Structural Biochemistry/Stem Cells

In cell biology, pluripotency is defined as "the potential of a cell to develop into more than one type of mature cell, depending on environment". So if a cell is pluripotent, it has the potential to transform itself into a lung cell, heart cell, etc. Pluripotency describes a cellular state of specific cells within the early embryo. Stem cells represent a category of unspecialized cells that possess the unique capability of developing and maturing into a variety of specific types of cells with various functions. Through the maturation and cell division processes, stem cells can specialize, grow, and replace unhealthy cells within living organisms. After cell replication, these cells have the ability to either mature and specialize in function, or remain an unspecialized stem cell, which preserves the cell’s potential to specialize as it is needed in the body. Not only are these cells able to specialize in function, but they are also capable of joining together to replace damaged tissues. The two types of stem cells scientists are currently studying are: embryonic stem cells and adult stem cells. In 1981, scientists first discovered methods to derive embryonic stem cells from mouse embryos, and in 1998, scientists made further developments and developed methods to derive and harvest stem cells from human embryos. Since stem cells are capable of self- replication, they offer great potential for treatment of diseases such as diabetes and heart disease. The use of these cells for treatment of certain diseases is called reparative medicine, and is currently a topic of much debate in present society and culture. Stem cells also offer the potential for use in screening new drugs as well as the study of how and why certain birth defects develop. Stem cell therapy offers a great deal of insight into medicine as well as human growth and development, and research is currently underway in order to find methods to fully implement all the advancements these cells could provide to modern medicine.

Stem cells are characterized by their ability to differentiate into a diverse range of specialized cells and to grow indefinitely. They have two properties:


 * Self-renewal: The ability to undergo cell division indefinitely while maintaining their undifferentiated state. Stem cells are capable of doing so due to having active telomerase.
 * Potency: The ability to differentiate into specialized cell types in response to a signal. Skin cells, heart cells, and neurons all have the same genome, but are all different cells.  This is due to differentiation and different RNA/protein combinations.

Stem cells are often classified into one of two categories: embryonic stem cells, or adult stem cells.

Totipotency
Totipotency is defined as a cell's ability to transform into all cell types of the organism. Both oocyte (the female germ cell involved in reproduction. This is an immature ovum) and zygote (this is the initial cell created after tow gamete cells are merged by means of sexual reproduction) do not possess the potential to be totipotent. However, the fertilized egg becomes equipotent blastomeres from its specialized cell type with a limited fate. Blastomeres are a type of cell created by cleavage of the zygote after fertilization.

What researchers have found through a series of transplantation experiments 1. Maternal proteins and RNAs are taken into the oocyte and deposited there 2. Those maternal proteins and RNAs have the capability to reset the epigentic state of somatic chromatin 3. They also serve to turn the oocyte into totipotent blastomeres after fertilization 4. So far, it has not been achieved to make totipotent cell lines in vitro. 5. This may be impossible due to the fact that in vivo, that is within living body, the transient nature of the totipotent is extremely different. The pre-embryonic stage of embryonic development offers blastomeres that are not actually self-renewin but they are generated by means of cleavage

Primordial Pluripotency
Slowly, a restriction in development potency become apparent as cleavage of the early embryo approaches to the 16-cell stage. Then the cells undergo two different lineages, the trophoblast lineage and the inner cell mass. The question is how the cell differentiation begins. Differentiation begins as blastomeres flatten and as they strengthen cell-to-cell contact. This process is called compaction.

Embryonic Stem Cells
Embryonic stem cells are cells that are derived from embryos that have typically been harvested from eggs through in-vitro fertilization. Embryonic stem cells are known as pluripotent, or having the ability to differentiate into every cell line within the organism. The embryos are 3–5 days old and consist of collections of hollow balls of cells that are called blastocysts.

