07-28-2011, 09:21 AM
(This post was last modified: 08-28-2013, 08:37 PM by Administrator.)
Stem cells, by definition, can undergo infinite number of cell division while remain in an undifferentiated state. However, they can also give rise to differentiated daughter cells that are committed to specialized cell fate in all three primary germ layers: ectoderm, mesoderm and endoderm. During embryogenesis, the degree of this differentiation becomes gradually restricted as the fetus develops and the potential of various stem cells is limited by the class they belong to. While both totipotent and pluripotent stem cells can generate every type of cells found in the body, only totipotent ones are able to form an entire organism. In contrast, multipotent stem cells are more mature in developmental age compared to totipotent and pluripotent stem cells, and therefore, can only give rise to a limited population of cells within a specific lineage.
The particularly extrinsic and intrinsic molecular signaling network that confers their self-renewal and differentiation remain fairly uncharacterized. However, a consistent requirement for the Oct4, Nanog and Sox2 transcription factors in maintaining pluripotency seems to be evolutionarily conserved between mouse and human.
There are two types of stem cells in mammals. One is the embryonic stem cell, which needs to be isolated from the blastocyst’s inner cell mass. The other type is the adult stem cells, which are located at various tissues in the body.
For an autologous process, there are three most accessible sources. These are:
1. Blood – wherein pheresis is used for extraction. When blood is extracted, it passes through a machine that separates only the stem cells, but returns other parts of the blood back to the donor;
2. Adipose tissue or lipid cells – which is extracted via liposuction;
3. Bone marrow – where harvesting is required through drilling into a bone, such as the iliac crest or femur; and
4. Umbilical cord – for newborns.
There are two properties that a stem cell is required to have. This is its ability of self-renewal, where it goes through several cycles of cellular division but remaining in its undifferentiated state. In its self-renewal process, a single stem cell can divide into either a mother cell or a daughter cell, where the former is identical to the original one and the latter is differentiated. This mechanism is called obligatory asymmetric replication. It can also have a stochastic differentiation, where the stem cell becomes two differentiated daughter cells.
The second property, aside from self-renewal, is its potency, which was discussed in the article above. This is defined as the ability of a stem cell to differentiate into specific or specialized cell types. These cells arise when fertilization occurs, along with the cells in its first few divisions.
Each stem cell has a differentiation potential. It can be totipotent or omnipotent where such a cell can produce an entire viable organism. Meanwhile, pluripotent cells are those from totipotent cells, or any of the three germinal epithelia during embryogenesis. Such cells can be differentiated into almost any cell. Pluripotent cells are the ones typically used in cell cultures.
Multipotent stem cells can differentiate into a certain number of cells closely related to its origin. Oligopotent cells, on the other hand, can differentiate into a lesser number of cells such as myeloid or lymphoid stem cells. Lastly, unipotent cells are those that can produce only a single cell type, which is their own. However, such cells may have the property of self-renewal.