Embryonic stem cell
Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre-implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells. Isolating the inner cell mass using immunosurgery results in destruction of the blastocyst, a process which raises ethical issues, including whether or not embryos at the pre-implantation stage have the same moral considerations as embryos in the post-implantation stage of development.
Researchers focus heavily on the therapeutic potential of embryonic stem cells, with clinical use being the goal for many laboratories. Potential uses include the treatment of diabetes and heart disease. The cells are studied to be used as clinical therapies, models of genetic disorders, and cellular/DNA repair. However, adverse effects in the research and clinical processes such as tumors and unwanted immune responses have also been reported.
Properties
Embryonic stem cells, derived from the blastocyst stage of early mammalian embryos, are distinguished by their ability to differentiate into any embryonic cell type and by their ability to self-renew. It is these traits that makes them valuable in the scientific and medical fields. ESCs have a normal karyotype, maintain high telomerase activity, and exhibit remarkable long-term proliferative potential.Pluripotent
Embryonic stem cells of the inner cell mass are pluripotent, meaning they are able to differentiate to generate primitive ectoderm, which ultimately differentiates during gastrulation into all derivatives of the three primary germ layers: ectoderm, endoderm, and mesoderm. These germ layers generate each of the more than 220 cell types in the adult human body. When provided with the appropriate signals, ESCs initially form precursor cells that in subsequently differentiate into the desired cell types. Pluripotency distinguishes embryonic stem cells from adult stem cells, which are multipotent and can only produce a limited number of cell types.Self renewal and repair of structure
Under defined conditions, embryonic stem cells are capable of self-renewing indefinitely in an undifferentiated state. Self-renewal conditions must prevent the cells from clumping and maintain an environment that supports an unspecialized state. Typically this is done in the lab with media containing serum and leukemia inhibitory factor or serum-free media supplements with two inhibitory drugs, the MEK inhibitor PD03259010 and GSK-3 inhibitor CHIR99021.Growth
ESCs divide very frequently due to a shortened G1 phase in their cell cycle. Rapid cell division allows the cells to quickly grow in number, but not size, which is important for early embryo development. In ESCs, cyclin A and cyclin E proteins involved in the G1/S transition are always expressed at high levels. Cyclin-dependent kinases such as CDK2 that promote cell cycle progression are overactive, in part due to downregulation of their inhibitors. Retinoblastoma proteins that inhibit the transcription factor E2F until the cell is ready to enter S phase are hyperphosphorylated and inactivated in ESCs, leading to continual expression of proliferation genes. These changes result in accelerated cycles of cell division. Although high expression levels of pro-proliferative proteins and a shortened G1 phase have been linked to maintenance of pluripotency, ESCs grown in serum-free 2i conditions do express hypo-phosphorylated active Retinoblastoma proteins and have an elongated G1 phase. Despite this difference in the cell cycle when compared to ESCs grown in media containing serum these cells have similar pluripotent characteristics. Pluripotency factors Oct4 and Nanog play a role in transcriptionally regulating the embryonic stem cell cycle.Uses
Due to their plasticity and potentially unlimited capacity for self-renewal, embryonic stem cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease. Pluripotent stem cells have shown promise in treating a number of varying conditions, including but not limited to: spinal cord injuries, age related macular degeneration, diabetes, neurodegenerative disorders, AIDS, etc. In addition to their potential in regenerative medicine, embryonic stem cells provide a possible alternative source of tissue/organs which serves as a possible solution to the donor shortage dilemma. However, there are ethical controversies surrounding this. Aside from these uses, ESCs can also be used for research on early human development, certain genetic disease, and in vitro toxicology testing.Utilizations
According to a 2002 article in PNAS, "Human embryonic stem cells have the potential to differentiate into various cell types, and, thus, may be useful as a source of cells for transplantation or tissue engineering."Tissue engineering
In tissue engineering, the use of stem cells are known to be of importance. In order to successfully engineer a tissue, the cells used must be able to perform specific biological functions such as secretion of cytokines, signaling molecules, interacting with neighboring cells, and producing an extracellular matrix in the correct organization. Stem cells demonstrates these specific biological functions along with being able to self-renew and differentiate into one or more types of specialized cells. Embryonic stem cells is one of the sources that are being considered for the use of tissue engineering. The use of human embryonic stem cells have opened many new possibilities for tissue engineering, however, there are many hurdles that must be made before human embryonic stem cell can even be utilized. It is theorized that if embryonic stem cells can be altered to not evoke the immune response when implanted into the patient then this would be a revolutionary step in tissue engineering. Embryonic stem cells are not limited to tissue engineering.Cell replacement therapies
Research has focused on differentiating ESCs into a variety of cell types for eventual use as cell replacement therapies. Some of the cell types that have or are currently being developed include cardiomyocytes, neurons, hepatocytes, bone marrow cells, islet cells and endothelial cells. However, the derivation of such cell types from ESCs is not without obstacles, therefore research has focused on overcoming these barriers. For example, studies are underway to differentiate ESCs into tissue specific cardiomyocytes and to eradicate their immature properties that distinguish them from adult cardiomyocytes.Clinical potential
- Researchers have differentiated ESCs into dopamine-producing cells with the hope that these neurons could be used in the treatment of Parkinson's disease.