In order to produce these blastocytsts, the embryonic stem cells must be harvested. The process of growing these stem cells is called cell culture, and is performed in the laboratory. The inner cell mass of human embryonic stem cells are placed onto a culture dish that has been coated with a medium that provides an adhesive surface to which the inner cell masses can stick. This medium is called a feeder layer, and is oftentimes composed of mouse skin cells. The human embryonic stem cells are then allowed to divide, spread, and attach to the medium of the culture dish in order to create a colony of embryonic stem cells. If the stem cell colony grows, divides and survives, the cells are plated onto new, clean culture dishes many times for the duration of many months in order to yield millions of embryonic stem cells, which is called a stem cell line. A sign that a stem cell line has developed successfully is if the cells have remained unspecialized for the duration of at least six months. If this is the case, the stem cell line can then be used for further testing and experimentation.

Throughout the harvesting process the embryonic stem cell colony must undergo a process called characterization, which involves a series of tests performed by scientists to see whether or not the cells have in fact developed correctly. One such test involves harvesting and examining the stem cell colony for many months in order to verify that the cells are healthy and capable of successful renewal. These cells are also analyzed to make sure they remain unspecialized. A way to test whether or not the cells have specialized is to test for the presence of transcription factors that are produced by unspecialized cells. Two examples of these transcription factors are Nanog and Oct4. The presence of these transcription factors indicate the cells have remained unspecialized and that they are capable of self replication and renewal.

Another test performed on embryonic stem cell cultures involves freezing, thawing and re-plating the cells to see whether or not they are capable of self renewal and re-growth.

An important test to examine whether or not the cells are functioning properly involves injecting a collection of embryonic stem cells into a mouse with a suppressed immune system followed by tests that indicate the formation of teratomas, which are benign tumors. If the injected stem cells successfully form a teratoma, scientists can be confident that the cells are capable of growth and differentiation, since teratomas contain various specialized cells.

Embryonic stem cells (ESC) are harvested from a blastocyst, which is an early stage in embryonic development(approximately four to five days in humans) that consists of 100-150 cells. ESC give rise to all three primary germ layers during development: ectoderm, endoderm, and mesoderm; which means they can develop into more than 200 specialized cell types. They can grow indefinitely under the right conditions due having active telomerase. Telomerase is inactive in adult cells, and as a result, cannot grow indefinitely.

Embryonic stem cells have proven to be usable in different situations:


 * Embryonic development: Understanding ESC aids scientists in further understanding embryonic development.
 * Drug testing: Certain cells cannot be cultured because they will die if removed from an organ or tissue.  For example, in designing drugs for the heart, scientists cannot culture heart cells because once the heart cells are removed, they die.  However, ESC can be cultured, and the ones that are differentiated into heart cells can be sustained in a culture to be used to test the drug.
 * Regenerative Medicine: This is not something ESC are currently being used for, but something that scientists are striving towards. The theory, or goal, is to be able to remove the nuclei in an embryonic stem cell, insert a patient's DNA into the cell, and have it differentiate according to the patient's needs.  For example, Parkinson's disease is a degenerative disease of the central nervous system, and in theory, by adding back new neurons, it should be able to cure Parkinson's.
 * Paralysis: In addition, those who suffer from paralysis and unmovable body parts could regenerate body motor control through the study of stem cell research.
 * Leukemia: Patients suffering from Leukemia who may be in need of bone marrow transplant or cord blood transfusion can benefit from these stem cell researches. These products could be reproduced without having the need to constantly find a matchable donor. Although this may not treat the disease, it can diminish the symptoms greatly.

Specialization
The purpose of harvesting embryonic stem cells is to create a colony of self replicating cells that have the potential to specialize into specific cells of interest that can be used to treat diseases.While they are being grown, the cells remain unspecialized but once they are allowed to clump together under certain specific conditions, they begin to form various specialized cells and tissues. The challenge scientists are currently facing is figuring out how to control the specialization to create specific cells of interest as opposed to spontaneous specialization that can result in the formation of various cells that carry unwanted specific functions. One such way to control specialization is by subjecting the colonies of embryonic stem cells to changes in chemical composition within the culture medium, changing the surface of the culture dish, or inserting specific genes into the culture to change the composition of the harvested cells. By transplanting the successfully specialized embryonic stem cells into humans, scientists can formulate treatments for certain diseases such as Parkinson’s Disease, diabetes, spinal cord injury, heart disease as well as vision and hearing loss.

