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What is a stem cell...

What is a STEM CELL?


 [ˈstem ˌsel]

NOUN

biology

an undifferentiated cell of a multicellular organism which is capable of giving rise to indefinitely more cells of the same type, and from which certain other kinds of cell arise by differentiation.



In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts in early embryonic development, and adult stem cells, which are found in various tissues of fully developed mammals. 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. 

There are three known accessible sources of autologous adult stem cells in humans: bone marrow, adipose tissue, and blood. (Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell therapy types, autologous harvesting involves the least risk.) (WEB SOURCE)


Stem cells are cells that have two essential characteristics:

1. Self-renewal: In other words, these cells are able to create new stem cells.
2. Ability to differentiate into one or more types of cells.

The more types of cells the stem cell is capable of changing into, the greater its differentiation capacity. By using this characteristic, we can classify the different types of stem cells. Differentiation is nothing more than the process of adopting morphological and functional characteristics of a certain cell. This process takes place when a stem cell adopts the characteristic genetic configuration of the cell that it copies. Differentiation ultimately expresses only the genes of the particular cell.

Once the new genetic configuration is set, the stem cell gradually acquires the characteristics of the target cell. After a set of cellular divisions, the result is a completely functional differentiated cell. (Web Source)


In medicine and research, the interest in stem cells lies in their ability to differentiate. They offer an enormous potential to decode science’s biggest puzzles that are, even today, a burden for humankind.

1.       Understanding more about the progress of a disease. In many cases, we know exactly just what causes an illness. For example, we know that Alzheimer’s results from an accumulation of certain proteins that take apart the brain’s architecture. However, we don’t know how the process develops: When does it happen? Why? How does it affect the neuron function in the first stages of the disease? All of these questions could find an answer thanks to stem cells. By differentiating stem cells into neurons in vitro, scientists can reproduce the disease perfectly. By doing so, they can unveil exactly what happens in the long-term process of the disease.

2.       Generating healthy cells to replace sick ones. This practice is known as “regenerative medicine.” This practice could serve in a multitude of cases. To give an example, when a person suffers from a third-degree burn, they need to undergo a skin graft. By using stem cells from the patient’s own skin, scientists could generate new epidermic tissues to cover the affected area.

3.       Evaluating the efficiency of new medication. As we all know, new medication needs to pass many tests before being commercialized. One of these tests consists in human testing. By using stem cells, we could generate in vitro internal micro-environments very similar to those of human beings for testing purposes. For example, scientists could test a new medication for a heart disease treatment on heart tissue generated from stem cells.

Adult stem cells are frequently used in various medical therapies (e.g., bone marrow transplantation). Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves. Embryonic cell lines and autologous embryonic stem cells generated through somatic cell nuclear transfer or dedifferentiation have also been proposed as promising candidates for future therapies. (web Source)




A stem cell transplant, also called a bone marrow transplant, infuses healthy blood-forming stem cells into the body to replace those damaged by disease, chemotherapy or radiation. Stem cell transplants for cancer patients are most effective when the cancer is in remission, meaning all signs of the cancer have disappeared. Stem cell transplants allow patients to begin creating their own stem cells, which increases their ability to fight infection and prevent bleeding.


But before you go transfusing blood into each other, for a stem cell transplant to take place there are several factors that have match. 

HLA stands for “human leukocyte antigen.” These are protein molecules that we inherit from our parents. We currently know about nearly 600 different HLA molecules. Before you have a stem cell transplant, your HLA type must be determined. This is done by taking a blood sample.


The laboratory will also determine the HLA type of anyone who may donate stem cells to you. It is important in stem cell transplants to see how closely the HLA of the transplant patient matches the HLA of the stem cell donor. The HLA “match” is the number of HLA molecules that any two people have in common. HLA matching is usually based on six HLA molecules. The more molecules two people share, the better the match. When two individuals share the same HLA type, they are said to be a good match. That is, their immune systems will not see each other as “foreign” and are less likely to attack each other.

The most likely place to find an HLA match between two people is among siblings (brothers and sisters who have the same mother and same father). If two siblings inherit the very same HLA molecules from both parents, they are said to be an “HLA identical match.”

You have a 25 percent (1 in 4) chance of being an HLA identical match with your sibling. Why? Because there is a basic rule in HLA inheritance: You have a 25 percent chance of inheriting the same HLA molecules as your sibling, a 25 percent chance of inheriting none of the same HLA molecules as your sibling, and a 50 percent chance of inheriting half of the same HLA molecules as your sibling.

