Research with Human Embryonic Stem Cells

last update: February 2024
Contact: Aurélie Halsband


I. Introduction

What are stem cells?

The term “stem cells” covers a non-uniform group of cells which, at least, share the following two properties:

  • Stem cells are precursor cells of highly differentiated cells.
  • After the stem cells have divided, the daughter cells can be either stem cells again (capable of self-renewal) or can differentiate into specific tissue, e.g. cardiac, neuronal, skin or muscle cells.
  • Given that stem cells have the ability of self-renewal, they can, in theory, multiply indefinitely.
  • Stem cells can differentiate into specific tissue, e.g. cardiac, neuronal, skin or muscle cells.

Stem cells can be identified first in the process of early embryonic development. The zygote is a totipotent stem cell (fig. 1). It develops in the early embryonic stages and forms the basis for all human tissue that is developed at later stages through specialisation or “differentiation”. The further the specialisation process of the daughter cells of a stem cell advances, the more the spectrum of their differentiation potential into various types of tissue is restricted.

Stem cells also go on to exist throughout the human life span in various types of adult human tissue, playing an important role in tissue regeneration and repair. They maintain the functionality of tissues and organs by supplying differentiated cells to replace damaged or dead cells. In common language use, the term “adult stem cells” has prevailed for this type of cells.

Many insights into early embryonic development, and also many therapeutic goals, can only be achieved with embryonic stem cells (ES cells). This issue focuses on research with this type of stem cells.

How are human embryonic stem cells derived from blastocysts?

Currently, the technique most often used for the derivation of embryonic stem cells is that of in vitro fertilisation (IVF). The application of this technique has become an established procedure in reproductive medicine as a way of inducing pregnancy in cases of unwanted childlessness. During the infertility treatment, test tube embryos are directly inserted into the woman's uterus by catheter where they can then develop into a child (fig. 2). Early embryos created in vitro can, however, also be used for the derivation of embryonic stem cell lines.

Five to six days after fertilisation the zygote has matured into a blastocyst. It consists of an outer cell layer - the trophoblast, which forms the basis the foetal part of the placenta - and of the inner cell mass which develops into the foetus. In order to derive stem cells (fig. 3), the trophoblast is destroyed either by using antibodies or laser technology. This renders any further development of the embryo impossible. 

There are also other procedures which allow the extraction of ES cells without compromising the integrity and the embryo’s ability to develop. However, due to their low efficiency and the remaining reservations of the lawmaker, these procedures have not been used widely so far. Embryonic stem cells are mainly obtained by destroying the trophoblast as shown. The now accessible inner cell mass is removed and cultivated in a cell culture dish in a special nutrient medium. The ES cells can continue to grow under the cell culture conditions without differentiating. Embryonic stem cell lines have hitherto been produced mostly from embryos left over from IVF trials.

What are the goals of research involving human embryonic stem cells?

Basic research

In the context of basic research, the main focus lies on gaining insight into the molecular mechanisms of individual cell specialisation as well as on examining the organisation of cells in situ. Furthermore, the goal is an improved understanding of the development and regulation of early stem cell stages as well as of the mechanisms behind the ability to proliferate and differentiate.

Translational research

Translational research on stem cells is particularly focused on questions arising from basic research, whose answers are a prerequisite for the therapeutic use of stem cells. With regard to a possible clinical application, a number of key criteria must be met, such as immune compatibility or sufficient reproducibility.

Research on the differentiation of human embryonic stem cells
Since the gain of the first human ES cell lines in 1998, several advancements have been made in the field of research with embryonic stem cells. Through in vitro and in vivo differentiations of human embryonic stem cells it has been possible to generate both different progenitor cells and differentiated cells from human embryonic stem cells. Research in this area is taking place, in particular, on nerve cells, cardiac and vascular cells, blood cells, hepatic and pancreatic cells and gametes (egg cells and sperm).

Research on the development of diseases and new therapeutic options
Given the characteristics of differentiation of ES cells, these cells are especially suited for such research to thoroughly investigate a variety of developmental processes. For example, ES cells can be used to research the occurrence of specific diseases on a molecular level. Also, researchers hope to gain insights advancing the development of individually tailored treatments. Stem cell based models which help know specifics on efficacy and security more accurately than animal research are of interest for the development of new as well as existing drug treatments. In this context, so called organoids made from stem cells play an increasingly important role as model systems.

Researchers also hope that ES stem cells will help make the creation of tissue replacement possible, especially for tissue that shows little to no regeneration capability, such as nerve tissue. ES cells are suspected to be a sheerly inexhaustible source to replace cells and tissue due to their capability to multiply unboundedly.

