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©2021 by The International Society for Stem Cell Research. All rights reserved.

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2.2 Research Review Categories

CATEGORY 1

CATEGORY 2

CATEGORY 3

1A – Exempt from review by a specialized oversight process

  • Most in vitro pluripotent stem cell research
  • Most in vitro organoid research
  • Transfer of human stem cells into postnatal animal hosts

2 – Reviewed by a specialized oversight process

  • Procurement of embryos, or gametes for the creation of embryos, for in vitro research 
  • Derivation of cell lines from human embryos
  • Genetic alteration of embryos or gametes 
  • In vitro culture of human embryos for research until the formation of the primitive streak or 14 days from fertilization, whichever occurs first
  • Human cells transplanted into nonhuman embryos that are gestated in a non-human uterus
  • Integrated stem cell-based embryo models
  • Transferring human embryos following MRT into a human uterus

3A – Not allowed: currently unsafe

  • Heritable genome editing
  • Transferring mtDNA-modified (not including MRT) embryos into a uterus 
  • Using gametes differentiated from human stem cells for reproduction

1B – Reportable, but not typically reviewed by a specialized oversight process

  • Non-integrated stem cell-based embryo models
  • In vitro culture of chimeric embryos (human cells into non-human embryos) 
  • In vitro gametogenesis without fertilization or generation of embryos

3B – Not allowed: lacks compelling scientific rationale or is ethically concerning 

  • Gestating human stem cell-based embryo models
  • Human reproductive cloning
  • Breeding human-animal chimeras where there may be human germ cells.
  • Transferring human-animal chimeric embryo(s) to a human or ape uterus
  • Transferring human embryo(s), irrespective of origins, to an animal uterus


Recommendation 2.2: To ensure that human embryo and related stem cell research is proceeding with due consideration, to ensure consistency of research practices among scientists globally, and to specify the types of scientific projects that should be subject to review, the research review and oversight process should use the three categories described in this section.

Note: This table is designed to broadly describe the types of research contained under each category. See relevant sections for more information.

2.2.1 Category 1

2.2.1A: Category 1A. Research determined to be exempt from a specialized scientific and ethics oversight process after being assessed by the appropriate existing mandates and committees for laboratory research. Category 1A research includes the following activities:

  1. Research with human pluripotent stem cell lines that is confined to cell culture and involves routine research practices, such as differentiation into tissue-specific cell types.
  2. Research that entails the reprogramming of human somatic cells to pluripotency (for example, the generation of induced pluripotent stem cells). 
  3. Research that entails the use of human fetal tissue and cells, only if these cells and tissues have been procured according to the recommendations under section 2.3 below. 
  4. Research on stem cell culture systems that model specific stages of development or specific anatomic structures rather than the continuous development of an intact embryo or fetus. These would include but are not restricted to models of amnion formation, neural tube development, development of primordial germ cells, placental structures, 2D or 3D models of gastrulation or post-gastrulation events, and in vitro stem cell-derived organoids in culture that recapitulate most aspects of organ function, but not those that fall under subsequent categories.
  5. The transplantation of human stem cells, their derivatives, or other human cells into postnatal animal hosts (see Recommendation 2.2.1.1).

Organoid Research 
At this time, there is no biological evidence to suggest any issues of concern, such as consciousness or pain perception with organoids corresponding to CNS tissues, that would warrant review through the specialized oversight process. However, researchers should be aware of any ethical issues that may arise in the future as organoid models become more complex through long-term maturation or through the assembly of multiple organoids (Hyun et al. 2020).

2.2.1B: Category 1B. Research that is reportable to the entity or body responsible for the specialized scientific and ethics oversight process, but not normally subject to further or ongoing review, at the discretion of the entity responsible for the oversight process and subject to regulations and policies in the jurisdiction. Category 1B research includes the following activities: 

  1. Research that entails the in vitro formation of human stem cell-based embryo models that are not intended to represent the integrated development of the entire embryo including its extraembryonic membranes.
  2. Chimeric embryo research in which human pluripotent stem cells are transferred into non-human, mammalian embryos and cultured in vitro for the minimum time necessary to achieve the scientific objective without gestation. 
  3. Research on in vitro gametogenesis from human cells including genetically modified pluripotent stem cells, which does not involve attempts at fertilization and the generation of embryos.