Adipogenesis
Adipocytes are cells specialized for energy storage and so when the balance of energy intake versus expenditure is shifted so that more energy is taken in than is burned, the process of adipogenesis begins. This process involves adipocyte hyperplasia, the excessive recruitment of stem cells to become adipocytes, thereby increasing the number of adipocytes. In order to accomplish this, pluripotent mesenchymal stem cells (MSCs) become preadipocytes then are further developed into mature adipocytes.

The pluripotent MSCs located in the vascular stroma of adipose tissue and bone marrow first become preadipocytes, a process initiated by bone morphogenic proteins BMP4 or BMP2, then after several rounds of mitotic clonal expansion, they become adipocytes. While this process is activated by the BMP factors, other factors such as Hh provide inhibitory signals if the process becomes unnecessary.

Regulation and Signaling of Stem Cells
For stem cells, transcription factors are important to their functioning and can be categorized into three central groups; 1. ones affecting development and its hierarchy (Developmental Core Module), 2. ones controlling cell growth and homeostasis (Cell Growth Module), and 3. ones that control signaling pathways of the other two groups (Signaling Module). These three groups mostly function independently, though there is some interaction present.

Developmental Core Module
The best understood of transcription factors controlling cell pluripotency is Oct4 which is an octamer class protein that specifically recognizes the sequence ATGCAAAT. Oct4 is unique apart from the other transcription factors in that it is crucial for epigenetic reprogramming (differentiation of cells during development). If it is inhibited or silenced in any way, ESCs will instantly stop self-renewing and differentiation into trophoblast-like cells. Oct4 will interact with many cofactors in the form of transcription factors. These cofactors tend to interact with DNA and include Sall4, Sp1, Hdac2, Nanog, Dax1, Nac1, Tcfp2l1, and Essrb. Their interactions do not form one protein nexus but multiple complexes all with different composition of proteins. This differing of composition and complexes may be the reason for different states of pluripotency.

Sox2 is another transcription factor involved in development and cell pluripotency. It has an expression pattern that is, to some extent, similar to Oct4. However, when inhibited or silenced, this will mainly affect the later stages of embryogenesis. Sox2 silencing will only affect a small portion its target genes and will lead to trophoblast-like cells. This is mainly because Sox2 loss can affect the amount of Oct4 levels in the cells which is the main reason for the creation of the trophoblast-like cells.

Nanog is also a member in the core developmental module and is a homeodomain-containing DNA-binding factor that is not associated with the normal ones. Nanog expression alone is able to help keep cell pluripotency in extreme conditions where factors are missing. It has been shown in studies to have a role of starting cell pluripotency rather than maintaining it. Nanog has cofactor proteins it interacts with which includes Smad1, Small3, Nr0b1, Nac1, Essrb, Zfp281, Hdac2, and Sp1.

Cell Growth Module
This is the gene network that controls and maintains the growth and proliferation of pluripotent stem cells.

The proto-oncogene c-Myc is one member of this group is associated with activating transcription and opening chromatin. Its expression allows ESCs to self-renew even in extreme conditions. If it is inhibited ESCs will shut off their self-renewing mechanism. c-Myc also controls transcription of genes from multiple unconnecting functions such as cellular metabolism, control of cell cycle, and manufacture of protein. It has an inhibition factor also and will prevent the expression of Gata6 which is a prodifferentiation factor. It also has been found to overlap with functions from genes of the developmental module such as Tip60 and p400. It has also seen to function in some cases of cancer growth. c-Myc expression is regulated by the transcription factor Ronin (Thap11) which is part of a family of proteins containing an N-terminal THAP domain. It is well conserved between mice and humans which means its sequence and target sequence is very similar for both mammals. Ronin inhibition will led to cell apoptosis while overexpression will lead to an abundance of mESCs.

Signaling Module
There are several signaling pathways that will directly promote self-renewal and/or preserve pluripotency by blocking or signaling normal differentiation prompts.

