Human embryonic stem cells in cell culture
Pluripotent: Embryonic stem cells are able to develop into any type of cell, excepting those of the placenta. Only embryonic stem cells of the morula are totipotent: able to develop into any type of cell, including those of the placenta.

Embryonic stem cells (ES cells or ESCs) are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre-implantation embryo.[1][2] Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells. Isolating the embryoblast, or inner cell mass (ICM) results in destruction of the blastocyst, a process which raises ethical issues, including whether or not embryos at the pre-implantation stage should have the same moral considerations as embryos in the post-implantation stage of development.[3][4] Researchers are currently focusing heavily on the therapeutic potential of embryonic stem cells, with clinical use being the goal for many labs.[5] Potential uses include the treatment of diabetes and heart disease.[5] The cells are being 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 tumours and unwanted immune responses have also been reported.[6]

Properties

The transcriptome of embryonic stem cells

Embryonic stem cells (ESCs), derived from the blastocyst stage of early mammalian embryos, are distinguished by their ability to differentiate into any cell type and by their ability to propagate. It is these traits that makes them valuable in the scientific and medical fields. ESCs are also described as having a normal karyotype, maintaining high telomerase activity, and exhibiting remarkable long-term proliferative potential.[7]

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 include each of the more than 220 cell types in the adult human body. Pluripotency distinguishes embryonic stem cells from adult stem cells, which are multipotent and can only produce a limited number of cell types.

Propagation

Under defined conditions, embryonic stem cells are capable of propagating indefinitely in an undifferentiated state. Conditions must either prevent the cells from clumping, or maintain an environment that supports an unspecialized state.[2] While being able to remain undifferentiated, ESCs also have the capacity, when provided with the appropriate signals, to differentiate (presumably via the initial formation of precursor cells) into nearly all mature cell phenotypes.[8]

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 potential in treating a number of varying conditions, including but not limited to: spinal cord injuries, age related macular degeneration, diabetes, neurodegenerative disorders (such as Parkinson's disease), AIDS, etc.[9] In addition to their potential in regenerative medicine, embryonic stem cells provide an alternative source of tissue/organs which serves as a possible solution to the donor shortage dilemma. Not only that, but tissue/organs derived from ESCs can be made immunocompatible with the recipient. Aside from these uses, embryonic stem cells can also serve as tools for the investigation of early human development, study of genetic disease and as in vitro systems for toxicology testing.[7]

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."[10]

Embryoid bodies 24 hours after formation.

However, embryonic stem cells are not limited to cell/tissue engineering.

Cell replacement therapies

Current research focuses on differentiating ESCs into a variety of cell types for eventual use as cell replacement therapies (CRTs). Some of the cell types that have or are currently being developed include cardiomyocytes (CM), neurons, hepatocytes, bone marrow cells, islet cells and endothelial cells.[11] However, the derivation of such cell types from ESCs is not without obstacles, therefore current research is focused on overcoming these barriers. For example, studies are underway to differentiate ESCs in to tissue specific CMs and to eradicate their immature properties that distinguish them from adult CMs.[12]

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.[13][14]
  • ESCs have been differentiated to natural killer (NK) cells and bone tissue.[15]
  • Studies involving ESCs are underway to provide an alternative treatment for diabetes. For example, D’Amour et al. were able to differentiate ESCs into insulin producing cells[16] and researchers at Harvard University were able to produce large quantities of pancreatic beta cells from ES.[17]
  • An article published in the

    Besides becoming an important alternative to organ transplants, ESCs are also being used in field of toxicology and as cellular screens to uncover new chemical entities (NCEs) that can be developed as small molecule drugs. Studies have shown that cardiomyocytes derived from ESCs are validated in vitro models to test drug responses and predict toxicity profiles.[11] ES derived cardiomyocytes have been shown to respond to pharmacological stimuli and hence can be used to assess cardiotoxicity like Torsades de Pointes.[19]

    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, current research is focusing on establishing fully functional ESC-derived hepatocytes with stable phase I and II enzyme activity.[20]

    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 (PGD), modeling genetic disorders is something that has been accomplished with stem cells. This approach may very well prove invaluable 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 (genetic cytology), 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 (when the risk of genetically related disorders is higher). 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.[21]

    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 (NHEJ), an error prone DNA repair process, as the primary pathway for repairing double-strand breaks (DSBs) during all cell cycle stages.[22] 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.[23] 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 (HRR) to repair DSBs.[23] 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 (i.e. after metaphase/cell division but prior the next round of replication) have only one copy of each chromosome (i.e. sister chromosomes aren’t present). Mouse ES cells lack a G1 checkpoint and do not undergo cell cycle arrest upon acquiring DNA damage.[24] Rather they undergo programmed cell death (apoptosis) in response to DNA damage.[25] 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.[26] Consistent with this strategy, mouse ES stem cells have a mutation frequency about 100-fold lower than that of isogenic mouse somatic cells.[27]

    Clinical trial

    On January 23, 2009, Phase I clinical trials for transplantation of oligodendrocytes (a cell type of the brain and spinal cord) derived from human ES cells into spinal cord-injured individuals received approval from the U.S. Food and Drug Administration (FDA), marking it the world's first human ES cell human trial.[28] The study leading to this scientific advancement was conducted by Hans Keirstead and colleagues at the University of California, Irvine and supported by Geron Corporation of Menlo Park, CA, founded by Michael D. West, PhD. A previous experiment had shown an improvement in locomotor recovery in spinal cord-injured rats after a 7-day delayed transplantation of human ES cells that had been pushed into an oligodendrocytic lineage.[29] The phase I clinical study was designed to enroll about eight to ten paraplegics who have had their injuries no longer than two weeks before the trial begins, since the cells must be injected before scar tissue is able to form. The researchers emphasized that the injections were not expected to fully cure the patients and restore all mobility. Based on the results of the rodent trials, researchers speculated that restoration of myelin sheathes and an increase in mobility might occur. This first trial was primarily designed to test the safety of these procedures and if everything went well, it was hoped that it would lead to future studies that involve people with more severe disabilities.[30] The trial was put on hold in August 2009 due to FDA concerns regarding a small number of microscopic cysts found in several treated rat models but the hold was lifted on July 30, 2010.[31]

    In October 2010 researchers enrolled and administered ESTs to the first patient at Shepherd Center in Atlanta.[32] The makers of the stem cell therapy, Geron Corporation, estimated that it would take several months for the stem cells to replicate and for the GRNOPC1 therapy to be evaluated for success or failure.

