Biomedical Engineering Reference
In-Depth Information
multipotent cells, oligopotent cells, unipotent cells, and terminal cells. A pluripotent stem cell
can develop into cells from all three germinal layers (e.g. cells from the inner cell mass). Other
cells can be oligopotent, bipotent, or unipotent depending on their ability to differentiate into
few, two or one other cell type(s). Stem cell's differentiation is regulated in a feedback mech-
anism. In normal developments, stem cell differentiation observe strict lineage, that is, cells in
general do not “differentiate” back to their “parent” cell type(s), nor their “cousin” types.
Reproductivity is the ability of stem cells to divide and produce more stem cells of the
same type without differentiation. During early development, the cell division is symmet-
rical, i.e. each cell divides to two daughter cells each with the same potential. Later in devel-
opment, the cell divides asymmetrically to two cells with one of the daughter cells preserving
the stem cell type and the other a more differentiated cell.
Stem cells can be artificially grown and transformed into specialized cell types with char-
acteristics consistent with cells of various tissues such as muscles or nerves through cell
culture. Highly plastic adult stem cells are routinely used in medical therapies. Stem cells
can be taken from a variety of sources, including umbilical cord blood and bone marrow.
To ensure reproductivity, stem cells undergo two types of cell divisions. Symmetric division
gives rise to two identical daughter cells both endowed with the same stem cell properties (or
potency). For example, one cell of totipotency divides to two cells of totipotency. Asymmetric
division, on the other hand, produces only one stem cell (of the same potency) and a progenitor
cell with limited self-renewal potential (or lower potency). Progenitors can go through several
rounds of cell division before terminally differentiating into amature cell. Most adult stem cells
are lineage-restricted (multipotent), i.e. theydo not produce cells of higher potency. It is possible
that the molecular distinction between symmetric and asymmetric divisions lies in differential
segregation of cell membrane proteins (such as receptors) between the daughter cells.
Stem cell division is regulated by a feedback mechanism. Stem cells remain undifferenti-
ated due to environmental cues in their particular niche. Stem cells differentiate when they
leave that niche or no longer receive those signals to reproduce their own type. For example,
studies in Drosophila germarium have identified the signals dpp and adherens junctions that
prevent germarium stem cells from differentiating.
The signal pathways that lead to reprogramming of cells to an embryonic-like state have
been found to include several transcription factors including the oncogene c-Myc. Initial
studies indicated that transformation of mice cells with a combination of these antidifferen-
tiation signals can reverse differentiation and may allow adult cells to become pluripotent.
However, the need to transform these cells with an oncogene may prevent the use of this
approach in therapy.
Challenging the integrity of lineage commitment, researchers have been attempting to alter
fermentation conditions to affect different outcomes. The somatic expression of combined
transcription factors were found to directly induce other defined somatic cell fates; researchers
identified three neural-lineage-specific transcription factors that could directly convert mouse
fibroblasts (skin cells) into fully functional neurons. This “induced neurons” (iN) cell research
inspires the researchers to induce other cell types implies that all cells are totipotent: with the
proper tools or environmental conditions, all cells may form all kinds of tissue.
The nature of stems makes them unique in medical applications. Since stem cells can
differentiate into cells that had been damaged or not regeneratable naturally, one can artifi-
cially apply stems to grow new cells of desire to heal damaged cells.
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