Lif (Leukemia inhibitory factor) stimulates the self-renewal of mESCs. It does not directly increase the growth rate of mESCs but rather increases the probability that the cells with self-renew rather than differentiate. The Lif-signaling effect is controlled by a heterodimeric complex that is made of a Lif receptor (LifR) chain and the protein gp130. It has been shown to signal three pathways which are the Jak/Stat3, the PI3K-Akt, and Mapk pathways. Of the three, only the Jak/Stat3 pathway completely depends on Lif for its signal. The other two pathways require multiple signals from multiple sources in other to start the process. Signaling of the Jak/Stat3 pathway and PI3K pathways will promote expression of Klf4 and Tcf3 which lead to the creation of greater transcription of Sox2 and Nanog which as stated earlier are involved in differentiation of stem cells and development of the body. For the Mapk pathway, its signaling will lead to less amounts of Tbx3 which will cause a decrease of transcriptional activity. FgfR1 (Fibroblast growth factor signaling) is the most ample receptor of ESCs in the body. It is involved in signaling the Jak/Stat, phophoinositide phospholipase C-y, PI3K, and Erk pathways. A similar protein, Fgf2 is important for the maintenance of hESC. For example, in mouse feeder cell cultures a hot amount of Fgf2 is required to maintain its self-renewal and prevent differentiation.

Wnt signaling is very important during embryonic development and controls differentiation of stem cells. Its can induce expression of mesodermal and endodermal markers that, for mESCs, includes Barchyury, Flk1, Foxa2, Lxh1, and Afp. Wnt signaling also maintains cell stem pluripotency by controlling the expression of Oct4, Sox2, and Nanog which are all transcription complexes involved in maintaining pluripotency and development.

Pluripotency-Associated Chromatin Structure
It should be noted that every cell has a unique chromatin structure. Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. In pluripotent cells, heterochromatic-associated foci seems to be more spaciously distributed. The associated histones are usually hyperacetylated. There exists chromatin-modifying enzymes that play a role with the transcriptional modules. They do so by controlling genes that encode key enzymes capable of covalently modifying proteins. For instance, the transcription of the H3K9 methyltransferase SetDB1 is controlled by Oct4. It is known that histone labels characteristic of this enzyme. When this happens, the following things occur; 1. Polycomb group proteins are introduced 2. Polycomb group proteins trigger subsequent events which involve methylation of H3K27 residues 3. Then further additional chromatin-associated modifications are enabled.

1. In Vitro Fertilization
The largest potential source of blastocysts for stem cell research is from in vitro fertilization (IVF) clinics. The process of IVF requires the retrieval of a woman's eggs via a surgical procedure after undergoing an intensive regimen of "fertility drugs," which stimulate her ovaries to produce multiple mature eggs.

When IVF is used for reproductive purposes, doctors typically fertilize all of the donated eggs in order to maximize their chance of producing a viable blastocyst that can be implanted in the womb. Because not all the fertilized eggs are implanted, this has resulted in a large bank of "excess" blastocysts that are currently stored in freezers around the country. The blastocysts stored in IVF clinics could prove to be a major source of embryonic stem cells for use in medical research. However, because most of these blastocysts were created before the advent of stem cell research, most donors were not asked for their permission to use these left-over blastocysts for research.

The IVF technique could potentially also be used to produce blastocysts specifically for research purposes. This would facilitate the isolation of stem cells with specific genetic traits necessary for the study of particular diseases. For example, it may be possible to study the origins of an inherited disease like cystic fibrosis using stem cells made from egg and sperm donors who have this disease. The creation of stem cells specifically for research using IVF is, however, ethically problematic for some people because it involves intentionally creating a blastocyst that will never develop into a human being.

2. Nuclear Transfer
The process called nuclear transfer offers another potential way to produce embryonic stem cells. In animals, nuclear transfer has been accomplished by inserting the nucleus of an already differentiated adult cell-for example, a skin cell-into a donated egg that has had its nucleus removed. This egg, which now contains the genetic material of the skin cell, is then stimulated to form a blastocyst from which embryonic stem cells can be derived. The stem cells that are created in this way are therefore copies or "clones" of the original adult cell because their nuclear DNA matches that of the adult cell.As of the summer of 2006, nuclear transfer has not been successful in the production of human embryonic stem cells, but progress in animal research suggests that scientists may be able to use this technique to develop human stem cells in the future.