Scientists pursuing Category 1A and B research are recommended to consult with appropriate institutional review committees or the committee or body responsible for the specialized scientific and ethics review process (see Recommendation 2.1.3) to determine the categorization of new research proposals. Committees with the jurisdiction over the relevant research should oversee the provenance of cells, tissues and derived human pluripotent stem cell lines to ensure that procurement and derivation is deemed acceptable according to the principles outlined in this document (see Section 2.3 and Section 2.4) and is in compliance with rigorous scientific, legal, and ethical standards.

Category 1B covers in vitro chimeric embryo research and in vitro gametogenesis for which there is no intent to generate a human embryo or fetus. Researchers are encouraged when possible to report on existing or planned in vitro experiments to the committee responsible for the specialized scientific and ethics oversight process to help identify cases that may warrant full review in the future.

Studies Transplanting Human Stem Cells or Their Direct Derivatives into the Central Nervous Systems of Animals 
Recommendation 2.2.1.1: Research involving the transfer of human stem cells or their direct neural and/or glial derivatives into the central nervous systems of postnatal animal hosts requires review by institutional animal research oversight committees supplemented by reviewer expertise in stem cell or developmental biology. (ISSCR Guidelines, 2006; Academy of Medical Sciences, 2011). Such oversight should weigh the potential benefits of the research and should utilize available baseline non-human animal data grounded in rigorous scientific knowledge or reasonable inferences and involve a diligent application of animal welfare principles.

Institutions should determine whether research involving human cells that have the capacity to integrate into the nervous systems of laboratory animals requires supplementation of the pre-existing animal research review process with scientists and ethicists that have relevant topic-specific expertise.

To assist review and oversight of stem cell-based human-to-non-human animal research, the ISSCR has provided an advisory report that guides reviewers through a series of considerations not typically covered by institutional animal research committees, but that are relevant for review (Hyun et al., 2021). Past experiences with genetically altered laboratory animals have shown that reasonable caution might be warranted if changes carry the potential to produce new defects and deficits. Current best practices dictate that research involving modified animals must involve the following: 

  1. the establishment of baseline animal data; 
  2. ongoing data collection during research concerning any deviation from the norms of species-typical animals; 
  3. the use of small pilot studies to ascertain any changes in the welfare of modified animals; and 
  4. ongoing monitoring and reporting to animal research oversight committees authorized to decide the need for real-time changes in protocols and, if necessary, the withdrawal of animal subjects. 

Research that may result in the presence of human gametes and their precursors in the gonads of laboratory animals is of not of significant ethical concern per se, as long as the animals are not allowed to breed (see Category 3 below).

Reviewers and investigators should follow the proposed ethical standards presented in the 2020 white paper and Appendix 1, while exercising appropriate judgment in individual situations. Research involving animals should also generally comply with the principles of the 3Rs (see: www.nc3rs.org.uk) and follow the ‘ARRIVE guidelines’ (Percie du Sert et. al 2020). 

2.2.3 Category 2

2.2.2 Category 2

2.2.2 Category 2. Forms of research with embryos, certain chimeras, and stem cell-based embryo models that are permissible only after review and approval through a specialized scientific and ethics review process. A comprehensive review should be coordinated with other relevant oversight, such as that provided by human subjects review committees, in vitro fertilization (IVF) clinic oversight bodies, and animal research review processes and the research should comply with local law and policy. All such research should have a compelling scientific rationale and necessitate the use of these materials rather than employ alternative models. The research should use the minimum number of embryos necessary to achieve the scientific objective. Forms of research requiring comprehensive review by a specialized review process encompass the following activities:

  1. Procurement and use of IVF human embryos for research in vitro.
  2. Procurement of human gametes to create research embryos in vitro.
  3. Research that generates human gametes from any progenitor cell type in vitro, when this entails performing studies of fertilization that produce human zygotes and embryos. The gametes may be derived from human pluripotent stem cells, oogonia, or spermatogonial stem cells that have been maintained in vitro, and they may be genetically modified or not. Any human embryos obtained from such gametes must only be studied in vitro, or be used to derive stem cell lines, such as embryonic stem cells.
  4. Research involving the genetic alteration of human embryos or gametes used to make embryos in vitro.
  5. Derivation of new cell lines from human embryos (not confined to pluripotent cell lines).
  6. Research involving the in vitro culture of human embryos where embryos are maintained in culture until the formation of the primitive streak or 14 days from fertilization, whichever occurs first. 
  7. Generation of stem cell-based embryo models that represent the integrated development of the entire embryo including its extraembryonic membranes.  These integrated stem cell-based embryo models should be maintained in culture for the minimum time necessary to achieve the scientific objective.
  8. Research aimed at generating human totipotent cells that have the potential to sustain embryonic development in vitro.
  9. Chimera research in which human pluripotent stem cells or their derivatives with broad potential are introduced into a) a non-human embryo or fetus in utero or b) a non-human embryo in vitro followed by transfer into a non-human uterus. Such experiments – if they are scientifically justified for the use of non-human primates above all other laboratory species – must exclude great and lesser ape species hosts (i.e., chimpanzees, gorillas, orangutans, bonobos, gibbons, and siamangs), as apes are prohibited from being used for invasive research in most parts of the world.
  10. Transferring human embryos to a human uterus following mitochondrial replacement. 

