Stem Cells and Tissue Renewal
MEDICAL BIOLOGY
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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:
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MEDICAL BIOLOGY
Stem Cells and Tissue Renewal
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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
Embryonic stem cells

4–5 days post fertilization at which time they consist of 50–150 cells

derived from the inner cell mass of a blastocyst, an early-stage embryo
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MEDICAL BIOLOGY
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Stem Cells and Tissue Renewal
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They do not contribute to the extra-embryonic membranes or the placenta
(pluripotent).
Adult stem cells
There are three known accessible sources of adult stem cells in humans:
1. Bone marrow.
2. Adipose tissue (lipid cells.
3. Blood

Most adult stem cells are multipotent and are generally referred to by their tissue origin
(mesenchymal stem cell, adipose-derived stem cell, endothelial stem cell, dental pulp
stem cell, etc.).
Cell Differentiation
 The human organism consists of hundreds of different cell types, all derived from the
zygote.
 During their specialization process, called cell differentiation, cells synthesize increased
quantities of specific proteins and become very efficient in specialized functions, often
changing their shape accordingly.
 For example, muscle cell precursors elongate into fiber-like cells containing large arrays
of actin and myosin. All animal cells contain and use actin filaments and myosins, but
muscle cells are specialized for using these proteins to convert chemical energy into
forceful contractions.
Stem Cell Applications
Diabetes
Rheumatoid arthritis
Parkinson’s disease
Alzheimer’s disease
Osteoarthritis
Stroke and traumatic brain injury repair
Learning defects
Spinal cord injury repair
Heart infarction
Anti-cancer
Baldness
Repair hearing
Restore vision
Wound healing
H.W: SCNT
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