Scientists believe that if they are able to use nuclear transfer to derive human stem cells, it could allow them to study the development and progression of specific diseases by creating stem cells containing the genes responsible for certain disorders. In the future, scientists may also be able to create "personalized" stem cells that contain only the DNA of a specific patient. The embryonic stem cells created by nuclear transfer would be genetically matched to a person needing a transplant, making it far less likely that the patient's body would reject the new cells than it would be with traditional tissue transplant procedures.

Although using nuclear transfer to produce stem cells is not the same as reproductive cloning, some are concerned about the potential misapplication of the technique for reproductive cloning purposes. Other ethical considerations include egg donation, which requires informed consent, and the possible destruction of blastocysts.

Adult Stem Cells
Adult stem cells are known as mesenchymal stem cells. Just like embryonic stem cells, adult stem cells are unspecialized cells capable of renewal and specialization, but unlike embryonic stem cells, adult stem cells are typically found in tissues surrounded by other specialized cells. This makes the stem cell only multipotent; it can only differentiate into certain cell lines as opposed to pluripotent embryonic stem cells which can differentiate into almost any cell line. The role of adult stem cells is to replenish damaged cells within tissues and organs when needed. Adult blood-forming stem cells from bone marrow have been used for transplants, and current research is underway to determine whether or not differentiation of stem cells found in the brain and heart can be controlled and used for various other forms of transplantation.

Adult stem cells are found in areas called “stem cell niches” in various organs and tissues. These cells remain dormant until there is a need for them, which could include a requirement for new cells to replace damaged tissue due to injury or disease. Like embryonic stem cells, scientists are developing methods to extract these adult stem cells, harvest them in colonies, and manipulate them to create colonies of specialized cells that can be used for treatments of certain diseases. The challenge arises because once extracted from the human body, these cells have limited capacities to grow and reproduce.

A way scientists identify adult stem cells is first by extracting cells from a living organism. Next, the cells are labeled in a cell culture and transplanted back into a different organism. If the cells reproduce in the organism, scientists can verify the presence of properly functioning stem cells. Scientists have been conducting research to identify various differentiation pathways, which are the processes by which adult stem cells differentiate and convert into highly specialized cells with specific functions.

Different types of adult stem cells include the following:
 * Hematopoietic stem cells: develop into bone cells, cartilage cells, fat cells
 * Neural stem cells: develop into neurons, astrocytes and oligodendrocytes
 * Skin stem cells: develop into epidermal stem cells and hair follicles

Adult stem cells are found in developed organisms and have the ability to divide and create another cell like itself, or create a cell more differentiated than itself.

They have a few differences from embryonic stem cells, which include:


 * They cannot grow indefinitely due to having inactive telomerase. Some can actually only double once in their cellular life span.
 * Adult stem cells have a limited ability do differentiate, unlike ESC.
 * They exist in much smaller quantities than ESC do. There exists about 1 adult stem cell for every 10,000-15,000 cells in an adult human.
 * There also exists less ethical issues regarding adult stem cells because they are extracted from adults who are able to make their own decisions and give consent.

Potential exists for adult stem cells and regenerative properties, such as:


 * Healing organs: In the event of a heart attack, heart cells die and are replaced by scar tissue.  Adult stem cells have been used to partially heal the scar tissue in animals by removing the damaged heart tissue, culturing adult stem cells in a petri dish, and putting them back in the damaged heart.
 * Organ regeneration: In 1999, Dr. Anthony Atala, Urologist and Director of the Wake Forest Institute for Regenerative Medicine, was studying patients with a bladder defect. He took a small piece of bladder, removed the stem cells, and actually grew it into a new bladder. He discovered that he could stick them back in the original patient, and seven years later, the patients were either better, or like normal.  The bladder he was able to create was functional enough to keep the patients healthy and alive.

