Stem cells
Overview.
Embryonic stem cells.
Adult stem cells.
Stem cell characteristics.
Self renewal.
Potency.
Call regeneration.
Induced pluripotent stem cells.
Potential of stem cells.
Overview.
Our body comprises of trillions of cells.
All these cells originated from a single fertilised egg,
then went on to differentiate into the trillions of specialised cells,
we have now.
We have specialised tissues and organs in the body.
For example, brain, heart, lungs, liver, kidney, muscles, etc..
The cells in these organs are full differentiated and specialised for each organ.
All cells in the body, have the same DNA in the nucleus.
Specialised cells, express different set of genes.
There are about 200 basic cell types in the body.
Each of them express a unique set of genes.
The process of differentiating into different cells,
takes place in many stages.
In general stem cells are those, which are capable of differentiating,
into many types of specialised cells.
Specific chemical signals, called transcription factors,
induce the stem cells to specialise, into somatic tissue and organ cells.
Broadly stem cells are of two types.
Embryonic stem cells.
Adult stem cells.
Embryonic stem cells.
The single fertilised egg, is called the zygote.
In a few days it develops into, what is called as blastocyst.
Embryonic stem cells are found in the blastocyst.
These stem cells are capable of differentiating into any of the cells,
found in our tissues and organs.
These cells are said to be pluripotent.
During embryonic development these cells continuously divide,
and become more specialised.
They can develop into any of the more than 200 cell types in the body.
Biochemical molecules called transcription factors,
regulate the differentiation, into specialised cells.
Theoretically it is possible to harvest embryonic stem cells,
and use it to generate or repair any tissue or organ in the body.
Due to ethical considerations, and other constraints,
the use of embryonic stem cells, is currently limited.
Adult stem cells.
Adult stem cells are found in various tissues of the body.
These stem cells are already partially differentiated.
These stem cells can develop into certain types of cells.
For example, hematopoietic stem cells, are found in the bone marrow.
These stem cells can generate all the types of red blood cells, and white blood cells.
They types of stem cells are said to be multipotent.
The adult body maintains stem cells,
for most of the tissues and organs, in the body.
A healthy adult maintains a steady pool of stem cells.
These cells are used by the body to regenerate and repair tissues and organs.
Stem cells constitute a very small proportion of the cells in the body.
For example, only about .01 percent of bone marrow cells, are stem cells.
It requires considerable effort to locate and identify them.
Umbilical cord blood has a variety of stem cells.
Some are pluripotent and some are multipotent.
Young stem cells are more pristine and potent.
Some of the better know sources of adult stem cells are :
- Bone marrow.
- Adipose tissue.
- Blood.
- Umbilical cord blood.
- Amniotic fluid.
Adult stem cells are already used in medical therapies.
Stem cell characteristics.
There are two main characteristics of stem cells.
- Self renewal.
- Potency.
Self renewal.
Stem cells are a source of supply of new somatic cells.
The healthy body requires a constant reserve of stem cells.
A typical stem cell divides into one mother cell, which is a stem cell,
and a daughter cell, which is differentiated cell.
Since every division produces a new stem cell,
the reserve of stem cells is maintained.
Potency.
A potency of a stem cell, defines the number of types of cells,
that the stem cell can differentiate into.
Multipotent stem cells can differentiate, into a number of cell type,
with in a family.
Transcription factors can turn genes on or off.
They play a role in differentiating stem cells,
into specialised somatic cells.
Uni-potent stem cell can produce only one cell type,
which is their own cell type.
Cell regeneration.
It is believed that the body regenerates itself in 7 to 15 years.
Different cells have different life spans.
It is believed that cells replicate about 50 times.
After that cellular senescence takes place.
The cells in the body are designed to age,
and ultimately die.
For example, Red blood cells live for about 120 days.
The body is constantly regenerating red blood cells,
at the rate of more than 2 million cells every second.
The liver renews itself in about 2 years.
Skeletal bones last longer, and renew in about 10 years.
The renewal process is continuous.
The rate of renewal slowed down with age.
Some type of cells are permanent and do not renew at all.
Some of these cells in the brain, nerves, heart, and muscles have this property.
On the other hand, some cells renew very rapidly.
Skin cells, stomach cells, small intestine cells, duct cells,
are examples of rapidly renewing cells.
About 40,000 skin cells fall of every hour.
Stem cells located under the top layer of the skin, generate new skin cells,
to replace the dead cells.
Stem cells can remain quiescent.
When it receives a cue from the environment, it starts to regenerate.
One example, is when the organ is damaged ,
it generates a signal for the stem cell to regenerate.
The normal circumstances stem cell regeneration is carefully regulated.
Unregulated regeneration can lead to tumours.
Induced pluripotent stem cells.
Stem cells are difficult to locate and identify.
Scientists have discovered a way to modify normal somatic cells,
into stem cells.
They do this by introducing specific genes, into somatic cells.
Specific chemical signals induce the stem cell to specialise.
The somatic cells get reprogrammed, and behave like embryonic stem cells.
These are called induced pluripotent stem cells or IPS.
IPS has tremendous potential in regenerative medicine.
It can theoretically be used to replace damaged organs.
Normally somatic cells have markers on their body,
to enable them to identify foreign bodies.
When an organ is transplanted from a donor, there is a possibility,
of rejection of the transplanted organ.
Since the somatic cells can be derived from the patient's body,
IPS does not have problems related to transplant rejection.
Potential of stem cells.
Stem cell research is at relatively early stage.
Stem cells are already in use for medical therapy.
Stem cells can be injected into an injured organ.
This under the right conditions, can help to repair the organ.
One example of a therapy already in use, is bone marrow transplantation.
Many scientists are working on advanced sophisticated use of stem cells.
Theoretically any human organ, can be cultivated using stem cells.
This could be used to transplant damaged or diseased organs.
This research is still in the lab stage, for human organs.
The precursor to heart and liver organs have been developed in the laboratory.
We can expect exciting developments in stem cell therapy,
in the coming decades.