Stem Cell Research

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The research of stem cells, particularly embryonic stem cells is a controversial issue in  our society today. Stem cell research has an incredibly large potential in medicine and could dramatically affect the treatment of many human illnesses that would have previously been incurable. However there are many groups of people within our society that protest against the use of embryonic stem cells because they believe that it is not right to destroy living cells that have the potential to develop into living human beings. Numerous debates have occurred because of peoples’ differing views on this topic.

What Are Stem Cells?

Stem cells are undifferentiated cells; meaning they are cells in the body that have not yet become specialised. They have the potential to develop into any of the 220 specialised cells in the human body. For example stem cells can develop into cardiac muscle cells in the heart in the circulatory system, or into nerve cells in the nervous system. Each type of specialised cell has a unique structure that is specifically suited for its particular function. For example, nerve cells have thin, long extensions, which help them to transport electrical messages around the body more efficiently.  Once cells specialise, they cannot revert back to their previous stem cell state. This means that they can no longer specialise to form any other cell. Therefore it is vital that stem cells are present in all tissues in the human body for growth, renewal and repair, as they have the ability to divide and specialise.3  Specialised cells (such as nerve cells) constantly need to be replaced, as they get damaged or die often. This occurs regularly, as somatic cells have a limited lifetime and are constantly being used, leaving them prone to damage. For example, muscle cells are regularly contracting, and are often worn and damaged because of this. This means that stem cells are responsible for the replacement of damaged or dead cells in the human body, providing a constant supply of specialised cells in order to keep the human body functioning regularly. If a specialised cell is damaged or if it dies (through normal wear and tear), then a stem cell will divide and specialise to produce another to replace the dead or damaged cell. Another important characteristic of a stem cell is its ability to continue to duplicate itself indefinitely, as well as producing specialised cells. This is to ensure that there is always a sufficient amount of stem cells available. A sufficient amount of stem cells leads to the successful growth and repair of living tissues, hence the body is able to function properly. There are 2 different main categories that stem cells are sorted into, depending on their defining properties and their location in the body. These are “embryonic stem cells” and “adult stem cells”. Another type of stem cell that has been recently discovered is an “induced pluripotent stem cell”. Scientists have managed to induce somatic (body) cells, into stem cells that have the ability to specialise into numerous other cells. They have accomplished this by modifying the genome of existing somatic cells. These 3 types of stem cells have been discussed in more detail later on.

The Differentiation of Stem Cells

Scientists are just beginning to understand the signals inside and outside of cells that trigger differentiation. It is a very complicated process that still has many unanswered questions. Internal signals that affect the differentiation of a cell are controlled by the cell’s genes (specific sequence of bases in the DNA). Firstly, to initiate specialisation, specific genes must be activated. As shown in the below diagram, firstly, the transcription of these genes occurs within the nucleus, where messenger RNA is produced from the existing DNA template. Next, the mRNA moves into the cytoplasm, through the nuclear pores. Then, the mRNA moves along the ribosomes, and a specific amino acid that corresponds to the sequence of 3 bases ( a codon) is brought to the ribosome. This process continues until a large chain of chemically bonded amino acids is produced. This chain of amino acids forms a protein, which is then used for various different purposes in the cell. The type of cell that a stem cell will differentiate into depends on the specific genes that are activated (transcripted then translated); which determines the proteins that are produced. This is because the specific proteins found in a cell relate to its function. For example, a muscle cell is going to have different proteins produced compared to a skin cell, due to their differing functions. The activation of the internal genetic code is one of the factors that trigger stem cell differentiation. External signals also can cause a stem cell to differentiate. Chemicals excreted by neighbouring cells and physical contact with these neighbouring cells can trigger differentiation.17 They do this by activating the previously mentioned genes, stimulating the specialisation of these cells. Because of this, the stem cell’s differentiation depends on the external environment that it is placed in. The interaction of both the external and internal signals causes the restriction and expression of certain genes. This causes the cell to become specialised, as some genes will be expressed, and some will be masked. For example, a skin cell has genes expressed that allow certain proteins to be produced; however some genes that are not specific to skin cells will not be expressed. There is still much more to know about the differentiation of stem cells. Studying how stem cells differentiate would provide vital information on how birth defects and cancer take place (discussed later on). Also, with the right research, scientists could learn to cause stem cells to differentiate into a desired type of cell. This research is mostly “trial and error”, where scientists alter the different growth factors (both internal and external), so that the stem cells can differentiate into the desired cell. This opens many opportunities in regenerative medicine, which are later discussed.