CD34+ Cells
CD34+ cells contain the protein CD34. CD34 protein is a glycoprotein found on human cell surfaces and functions as an adhesive for cells to interact. It could also have a function in attaching stem cells to bone marrow or connective tissue cells. The protein is expressed early in hematopoietic cells. Another function is that it is needed for T cells to enter lymph nodes. CD34+ cells are commonly found in as hematopoietic stem cells in umbilical cords and bone marrow.

Uses
To isolate CD34+ cells from blood samples, immunomagnetic or immunofluorescent techniques are used. These hematopoietic stem cells can be purified for research or clinical use in bone marrow transplant, by using antibodies. These cells can differentiate into different blood cells. CD34+ cells can be injected into patients to treat diseases such as Liver Cirrhosis, Peripheral Vascular disease, and spinal cord injury.

Induced Pluripotent Stem Cells
Induced pluripotent stem cells or iPSCs are adult somatic cells that have been reprogrammed to a state similar to embryonic stem cells. Although iPSCs are theoretically pluripotent cells, it remains unknown if they differ at all from embryonic stem cells. Induced pluripotent stem cells were first produced in 2006 from mouse cells and human cells followed in 2007.

Production of iPSCs
iPSCs are produced through a transfection which is process of introducing nucleic acids into a cell. In the case of iPSCs, this done through retroviruses. Other methods do exist in the production of iPSCs such as the use of adenoviruses and plasmids. Adenoviruses differ from retroviruses because it does not incorporate its own genetic information into the adult cell. Adenoviruses are used to transport the necessary transcription factors into the DNA of the adult cells. Adenoviruses present some benefits such as the fact that they avoid any potential for mutagenesis that may stem from the insertion as well as the fact that adenoviruses only need to be present for a small period of time to reprogram the adult cell. Plasmids can also be used to reprogram the adult cell and in this method, two plasmids are required to carry the necessary transcription factors. The main benefit of using plasmids is the simple fact that viruses do not have to used. Two main problems exist for the use of plasmids though and they are the fact that plasmids demonstrate low efficiency as well as the fact that plasmids have a continued risk of insertional mutagenesis and use cancer-promoting genes during reprogramming.

Through Cell Fusion
IPSCs have been created successfully by fusing EGCs (embryotic germ cells) or ESCs (embryotic stem cells) with somatic cells. Somatic cell nuclei can be reprogrammed by EGCs and mESCs after they are fused with thymic lymphocytes. After reprogramming, the procedure leads to tetraploid cells that resemble EGCs where the female cells have their inactive X chromosomes became active again. These tetraploids can then be injected into blastocyst to form chimeric embryos. Specifically for humans, this can, in the future, be used for both bone marrow cells and brains cells in the central nervous system that have been isolated. These cells can then be fused with mESCs in culture to create artificial cells that are similar to real ones in morphology and characteristics.

Through Cell Reprogramming
iPSCs can be also be obtained through reprogramming somatic cells (body cells of an organism) into pluripotent stem cells. This procedure is performed by the use of transcription factors which are proteins that bind to specific DNA sequences. The four transcription factors that are used in mouse fibroblasts via retroviral infection are Oct4, Sox2, Klf4, and c-Myc. For humans, different combinations of transcription factors have been used which includes Nanog, Lin28, or Nr5a2. The exogenous expression of the transcription factors is followed by selection in a mESC (mouse embryotic stem cell) agent to finally lead to the iPSCs. The resulting iPSCs have characteristics of normal ESCs (embryotic stem cells) such as reactivation of both X chromosomes in female cells, expression of endogenous pluripotency indicators, and the ability to create chimeric animals. The molecular events are still poorly understood, but how reprogramming occurs can be investigated. During reprogramming, epigenetic marks are expansively altered. Studies have shown that iPSCs clones to be impossible to tell apart from ESCs though there are still small differences that need to be tested before they can be used in regenerative medicine.

