. It was during the nineteenth century that the pathologists
Durante and Conheim for the first time hypothesized the existence of stem cells in
adults as “embryonal rests” in order to include the survival of embryonic-like stem
cells in adult tissues (Sell 2004). In 1981, Martin Evans and group set an important
milestone by establishing an in vitro culture for non-malignant pluripotent cells
derived from mouse embryos (Evans and Kaufman 1981). During the same year Gail
Martin too isolated cells from the mouse embryo and named “stem cells,” since it
seemed that the isolated cells could grow into nearly all cell types (Martin 1981).
The fertilized egg, which is considered to be the ultimate stem cell, is formed
from the fusion of haploid germinal stem cells. In adults, the tissues are renewed by
proliferation of the stem cell pool, which divide stem cells. In adults, the tissues are
renewed by proliferation of the stem cell pool, which divide asymmetrically yielding
an adult stem cell and a progenitor cell. The progenitor cell divides further to
ultimately produce terminally differentiated cells of the tissue with a limited lifespan
in the tissues which are lost by wear or through apoptosis.
The differentiating progeny of the stem cells leads to tissue renewal. It has been
demonstrated that stem cells have the ability to produce progenitor cells for tissue
repair and renewal which opens up vast panorama of applications in the regenerative
biology. Determination or lineage commitment is a specialized function acquired by
stem cells during differentiation. As the stem cells differentiate to give rise to
specialized cells, with decreasing differentiation potential and finally to the terminally differentiated cells. Classic embryology considers determination as an irreversible process; once a cell is differentiated into adult, it is completely stable and not
able to revert to stemness (Surani 2001). However, it has been demonstrated by the
researchers that this differentiated cells can be reversed to stemness a process called
“de-differentiation.” One way to do de-differentiation is through Somatic Cell
Nuclear Transfer (SCNT) where a nucleus is transferred from differentiated adult
cell into an enucleated oocyte which results in restoration of totipotency of the
nucleus in the oocyte (Hakelien and Collas 2002; Hochedlinger and Jaenisch
2002; Surani 2001; Wilmut et al. 1997).
More recently Shinya Yamanaka has developed the concept of iPSCs (Induced
Pluripotent Stem Cells). His lab has developed a methodology to reprogram the
differentiated cells into stem cells which they have referred to as—induced pluripotent stem cells (iPSCs) (Takahashi and Yamanaka 2006). iPSCs have the potential to
present themselves as an ideal source of stem cells source for cell-based therapy and
regenerative medicine in future (Novak et al. 2010).
Embryologists and developmental biologists have known the power of stem cells
for tissue development, regeneration, and renewal for many years (Sell 2004). In the
recent years, a lot of research and development work has been focussed on understanding the biology of stem cells derived from different sources. Researchers are
working on different aspects of stem cells including differentiation, transdifferentiation, and de-differentiation and more recently the work is initiated on
iPSCs and their role in regenerative medicine. All these developments have brought
252
A. Kumar et al.
and postulated that the cells between the fertilized egg and committed germ cells can
be considered as stem cells. It was during the nineteenth century that the pathologists
Durante and Conheim for the first time hypothesized the existence of stem cells in
adults as “embryonal rests” in order to include the survival of embryonic-like stem
cells in adult tissues (Sell 2004). In 1981, Martin Evans and group set an important
milestone by establishing an in vitro culture for non-malignant pluripotent cells
derived from mouse embryos (Evans and Kaufman 1981). During the same year Gail
Martin too isolated cells from the mouse embryo and named “stem cells,” since it
seemed that the isolated cells could grow into nearly all cell types (Martin 1981).
The fertilized egg, which is considered to be the ultimate stem cell, is formed
from the fusion of haploid germinal stem cells. In adults, the tissues are renewed by
proliferation of the stem cell pool, which divide stem cells. In adults, the tissues are
renewed by proliferation of the stem cell pool, which divide asymmetrically yielding
an adult stem cell and a progenitor cell. The progenitor cell divides further to
ultimately produce terminally differentiated cells of the tissue with a limited lifespan
in the tissues which are lost by wear or through apoptosis.
The differentiating progeny of the stem cells leads to tissue renewal. It has been
demonstrated that stem cells have the ability to produce progenitor cells for tissue
repair and renewal which opens up vast panorama of applications in the regenerative
biology. Determination or lineage commitment is a specialized function acquired by
stem cells during differentiation. As the stem cells differentiate to give rise to
specialized cells, with decreasing differentiation potential and finally to the terminally differentiated cells. Classic embryology considers determination as an irreversible process; once a cell is differentiated into adult, it is completely stable and not
able to revert to stemness (Surani 2001). However, it has been demonstrated by the
researchers that this differentiated cells can be reversed to stemness a process called
“de-differentiation.” One way to do de-differentiation is through Somatic Cell
Nuclear Transfer (SCNT) where a nucleus is transferred from differentiated adult
cell into an enucleated oocyte which results in restoration of totipotency of the
nucleus in the oocyte (Hakelien and Collas 2002; Hochedlinger and Jaenisch
2002; Surani 2001; Wilmut et al. 1997).
More recently Shinya Yamanaka has developed the concept of iPSCs (Induced
Pluripotent Stem Cells). His lab has developed a methodology to reprogram the
differentiated cells into stem cells which they have referred to as—induced pluripotent stem cells (iPSCs) (Takahashi and Yamanaka 2006). iPSCs have the potential to
present themselves as an ideal source of stem cells source for cell-based therapy and
regenerative medicine in future (Novak et al. 2010).
Embryologists and developmental biologists have known the power of stem cells
for tissue development, regeneration, and renewal for many years (Sell 2004). In the
recent years, a lot of research and development work has been focussed on understanding the biology of stem cells derived from different sources. Researchers are
working on different aspects of stem cells including differentiation, transdifferentiation, and de-differentiation and more recently the work is initiated on
iPSCs and their role in regenerative medicine. All these developments have brought
252
A. Kumar et al.