Stem Cells and Tissue Renewal

Stem cells are undifferentiated cells that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells. They are found in multicellular organisms. In mammals, there are two broad types of stem cells:
1- Embryonic stem cells: which are isolated from the inner cell mass of blastocysts, and
2- Adult stem cells: which are found in various tissues.
In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues.
• In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
• Many stem cells divide infrequently but the divisions are always asymmetric, that is, one daughter cell remains as a stem cell while the other becomes committed to a path that leads to differentiation.
• Stem cells of many tissues are found in specific locations where the microenvironment helps maintain their uniquely undifferentiated properties; they are often rare and not noticeable by routine histological methods.
• The differentiation may quickly yield a new specialized cell that is functionally integrated into the tissue or organ.
• In some tissues, such as the blood and epidermis, cells become terminally differentiated, meaning they cannot renew cycling and exist for a short period of time. Such tissues have rapidly renewing cell populations.
• Most of these dividing cells are not the stem cells but the more rapidly dividing progeny of the cells committed to differentiation. They are commonly called progenitor cells or transit amplifying cells because they are in transit along the path from the stem cell to a differentiated state, while still amplifying the number of new cells available for the differentiated tissue by mitosis.
Characteristics of stem cells
The classical definition of a stem cell requires that it possess two properties:
1- Self-renewal: the ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
2- Potency: the capacity to differentiate into specialized cell types. This requires stem cells to be either totipotent or pluripotent—to be able to give rise to any mature cell type, although multipotent or unipotent progenitor cells are sometimes referred to as stem cells.
• stem cell function is regulated in a feedback mechanism.
Degrees of plasticity (potency)
1. Totipotent:
• Can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism.
• Produced from the fusion of an egg and sperm cells or by the first few divisions of the fertilized egg.
2. Pluripotent:
• Descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers.
3. Multipotent:
• Can differentiate into a number of cell types, but only those of a closely related family of cells, e.g. hematopoietic stem cells.
4. Unipotent:
• Cells can produce only one cell type, their own, but have the property of self-renewal, which distinguishes them from non-stem cells (e.g. progenitor cells, muscle stem cells).
Kinds of Stem Cells