The Clinical Future of iPSCs
Induced pluripotent stem cells promise the almost infinite possibilities of embryonic stem cells without the nagging moral implications of the destruction of human embryos, and for this reason offers a promising place for research for regenerative medicine. iPSCs also hold the promise of other benefits over other types of stem cells. The main one being the fact that iPSCs can be developed from a patients own cells which mean that any treatments derived from the iPSCs should avoid any immunogenic responses. Even with the enormous possibilities of iPSCs in a clinical setting, a major concern hovers over the future which is the fact that iPSCs have an increased tendency to form tumors. From experiments, it was discovered that iPSCs injected into mice formed teratomas which are tumors with tissue or organ components inside. Moving toward the future, research is focused on not only coming up with potential treatments using iPSCs but on producing methods with higher efficiency as well as decreased occurrences of tumors using recombinant proteins.

Uses of Stem Cells
Both embryonic and adult stem cells have the potential to provide both treatments for various diseases as well as insight into human growth and development. One specific area of stem cell therapy is the use of these cells to treat people with type I diabetes. Type I diabetics are unable to produce insulin in the pancreas. Current research is underway to determine whether or not embryonic stem cells can be selectively specialized to give rise to insulin producing colonies that can be transplanted into diabetic patients. In order for such a procedure to be successful, stem cells must be capable of reproducing and generating enough of the tissue/cells of interest. They must also be able to survive in the recipient host, function properly, and maintain the capability to reproduce, while leaving the recipient unharmed. All of these requirements must be met without the recipient rejecting the cells, which is a challenge researchers are presently trying to overcome.

Human stem cells could also be used to test new drugs. New medications could be tested for safety on differentiated cells generated from human pluripotent cell lines. Cancer cell lines for example are used to screen potential anti-tumor drugs. Using stem cells scientists are also developing new ways to grow brain cells in the laboratory that could be used to treat patients with Parkinsons disease. Among the various approaches one is to grow stem cells into brain cells and transplant them into patients. If stem cells can be cultivated to become dopamine-producing nerve cells, researchers believe that they could replace the lost cells.

By using stem cells one could observe the specailization of specific cells such as nerve and muscle cells. By observing this process some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process of specialization. Another potential application of stem cells is making cells and tissues for medical therapies.

Cancer cells have the ability to divide indefinitely and spread to different parts of the body during metastasis. Embryonic stem cells can self-renew and, through differentiation to somatic cells, provide the building blocks of the human body. Embryonic stem cells offer tremendous opportunities for regenerative medicine and serve as an excellent model system to study early human development.

Controversy Surrounding Research
There is much ethical controversy regarding embryonic stem cell research. The number one being how ESC are harvested because once ESC are retrieved, the blastocyst dies. There is no way to harvest them without destroying the embryo. This brings to the table the question of when does life begin? There are many different beliefs as to when life begins, whether it be scientific, religious, etc., and so it is difficult to set a definition that everyone can agree upon.

Most of the blastocysts that are used for stem cell research come over from excess invitro fertilization. People who have fertility issues and decide to have invitro fertilization. This is where sperm and an egg are combined in a test tube, which results with a number of embryos, some of which are implanted in the mother. The left over embryos are then frozen for a variety of reasons; whether the first implantation doesn't take, or the couple decides to have a second child later on. Some decide to discard the embryos, but a vast majority are left frozen in labs. Another controversial question that arises is that whether it is alright to use the frozen embryos for ESC.

Political controversy in research arises as well:


 * late '90s: The first embryonic stem cell line was created.
 * 2001: Bush outlaws the creation and studying of more embryonic stem cell lines with the use of federal money. There were 21 distinct ESC lines at the time.
 * 2009: Obama creates the National Institute of Health (NIH) panel that will approve whether or not federal funding can be used to study certain stem cell lines. Established guidelines state that the stem cells must come from embryos that were created through invitro fertilization but were frozen and not going to be used anymore.  Parents must provide consent for their frozen embryos to be used for stem cell research, and once approved by the NIH federal money can be used to study them.  However, federal money cannot be used to create new stem cell lines.  They must be created through private funding, and once they are created, federal funding can be used to study it.

An issue of the overall safety of the use of stem cells exists as well. Researchers do not necessarily know what will happen when stem cells are inserted into a person, they know what they WANT to happen, but cannot guarantee what will. Scientists have yet to be able to precisely control what cells an embryonic stem cell will differentiate into.