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Bioinformatics of the Brain

more restricted differentiation potential and self-renewal than PSCs, differen-

tiating into kindred cell lineages that generally originate from the same germ

layer. These multipotent SCs lose their ability to specialize in miscellaneous

and unrelated cell lineages. For instance, hematopoietic stem cells (HSCs)

can develop into all blood cells; however, they are not able to install divergent

cell lineages in practice. When multipotent cells become one step more spe-

cialized, they are henceforth called oligopotent SCs. Oligopotency conduces

to form discrete cell types descended from a similar progeny. The narrow-

est range of differentiation is defined as unipotency. Unipotent cells can no

longer be considered stem cells because they can only be sustained to consti-

tute identical cells by dividing. Another SC classification is grounded on the

specific location or origin where stem cells can be achieved. Thus, the unique

character of each stem cell type, especially its ability to differentiate, makes

SCs useful and precious for multiple applications in life sciences.

2.2.1

Embryonic Stem Cells (ESCs)

Following fertilization, the zygote undergoes a series of arrangements in hu-

mans. Embryonic stem cells (ESCs) in the pluripotent state migrate into the

inner layer (ICM; inner cell mass) of the blastocyst, an embryonic configura-

tion acquired by rapid cell division and mobility during early development.

ESCs develop into three embryonic germ layers: the endoderm, mesoderm,

and ectoderm. Subsequently, all somatic (body) cells and primordial germ

cells (PGCs) arise from these ESC-derived germ layers by passing a hierarchi-

cal, multi-step course.

Human ESCs (hESCs) were first discovered and characterized by Thom-

son’s group in 1998. They demonstrated that ESCs can be maintained in cul-

ture conditions by preserving developmental potential. Cultured ESCs were

able to develop into embryonic germ layers and then various somatic cell types

in vitro [2]. A great number of hESC lines were brought out of donated hu-

man embryos to study the developmental aspects of human beings and their

potential in regenerative medicine. Because ESCs are embedded inside the

ICM, they can be isolated by disrupting a living embryo. This is the major

disadvantage and discrepancy of using ESCs for scientific research and regen-

erative approaches. Although the competence of ESCs in clinical studies has

been documented many times [3], their creation and usage have been restricted

because of legal regulations and ethical concerns in societies.

2.2.2

Adult Stem Cells (ASCs)

During the embryonic-fetal transition that orientates a body formation, some

quiescent precursor cells in multipotent and oligopotent states are stored

in confidential compartments, namely stem cell niches, within the tissues

and organs. These niches anatomically and physiologically ensure supportive

microenvironments through cell-cell and cell-extracellular matrix (ECM) in-

teractions specific to stem cell types. Adult stem cells (ASCs) start to