Adult Stem Cells

Adult stem cells are found only in mature tissues, such as the blood, the skin and the brain. They are the only stem cells that are found in humans that have grown past the embryo phase. They can usually only form the particular cells of the tissue that they are present in.  They are “multipotent”, meaning they are limited to differentiating into only a few specific cell types.  For example, adult stem cells in the bone marrow (hematopoietic stem cells) are responsible for replenishing blood cells on a regular basis.  These hematopoietic stem cells can only differentiate to form cells that are found in the blood, such as red blood cells and white blood cells. It is the transplantation of these stem cells that helps to rebuild the damaged blood system of leukaemia sufferers after successful bone marrow transplants. This is a prime example of how stem cells can be used to treat illnesses and diseases (more of which will be discussed in more detail later on). Adult stem cells are relatively hard to culture on a culturing plate because they are quite rare in the tissues they are present in. They have been found in many tissues in the human body, including the brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis. However, their density is significantly smaller than the cells that reside in the tissue they are found in. Therefore most types of adult stem cells are going to be incredibly hard to isolate from the other cells in their particular tissue, hence they will be difficult to culture. However, for some adult stem cells, processes have been found to isolate and culture them.

How Adult Stem Cells are Harvested, Isolated and Cultured

Scientists have been able to isolate and culture certain types of adult stem cells.

For example, hematopoietic stem cells (found in the blood) are able to be harvested from the human body. Firstly, “growth factors” are injected into a donor to stimulate the growth of stem cells, and to cause them to move from the bone marrow into the peripheral blood (the blood that circulates around the body).  This increases the concentration of these stem cells in the blood, maximising the amount that are able to be harvested. After a few days of injecting this “growth factor” into the donor, their stem cells are ready to be collected.  A very thin, sterilised, flexible tube (called a catheter) is put into one of the donor's veins. This catheter is then attached to tubing that goes to a special machine. The donor's blood passes through this machine, which separates and keeps only the stem cells. This successfully isolates the hematopoietic stem cells from the other cells found in the blood. The rest of the blood is returned to the donor. This process can take several hours, and may need to be repeated for a few days to get a sufficient amount of stem cells. The stem cells are filtered, stored in bags, and frozen until they are required. There are more processes for collecting hematopoietic stem cells. These include collection from umbilical cord blood (from new born children) and also collection from a person’s bone marrow. Once the hematopoietic stem cells have been collected, they can then either be used in medical treatments (which are later discussed), or cultured for research purposes. To be cultured, firstly the adult stem cells harvested must be placed on a plastic culturing dish. They then must be left to divide and multiply. Adult stem cell’s ability to divide and multiply is quite limited however. Therefore, it is quite hard to grow large quantities of them in the laboratory.  Scientists are now researching new ways in which adult stem cells can be cultured.12

Embryonic Stem Cells

Embryonic stem cells occur only at the earliest stages of human development, as the embryo is still developing.  They are not present in any tissues found in any human that has been born ( and passed the embryo phase). As shown on the diagram below, they are derived from the blastocyst; an early staged embryo that is 4-5 days old and consists of approximately 100 cells. Embryonic stem cells are pluripotent, meaning they have the ability to become any one of the 220 types of cells in the human body.4 This is a large advantage that they have over adult stem cells, which can only form the cells of the tissue that they are in. Because they are pluripotent, embryonic stem cells are responsible for the growth of a human foetus, ensuring that all organs form correctly. This is because they form every specialised cell in the foetus, which in turn forms every working system in the body.  Embryonic stem cells have caused a lot of controversy due to the way that they are harvested.

How Embryonic Stem Cells are Harvested and Cultured.

Embryonic stem cells are easily isolated and cultured by scientists. A blastocyst must first be obtained, either through creating one especially for research purposes, or by other means, such as discarded IVF embryos. Either way, a sperm fertilises an egg, forming a zygote. This zygote is left to develop for approximately 4-5 days, until it becomes a “blastocyst” consisting of around 100 cells. Firstly, 20-30 embryonic stem cells are removed from within the blastocyst (destroying the potential foetus). These embryonic stem cells make up the “inner mass” of the blastocyst (as shown in the diagram); hence, the blastocyst must be ruptured to retrieve the embryonic stem cells. Because of this, once the embryonic stem cells are harvested, the development of the embryo is ended, and it is destroyed. An implication of this is that ethical issues are raised regarding the destruction of these blastocysts (which are later discussed). The embryonic stem cells are then transferred onto a plastic culturing dish. They then grow and divide until eventually millions of embryonic stem cells are present.4 It will take approximately a week for visible cell colonies to become apparent. Embryonic stem cells have a far greater ability to multiply indefinitely, compared to adult stem cells. The resulting collection of cells is called a “stem cell line”. Experimentation can then occur with this culture and possible stem cell treatments can be investigated. For example, study areas include drug effects on the body, the differentiation of cells and the treatment of diseases such as Parkinson’s disease, macular degeneration and leukaemia. These experiments and possible treatments are discussed in the “medical potential” paragraph.

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Advantages and Disadvantages of Adult and Embryonic Stem Cells

Both adult stem cells and embryonic stem cells have disadvantages and advantages relating to their possible use in therapeutic stem cell treatments. The obvious difference is the difference in the variety of specialised cells that each can form. Embryonic stem cells can form any specialised cell in the human body; however adult stem cells are limited to forming only the cells of the tissue that they are in. Therefore, in regards to research and treatments, embryonic stem cells will be more useful in this case, as they are not limited ...

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