In the area of tissue replacement, research on adult stem cells and iPS cells also plays a central role. Some therapeutic procedures in which adult stem cells are used, such as blood stem cell transplantation, are already widely used in clinics.

Clinical application of stem cell research
Central foci of clinical research are, i.a., the efficacy, compatibility and patient safety of medical interventions. An early clinical application has been regarded unrealistic for a long time. Stem cell-based therapies were only offered by dubious private clinics in countries without counteracting regulations. In the meantime, several clinical studies were carried out which give occasion to revise the previous assessment.

II. Ethical Aspects

There is widespread agreement that the goals pursued in research involving human embryonic stem cells, both in fields of basic biological research and therapeutic research, are not only legitimate, but also eminent, i.e. “high-ranking”. Opinions differ, however, on the question of the justifiability of the means used in this research, if they involve the utilization and - according to state-of-the-art technology - also the destruction of human embryos.

One manifestation of this disagreement is, for instance, the great diversity of national, including the german one, and international regulatory models and their current discussion. Various ethical aspects and considerations come into play here, some of which are weighted differently by the participants in the debate.

One well-established method of ethically assessing an action is to examine the legitimacy of the objectives pursued by the action and the legitimacy of the means employed. The discussion mainly focuses on the question of how far human embryos are particularly worthy of protection and whether their protection status permits using embryos for the derivation of stem cells or even producing them especially for this purpose (1.). Granting a possible inadmissibility of consumptive embryo research leads on to the ensuing question of whether this also applies to ‘surplus’ embryos (2.) or to cell nuclear transfer embryos which are not created by the “conventional” nuclear fusion of two germ cells (3.). Finally, as assessing the justifiability of the means also depends on the availability of other means, the discussion of possible alternatives also plays an important role (4.).

1. The question of the worthiness of protection of the human embryo

The debate about the worthiness of protection of the human embryo is characterised by two different fundamental positions.

At the heart of the first position lies the conviction that from the moment in which nuclear fusion is completed, i.e. right from the beginning, the embryo is entitled to the same protection status as that accorded to human beings after birth on the basis of their personhood. The moral status of the early embryo is thus determined on the basis of the autonomous subject into which he or she may potentially develop. Accordingly, embryos must never be used as mere means to other ends, irrespective of the embryos' developmental stage or how eminent the purposes may be.

Following the second fundamental position, the embryo first needs to reach a specific stage in its development before it is entitled to the same protection status granted to human beings after birth on the basis of their personhood. Prior to this stage, embryos are only entitled to a graduated protection level.

Accordingly, consumptive embryo research using embryos that have not reached the relevant developmental stage requires justification, but is not ruled out entirely on moral grounds. As a consequence, this type of research is not only morally admissible, but is even imperative if the research goals are sufficiently eminent and if there are no alternative means. The same applies to the creation of embryos for research purposes.

Advocates of the first position generally draw on the following lines of reasoning. Firstly, they argue that right from the beginning the embryo has the potential to develop into a person (potentiality argument). What is more, following completion of nuclear fusion the embryo develops into a person in a continuous process. Hence, if one seeks to avoid arbitrary assertions, the commencement of protection worthiness can also only be located at this point in time (continuity and identity argument). Not only that, the proponents of this position point out that it would be a violation of the fundamental principle of human dignity to make worthiness of protection dependent on a property other than that of being a human being (species affiliation argument).

Supporters of the second position, which only accord the embryo full worthiness of protection when it reaches a specific stage of development, vary in their definition of this specific stage. Some consider the moment of nidation in the uterus as the decisive factor, because the embryo would only be truly able to develop from this point onwards. Others define the formation of the primitive streak as the decisive criterion: only then could the possibility of a multiple pregnancy be ruled out and the individuation process was completed. A third group of proponents considers the development of the neuronal preconditions for such capacities as pain sensation or the ability to develop interests to be the decisive factors. They argue that without these capacities it was impossible to establish any claims, even less so to justify any related claims to protection.

The German Embryo Protection Act is based on the first position which prohibits the creation of embryos for research purposes and the use of embryos for purposes other than their preservation. UK regulations - the Human Fertilisation and Embryology Act of 1990 and the Human Fertilisation and Embryology (Research Purposes) Regulations of 2001 - are based on the second position. Under certain further conditions, they not only allow the use of "surplus" embryos for defined research goals up to the formation of the primitive streak, but also the creation of embryos for these very purposes.