Culture of Human Embryos Beyond formation of the primitive streak or 14 Days

It is currently not technically feasible to culture human embryos beyond formation of a primitive streak or 14 days post-fertilization. However, culture systems are evolving, making this a possibility in the near future. Understanding the primitive streak, early germ layer development and primordial germ cell formation in humans is crucial to improve our understanding of and interventions for infertility, in vitro fertilization, pregnancy loss, and developmental disorders that occur or originate soon after implantation. Research using embryos is also crucial to validate integrated stem cell-based embryo models, which in the future may provide a more practical alternative to understanding some aspects of early human development.

Recommendation 2.2.2.1: Given advancements in human embryo culture, and the potential for such research to yield beneficial knowledge that promotes human health and well-being, the ISSCR calls for national academies of science, academic societies, funders, and regulators to lead public conversations touching on the scientific significance as well as the societal and ethical issues raised by allowing such research. Should broad public support be achieved within a jurisdiction, and if local policies and regulations permit, a specialized scientific and ethical oversight process could weigh whether the scientific objectives necessitate and justify the time in culture beyond 14 days, ensuring that only a minimal number of embryos are used to achieve the research objectives.

Human-Animal Chimeric Embryo Research

Recommendation 2.2.2.2: Chimeric embryo and in utero research described in ‘Category 2, i’ (see above) should proceed for the minimum time necessary to achieve the scientific aim. This research must proceed incrementally, stopping at well-defined timepoints to assess the degree and scope of chimerism during development before proceeding to full gestation, if full gestation is among the well-justified goals of the research. To avoid unpredictable and widespread chimerism, researchers should endeavor to use targeted chimerism strategies to limit chimerism to a particular organ system or region of the gestating chimeric animal.

Techniques such as blastocyst complementation, whereby a specific cell type or organ is effectively deleted as the host embryo develops, can lead to a specific cell type or organ being replaced entirely by derivatives from the donor-derived pluripotent stem cells. By itself this targeted chimerism may not prevent contributions elsewhere in the chimera, thus the need for an incremental approach. Nonetheless, if the host cells have an advantage over the donor cells, such as even a slightly faster rate of cell replication, then the donor cells will be disadvantaged and effectively selected against, leading to a little or no contribution outside the organ of choice.

As a general principle, non-human primate species should only be used when all other species, more distant in evolution from humans, are inadequate for the scientifically well justified research question being pursued. Appropriate research aims include understanding human development, understanding species barriers to chimerism, and treating disease. Any research involving non-human primates must utilize common laboratory species that are widely used in biomedical research (which excludes apes). Trained veterinary staff specializing in the care of non-human primates must be closely involved in the review and oversight of studies involving the transfer of human stem cells and their derivatives into nonhuman primate hosts.

Mitochondrial Replacement Techniques 
Recommendation 2.2.2.3: Further research should be undertaken to refine and assess the safety and efficacy of Mitochondrial Replacement Techniques (MRT), including minimizing a) the risk of mitochondrial carryover and b) disruptions to the interaction between mitochondrial and nuclear genomes. In addition, further research on polar body transfer techniques and the use of mitophagy or genome editing is needed to reduce or eliminate pathogenic mitochondrial DNA. Such research should be subject to review by a specialized oversight process as Category 2 Research (Section 2.1.5.2).

MRT is being explored to prevent the transmission of serious mitochondrial DNA-based diseases in at-risk pregnancies (see Section 3.4.8). MRT most commonly involves transferring the nuclear DNA from the prospective mother’s oocyte or fertilized oocyte (pronuclear stage) to that of a mitochondrial donor from which the nuclear DNA has been removed [maternal spindle transfer (MST) or pronuclear transfer (PNT), respectively]. The mitochondrial donor is selected to be free of known pathogenic mutations. Input from clinicians and scientists with relevant expertise in mitochondrial and embryo biology should augment the review process for evaluating clinical protocols that entail uterine transfer of human embryos for the purpose of human reproduction.