    In November 2011 Geron announced it was halting the trial and dropping out of stem cell research for financial reasons, but would continue to monitor existing patients, and was attempting to find a partner that could continue their research.[33] In 2013 BioTime (AMEXBTX), led by CEO Dr. Michael D. West, acquired all of Geron's stem cell assets, with the stated intention of restarting Geron's embryonic stem cell-based clinical trial for spinal cord injury research.[34]

    BioTime company Asterias Biotherapeutics (NYSE MKT: AST) was granted a $14.3 million Strategic Partnership Award by the California Institute for Regenerative Medicine (CIRM) to re-initiate the world’s first embryonic stem cell-based human clinical trial, for spinal cord injury. Supported by California public funds, CIRM is the largest funder of stem cell-related research and development in the world.[35]

    The award provides funding for Asterias to reinitiate clinical development of AST-OPC1 in subjects with spinal cord injury and to expand clinical testing of escalating doses in the target population intended for future pivotal trials.[35]

    AST-OPC1 is a population of cells derived from human embryonic stem cells (hESCs) that contains oligodendrocyte progenitor cells (OPCs). OPCs and their mature derivatives called oligodendrocytes provide critical functional support for nerve cells in the spinal cord and brain. Asterias recently presented the results from phase 1 clinical trial testing of a low dose of AST-OPC1 in patients with neurologically-complete thoracic spinal cord injury. The results showed that AST-OPC1 was successfully delivered to the injured spinal cord site. Patients followed 2–3 years after AST-OPC1 administration showed no evidence of serious adverse events associated with the cells in detailed follow-up assessments including frequent neurological exams and MRIs. Immune monitoring of subjects through one year post-transplantation showed no evidence of antibody-based or cellular immune responses to AST-OPC1. In four of the five subjects, serial MRI scans performed throughout the 2–3 year follow-up period indicate that reduced spinal cord cavitation may have occurred and that AST-OPC1 may have had some positive effects in reducing spinal cord tissue deterioration. There was no unexpected neurological degeneration or improvement in the five subjects in the trial as evaluated by the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) exam.[35]

    The Strategic Partnership III grant from CIRM will provide funding to Asterias to support the next clinical trial of AST-OPC1 in subjects with spinal cord injury, and for Asterias’ product development efforts to refine and scale manufacturing methods to support later-stage trials and eventually commercialization. CIRM funding will be conditional on FDA approval for the trial, completion of a definitive agreement between Asterias and CIRM, and Asterias’ continued progress toward the achievement of certain pre-defined project milestones.[35]

Concern and controversy

Adverse effects

The major concern with the possible transplantation of ESC into patients as therapies is their ability to form tumors including teratoma.[36] Safety issues prompted the FDA to place a hold on the first ESC clinical trial, however no tumors were observed.

The main strategy to enhance the safety of ESC for potential clinical use is to differentiate the ESC into specific cell types (e.g. neurons, muscle, liver cells) that have reduced or eliminated ability to cause tumors. Following differentiation, the cells are subjected to sorting by flow cytometry for further purification. ESC are predicted to be inherently safer than IPS cells created with genetically-integrating viral vectors because they are not genetically modified with genes such as c-Myc that are linked to cancer. Nonetheless, ESC express very high levels of the iPS inducing genes and these genes including Myc are essential for ESC self-renewal and pluripotency,[37] and potential strategies to improve safety by eliminating c-Myc expression are unlikely to preserve the cells' "stemness". However, N-myc and L-myc have been identified to induce iPS cells instead of c-myc with similar efficiency.[38] More recent protocols to induce pluripotency bypass these problems completely by using non-integrating RNA viral vectors such as sendai virus or mRNA transfection.

Ethical debate

Due to the nature of embryonic stem cell research, there are a lot of controversial opinions on the topic. Since harvesting embryonic stem cells necessitates destroying the embryo from which those cells are obtained, the moral status of the embryo comes into question. Scientists argue that the 5-day old mass of cells is too young to achieve personhood or that the embryo, if donated from an IVF clinic (which is where labs typically acquire embryos from), would otherwise go to medical waste anyway. Opponents of ESC research counter that any embryo has the potential to become a human, therefore destroying it is murder and the embryo must be protected under the same ethical view as a developed human being.[39]

History

  • 1964: Lewis Kleinsmith and G. Barry Pierce Jr. isolated a single type of cell from a teratocarcinoma, a tumor now known to be derived from a germ cell.[40] These cells isolated from the teratocarcinoma replicated and grew in cell culture as a stem cell and are now known as embryonal carcinoma (EC) cells.[41] Although similarities in morphology and differentiating potential (pluripotency) led to the use of EC cells as the in vitro model for early mouse development,[42] EC cells harbor genetic mutations and often abnormal karyotypes that accumulated during the development of the teratocarcinoma. These genetic aberrations further emphasized the need to be able to culture pluripotent cells directly from the inner cell mass.
Martin Evans revealed a new technique for culturing the mouse embryos in the uterus to allow for the derivation of ES cells from these embryos.