2. The question of the admissibility of research involving so-called ‘surplus’ embryos

In many countries where in vitro fertilisation treatments are permitted and carried out, the problem of the so-called "surplus" embryos arises. ‘Surplus’ embryos are embryos created in the course of in vitro fertilisation (IVF), but not transferred to the uterus. They are no longer needed by the parents for a subsequent transfer, either because one of the parents has developed a disease or has died or because the parents do not want to have any more children.

There are two legal options for their use: discarding the embryos or donating them to another couple. A third option being discussed is the use of the embryos by research (such as for the derivation of embryonic stem cells).

Should the use of surplus embryos for the derivation of stem cells and their destruction in this process be permitted?

Supporters of their use for stem cell derivation argue that this does not constitute an inadmissible use as means to an end (an inadmissible “instrumentalisation”). These embryos would not have the chance to develop into a child anyway and are consequently “doomed to die”. The only remaining options were to let them die or store them for an unlimited period.

Opponents have their doubts whether such embryos are necessarily “doomed to die”. They point out the possibility of later embryo adoption. Furthermore, they argue that permitting the use of such ‘surplus’ embryos for stem cell derivation could tempt IVF providers and also IVF users in future to produce even more embryos in order to make them available to research.

Those in favour of such a permission counter that the artificial creation of ‘surplus’ embryos could be prevented by adequate legal regulations. In its statement, the Leopoldina, as a proponent, proposes a control by a federal authority together with an ethics commission in the case of a legalisation of research on surplus embryos. The supporters reject embryo adoption with the argument that it causes a split in parenthood (so-called "split motherhood") since the person who donates the egg is not the same person as the one who will carry the embryo and subsequently will be the social parent. At the same time, this split is associated with considerable risks for the child.

3. The question of the admissibility of research with cell nuclear transfer embryos and their creation

It is hoped that the use of specific cloned stem cells will facilitate the development of multi-faceted, patient-specific therapeutic procedures for previously incurable diseases. The aim of this therapeutic cloning is to produce a genetically identical replacement tissue from the somatic cells of the patients which does not get rejected. In the so-called somatic cell nuclear transfer, the nucleus extracted from a somatic cell of the patient is inserted into a previously enucleated oocyte. This combination results in an embryo whose genome almost completely matches the patient’s DNA. It forms the basis for a possible later extraction of so-called SCNT stem cells. As the stem cells are almost completely genetically identical to the cells of the patient, it is expected that there will be fewer problems with immune tolerance, which can occur in the course of the therapeutic use of IVF-ES cells.

The technique for obtaining these stem cells (cell nuclear transfer) as well as the legal provisions governing their use and the relevant ethical debate are discussed in the In Focus section on Research Cloning. Leaving aside the ethical and legal problems, obtaining embryonic stem cells after cell nuclear transfer (so-called “cloning for research purposes” or “therapeutic cloning”) was technically not feasible for a long time. The stem cells obtained in this way are similar to those obtained from fertilised embryos and can differentiate into functional nerve cells, heart muscle cells and liver cells, among others.

The scientists involved in the SCNT procedure emphasize that their research is aiming at therapeutic cloning, not at reproductive cloning. Whether this procedure of obtaining embryonic stem cells will ever be applied in medical practice is yet controversial due to ethical concerns regarding the creation and destruction of embryos, the physiological stress of the oocyte-donators and a potential expansion to reproductive cloning.

4. The question of the alternatives to human embryonic stem cell research

Given the same goals, means which are ethically less problematic ought to be given priority over ethically more problematic means. In order to be ethically justifiable, the application of means which are ethically more problematic must, therefore, not only be adequate but must also be necessary to achieve the desired goals.

Adult stem cells as an alternative?

There are critics who doubt that research involving human embryonic stem cells has no alternatives. They believe that the goals of both basic and therapeutic research can also be achieved using tissue-specific adult stem cells. They argue that the derivation of these cells is ethically less problematic. These cells also have the advantage that cell transplants derived from adult stem cells are more likely to be immunotolerant, as these cells are taken directly from the organism of the transplant recipient. However, in the case of embryonic stem cells, the creation of autologous cell transplants is, so the argument goes, only possible by way of “therapeutic cloning”. Furthermore, the critics maintain that transplants produced from adult stem cells show a lower risk of tumour formation and that using adult stem cells has already led to recorded therapeutic successes. This, they point out, has not yet been achieved with embryonic stem cells. Some of the therapeutic procedures developed with adult stem cells have since even become the clinical standard. One area of research in this field is the cultivation of stem cells that were isolated from amniotic fluid. It has been reported that human fat, muscle, bone, nerve and liver cells have been gained from such cells.

