applications. Stem cell research opens-up the new field of ‘cell-based therapies’ and, as such, several safety
measures have also to be evaluated [19, 20].
STEM CELL TYPES
Stem cells are unspecialized cells and different kinds of somatic cells in the human body have been described. It
varies from single cells to more specialized cells, which form the base of the human organ or tissue. They can
also distinguish between any organism cell and possess the capacity to renovate itself. Stem cells were present
at various levels of specialization, both as embryos and adult cells. For each stage the developmental capacity is
diminished, so a unipotent stem cell cannot divide into as many cell types as a pluripotent cell [22].
The key characteristics of stem cells are multipotency and self-renewal. Multipotency is the ability to divide
between various cell types with a similar germ layer (endoderm, mesoderm, or ectoderm) whereas the ability to
create identical copies through cell division is self-renewal. Stem cells have a large ability to proliferate and It
may come from a vast variety of human tissues [23].
Mammalian stem cells are generally classified based on their tissue of origin. The ovary and testis are
known as stem cells of the gonads since they contain oogonia and spermatogonia. In adult mammals, only the
germ cells undergo meiosis to produce male and female gametes, which fuse to form the zygote that retains the
ability to make a new organism thereby ensuring the continuation of the germline. In fact, the zygote is the
most primitive cell that produces the first two cells by cleavage and these germ cells are known for this unique
characteristic of developmental totipotency. Intriguingly, the Oct 4-an embryonic transcription factor critical
for the maintenance of pluripotency-continues to be expressed in the germ cells but is absent in other
Mesenchymal stem cells can be categorized into multiple stem cell groups. Adult stem cells, for example,
can make up cartilage, bone, tendon, ligament, muscle, skin, and nerve cells, whereas blood cells are the only
origin for hematopoietic stem cell lines (red cells, white cells, and platelets), while neural stem cells originate
from cells of the nervous system [21].
Embryonic stem cells (ESCs) originate from blastocysts during the division of the 16-cell stage. Pluripotent
cells are cells that create all sorts of cells in the developing embryo. ESCs will supply cells for transplantation
virtually infinite. Surprisingly, after in vivo transplant, ESCs form teratomas. Purification of neural cells is
practicable by the elimination of pluripotent tumor cells, but the resultant populations of cells are likely to be no
different from NSCs from other outlets [7, 23].
Totipotent stem cells may be specialized into cells of different structures. A zygote is a totipotent cell that
has the greatest ability for distinguishing between cells and embryos. These cells may ultimately form a
placental, or become one of the three layers of germ. The inner cell mass of the blastocyst comes pluripotent
after about 4 days which is the foundation of pluripotent cells [2, 3, 19].
Pluripotent stem cells (PSCs) are cells that specialize in germ layers in any type except placental structures
which are extraembryonic. Inner cell mass in pre-implant embryos originates from the embryo stem cells
(ESCs). Induced pluripotent stem cells (iPSCs) extracted from an epiblast layer of implanted embryos are
another illustration. The pluripotency of these cells ranges from completely pluripotent cells including ESCs
and iPSCs to less potency-multi-, oligo- or unipotent cells and their efficacy is evaluated through teratoma
formation assay. Artificially produced from somatic cells, iPSCs act in a similar way to PSCs. Their cultivation
and use for current and future regenerative medicine is very exciting [8, 24, 25].
Multipotent stem cells are differentiated in contrast to PSCs into restricted cell types only but may be
specialized in various cells with particular cell lines. For instance, blood cell hematopoietic stem cells may vary
in different blood cell types. A hematopoietic stem cell is converted after transformation into an oligopolistic
cell. However, its differentiation abilities become restricted to its own cells type only. Any multipotent cells,
though, maybe transformed into unrelated cell types, indicating that they are called pluripotent cells [5, 26].
Oligopotent stem cells are stem cells that can be distinguished into just a few cell types, such as myeloid
stem cells, but not red blood cells.
Unipotent stem cells, such as dermatocytes, are only capable of becoming one cell type. There are special
kinds of stem cells that can break up twice, which makes them an outstanding therapeutic candidate in
regenerative medicine but less diverse from other types of stem cells [2, 19, 24].
Neural stem cells (NSCs) In the adult human brain, they are located mostly in the subventricular and
hippocampal regions. These cells are characterized by three groups of cerebral cells: neurons, astrocytes, and
oligodendrocytes. NSCs can be derived from many central nervous system areas from exogenous sources and