- ESCs have been differentiated to natural killer cells and bone tissue.
- Studies involving ESCs are underway to provide an alternative treatment for diabetes. For example ESCs have been differentiated into insulin-producing cells, and researchers at Harvard University were able to produce large quantities of pancreatic beta cells from ESCs.
- An article published in the European Heart Journal describes a translational process of generating human embryonic stem cell-derived cardiac progenitor cells to be used in clinical trials of patients with severe heart failure.
Drug discovery
ESC-derived hepatocytes are also useful models that could be used in the preclinical stages of drug discovery. However, the development of hepatocytes from ESCs has proven to be challenging and this hinders the ability to test drug metabolism. Therefore, research has focused on establishing fully functional ESC-derived hepatocytes with stable phase I and II enzyme activity.
Models of genetic disorder
Several new studies have started to address the concept of modeling genetic disorders with embryonic stem cells. Either by genetically manipulating the cells, or more recently, by deriving diseased cell lines identified by prenatal genetic diagnosis, modeling genetic disorders is something that has been accomplished with stem cells. This approach may very well prove valuable at studying disorders such as Fragile-X syndrome, Cystic fibrosis, and other genetic maladies that have no reliable model system.Yury Verlinsky, a Russian-American medical researcher who specialized in embryo and cellular genetics, developed prenatal diagnosis testing methods to determine genetic and chromosomal disorders a month and a half earlier than standard amniocentesis. The techniques are now used by many pregnant women and prospective parents, especially couples who have a history of genetic abnormalities or where the woman is over the age of 35. In addition, by allowing parents to select an embryo without genetic disorders, they have the potential of saving the lives of siblings that already had similar disorders and diseases using cells from the disease free offspring.
Repair of DNA damage
Differentiated somatic cells and ES cells use different strategies for dealing with DNA damage. For instance, human foreskin fibroblasts, one type of somatic cell, use non-homologous end joining, an error prone DNA repair process, as the primary pathway for repairing double-strand breaks during all cell cycle stages. Because of its error-prone nature, NHEJ tends to produce mutations in a cell's clonal descendants.ES cells use a different strategy to deal with DSBs. Because ES cells give rise to all of the cell types of an organism including the cells of the germ line, mutations arising in ES cells due to faulty DNA repair are a more serious problem than in differentiated somatic cells. Consequently, robust mechanisms are needed in ES cells to repair DNA damages accurately, and if repair fails, to remove those cells with un-repaired DNA damages. Thus, mouse ES cells predominantly use high fidelity homologous recombinational repair to repair DSBs. This type of repair depends on the interaction of the two sister chromosomes formed during S phase and present together during the G2 phase of the cell cycle. HRR can accurately repair DSBs in one sister chromosome by using intact information from the other sister chromosome. Cells in the G1 phase of the cell cycle have only one copy of each chromosome. Mouse ES cells lack a G1 checkpoint and do not undergo cell cycle arrest upon acquiring DNA damage. Rather they undergo programmed cell death in response to DNA damage. Apoptosis can be used as a fail-safe strategy to remove cells with un-repaired DNA damages in order to avoid mutation and progression to cancer. Consistent with this strategy, mouse ES stem cells have a mutation frequency about 100-fold lower than that of isogenic mouse somatic cells.