The counter argument produced by supporters of research with embryonic stem cells is the fact that adult stem cells seem to have a far lower differentiation potential than embryonic stem cells. The number of various tissue types which can be obtained from adult stem cells, would, therefore, probably be very limited. Apart from this, the argument goes, adult stem cells could not be proliferated on a scale required for the creation of therapeutically efficacious cell transplants. In order to understand better the mechanisms responsible for differentiation, redifferentiation and proliferation, research involving human embryonic stem cells is, therefore from their point of view, absolutely indispensable (This observation can be applied to all of the alternatives presented here). Gaining insight into these mechanisms would even be a necessary precondition for the further development of adult stem cell therapies.

iPS cells as an alternative?

Induced pluripotent stem cells have long been considered a beacon of hope for an ethically acceptable alternative to research on human embryonic stem cells.

By applying a procedure known since 2007, human somatic cells can be reprogrammed in such a way that they show essential characteristics of embryonic stem cells. Such cells are called induced pluripotent stem cells (iPS cells).

The techniques are connected with risks which have to be solved prior to therapeutic use. Various research groups are currently investigating to solve these. Previous techniques required the transportation of four genes (Oct4, Sox2, c-Myc and Klf4) into the respective cell for reprogramming. Viruses thereby served as a vehicle for transportation, as they infiltrate the DNA of the cell and modify it. Yet changes to the DNA can lead to genetic anomalies. These may affect single DNA elements or whole sections and can even alter the number of chromosomes. Depending on the technique being used, the mutations occur at different points of time and can even lead to a higher risk of cancer if they affect sections which control cell growth. A broad application of tissue cells obtained from reprogrammed cells for therapeutic use is therefore not feasible yet, despite clinical applications having already been conducted.

Ethically, induced pluripotent stem cells provide the advantage of being gained by reprogramming adult cells instead of by destroying the embryo. With the help of known methods such as tetraploid embryonic complementation, however, it is theoretically possible to generate completely viable organisms, i.e. reproductive clones, from adult cells and iPS cells. However, concerns have also been raised about their use. On the one hand, it has become apparent that due to epigenetic changes the similarity between embryonic stem cells and induced pluripotent cells is not as close as initially assumed and that the former exhibit a greater susceptibility to mutations. On the other hand, research successes on induced pluripotent cells with the process of tetraploid embryo complementation have raised doubts about the ethical acceptability of this cell type. Tetraploid embryo complementation has made it possible to create viable clones in mouse models with the aid of adult cells and, successively, induced pluripotent cells.

Some critics point to the fact that the possibility to derive embryos from iPS cells makes the use of iPS cells ethically comparable to ES cells. In October 2009, the Berlin-Brandenburg Academy of the Sciences (Berlin-Brandenburgische Akademie der Wissenschaften) together with the National Academy of the Sciences (Nationale Akademie der Wissenschaften (Leopoldina)) published a statement concerning this new technique of stem cell derivation. The German Ethics Council published an Ad Hoc Recommendation on this matter in September 2014.

Furthermore, there are some indications that iPS cells differ from ES cells with regard to changes in the genome. As a result, the specific processes of differentiation and its control, which are crucial for clinical application, can only be investigated using the ES cell lines themselves. iPS or adult stem cells are therefore not yet an alternative for this area. ES cells are still considered the gold standard for medical research, i.e. all alternatives are measured against them.

Transdifferentiated cells as an alternative?

Until now the relatively recent process of transdifferentiation of cells has not appeared to be critical. This process attempts to obtain other specialised cell types (e.g. nerve cells) from differentiated, adult cells (e.g. skin cells) by bypassing stem cells. It is also possible to produce progenitor cells of certain kinds of tissue. For the process of transdifferentiation no embryonic tissue is required. Given that transdifferentiated cells are the patient’s own, they help avoid immune and adverse reactions, making them especially suitable for applications in regenerative medicine. The genetic modifications needed for their generation, however, bear the risk of malignant degeneration. The technology used to reprogram cells is currently being used to research genetic diseases.

Conducting research on animals as an alternative?

Exclusively conducting research on animals is a further, occasionally discussed alternative to research on human embryonic stem cells. A usual argument for this is that the cultivation and mechanisms of differentiation of stem cells can be researched with stem cells from mice. Such research is additionally not regulated and pre-trials with mice often help plan and justify follow-up trials on human stem cells. Besides animal research being ethically controversial (see “Animal Experiments in Research”) there is a further problem concerning the applicability of their results to humans. The differentiation mechanisms of embryonic stem cells are partly controlled by different growth factors in mice than in humans.

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