However, two unrelated people can just happen to be a good HLA match, too. Although it is less likely, it is possible that you could have some of the same HLA molecules as a friend or as someone you don’t even know. If you and your friend share three HLA molecules, for example, then you are said to be a “three HLA antigen match.”(Web Source)

The Process of HLA Typing

HLA typing assesses the particular HLA genes that you have inherited (i.e., your string colors). Because there are a number of different HLA genes, as well as different variations of these genes, there are very many different possible color combinations that together make up your specific HLA type.

HLA typing also usually includes testing for antibodies targeted to specific HLA proteins. Antibodies are made by part of the immune system. If a person already has an antibody against an HLA protein (i.e., if it already is primed to attack a certain color string), it may attack that protein if it is transplanted. This may cause the transplant to fail. So generally, you shouldn’t receive a transplant from someone if you already have an antibody against one of their HLA proteins.(Web Source)

HLA is much more complicated than blood typing because there are many more HLA markers that make a person’s cells unique. There are only eight basic blood types, and many people can safely receive more than one type of blood (depending on their type). To receive only blood from a person, you do not need to be an HLA match, because HLA is not present on red blood cells.

However, to receive a solid organ transplant, the recipient must have a compatible blood type with the donor, as well as the best HLA match possible. For stem cell donations, one needs a very strong HLA match, but blood type is not as important as it is for solid organ transplants.

Because the HLA genes are located close together on your DNA, they are usually inherited as a group—you inherit a whole set of colors not just one individual color at a time. Your HLA type is composed of the set of HLA genes you inherited from your mother and the HLA genes you inherited from your father. In our analogy, the HLA genes contain information about the "color of the strings" your cells will have.

Biological parents always share half of their HLA proteins with their child. This is also called a “half match.” Conversely, a child always is a half match with their parents. In our analogy, a child would share half of the colors on his cells with each of his parents.1

Siblings who share both parents are most likely to be an identical HLA match. Such siblings have a 1 in 4 chance of being a perfect HLA match (with perfectly matching colored strings).

There is also about a one in two chance that siblings will share half of the HLA markers and be a half-match.

Because siblings only have a one in four chance of being HLA identical, it’s not uncommon for people not to have anyone in their family that is a close match.5

For a solid organ transplant (like a kidney) that can be given by living donors, it may be worth getting HLA typing for other members of the family as well: uncles, aunts, (and more) to help find a good match. Because stem cell donations require a higher percentage of HLA matches, it is less likely that a suitable match will be found this way.

Ethnicity

Groups of HLA “colors” run in certain ethnic groups. So even if someone in your family isn’t a good match, it may be more likely that someone from a shared genetic heritage will be a match for you. This is part of the reason it may be harder for some people to find a good HLA match than others.

For example, bone marrow registries currently contain fewer potential donors of African American descent. This may make it less likely that these individuals can find a good HLA match from a non-relative.5

How Is It Performed?

HLA typing is a genetic test. For the test, you’ll need to give some sort of tissue sample. This is usually from a swab from inside your cheek or from a blood sample drawn from a vein in your arm.6 Usually, no preparation for the test is necessary. The sample will probably need to be sent to a specialized center for analysis. Since HLA typing is not a commonplace blood test, you may want to check with your insurance carrier ahead of time to assess for coverage and cost.

How Many HLA Matches Do You Need?

Ideally, the donor and recipient would be perfectly HLA matched. However, this is not always possible. The details of this depend on the specific type of transplant and on other medical circumstances.

Stem cell transplantation is often a greater challenge than solid organ transplantation in terms of the importance of a good HLA match. In both, there is a risk that the cells of transplant recipient may attack the donated tissue. But in a stem cell transplant, there is also a chance that some of the donated cells may also attack the cells of the transplant recipient. This is known as graft-versus-host disease.7 So, people receiving stem cell donations usually need to have a higher percentage of matches than people receiving a solid organ.

People receiving solid organ donations tend to do better if they have a better HLA match. For example, 10 years after a kidney transplant, you are more likely to still have a functioning kidney if you received a kidney with a full HLA match than if you received only a half HLA match.

Different doctors and medical institutions may have different guidelines about the number of HLA matches needed to go ahead with a transplant. But in certain situations, you might still be able to have a transplant with a smaller number of matches.

Your doctor will work with you to find the best treatment option if you haven’t yet found a good transplant match. In some cases, you may want to go ahead with a transplant that isn’t a very good match. In other cases, you may want to receive other treatments while you wait for a better match to potentially become available. It’s challenging to wait, but sometimes that is the best option. (Web Source)