2.2.3 Category 3

2.2.3A Category 3A. Research activities currently not permitted. Research under this category should not be pursued at this time because the approaches are currently unsafe or raise unresolved ethical issues. There may be valid reasons for undertaking the research in the future, but this should not proceed until the safety and ethical issues are resolved. Such research includes:

  1. Research in which human embryos that have undergone modification to their nuclear genome are transferred into or gestated in a human uterus. Genome-modified human embryos include human embryos with engineered alterations to their nuclear DNA and embryos generated from a human gamete that has had its nuclear DNA modified, when such modifications could be inherited through the germline. While there are valid reasons for pursuing this line of research, which may include situations where correcting a deleterious gene variant is the only way that prospective parents may have a genetically-related child (National Academy of Medicine, National Academy of Sciences, and the Royal Society, 2020), conduct of such research will be dependent on appropriate policies, regulations, and oversight.
  2. Research in which human embryos that have undergone editing to their mitochondrial genome are transferred into or gestated in a human uterus, as current knowledge of such interventions is inadequate to ensure safety. 
  3. The use of human gametes differentiated from human stem cells for the purposes of fertilization and human reproduction. If the safety, policy, and regulatory issues are resolved, this approach may be desirable, for example in cases where treatment of childhood cancer has led to infertility, or as a route to heritable genome editing as indicated in (a) above.

2.2.3B Category 3B: Prohibited research activities. Research under this category should not be pursued because of broad international consensus that such experiments lack a compelling scientific rationale or are widely considered to be unethical. Such research includes:

  1. Transfer of human stem cell-based embryo models to the uterus of either a human or animal host. 
  2. Research in which human embryos produced by reprogramming of nuclei are implanted into a human or animal uterus (often referred to as human reproductive cloning). 
  3. Research in which animal chimeras incorporating human cells with the potential to form human gametes are bred to each other.
  4. Transfer of chimeric embryos mixing animal and human cells (whether predominantly animal or human) to the uterus of a human or great or lesser ape (i.e., chimpanzees, gorillas, orangutans, bonobos, gibbons, and siamangs).
  5. Transfer of a human embryo(s), irrespective of its origins, to an animal uterus.

Emerging Categories of Embryo Research That Merit Close Review: Heritable Genome Editing 
Recommendation 2.2.3.1: Until there is further scientific clarity regarding how to achieve desired genetic alterations, additional evidence for safety, and wider discussion and consensus on ethics (i.e., whether it should be done and, if so, under which circumstances), any attempt to edit the mitochondrial genome or modify the nuclear genome of human embryos for the purpose of human reproduction is premature and should not be permitted at this time (see Section 2.2.3A, Category 3A, a). 

Preclinical research that entails modifying the nuclear genomes of gametes, zygotes, and human embryos may be permissible under a rigorous specialized oversight process (Category 2). Such research promises to enhance fundamental knowledge and is essential to inform deliberations about the potential safety and use of nuclear or mitochondrial DNA genome editing in strategies aimed at preventing the transmission of serious genetic disorders. 

Scientists currently lack an adequate understanding of the fidelity and precision of techniques for genome editing of human embryos, as well as a full appreciation of the safety, ethics, and potential long-term risks and benefits to individuals born following such a process. This is described in more detail in the recent report, Heritable Human Genome Editing, from the International Commission on the Clinical Use of Human Genome Editing (National Academy of Medicine, National Academy of Sciences, and the Royal Society, 2020), which suggests a responsible translational pathway in certain circumstances, albeit one that cannot currently be met. Of note, the focus of this report was on developing a responsible translational pathway; it did not include a broad look at the societal and ethical issues, as these were outside its task. Such issues have been considered elsewhere (e.g. by the Nuffield Council on Bioethics). The forthcoming report from the WHO Advisory Committee on Developing Global Standards for Governance and Oversight of Human Genome Editing will bear in mind societal views and ethical principles, but this will focus on mechanisms of governance.

Basic and preclinical research is needed to minimize the potential harms resulting from intended and unintended edits, which could be passed to future generations, as well as direct or indirect effects of the editing process that could affect embryo viability or developmental potential. 

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