Glossary

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Definitions and discussion of terminology relevant to these guidelines is found in Glossary pages G.1 – G.5. by using the left navigation or the Next button. Other definitions can be found at http://stemcells.nih.gov.

• G.1 The term “embryo” and other terms used to describe early stages of development

  Blastocyst: The stage of preimplantation embryo development that, for humans, occurs around day 5–6 after fertilization or intracytoplasmic sperm injection. The blastocyst contains a fluid filled central cavity (blastocoele), an outer layer of cells (trophectoderm) and an inner cell mass (ICM). The trophectoderm cells attach the embryo to the uterine wall, and the ICM forms the embryo proper. The human blastocyst hatches from the zona pellucida (a surrounding glycoprotein shell) around days six-seven after fertilization. Thereafter, and coupled to implantation, the ICM of the blastocyst begins to organize itself into a flattened embryonic disc and associated amnion.

  Cleavage stage embryo (preimplantation stage): The embryonic stage that follows the first division of the zygote and ends upon morula compaction; precise stages include the two-cell, four-cell, eight-cell and 16-cell embryo. In humans, each cleavage division takes around 18-24 hours.

  Embryo: The term “embryo” has been defined and used differently in various biological contexts as discussed below.

  In this document, the term “embryo” is used generically to describe all stages of development from the first cleavage of the fertilized ovum to nine weeks post fertilization in the human, including the placenta and other extraembryonic membranes.

  More precise terms have been used to describe specific stages of embryogenesis; for example, the two, four and eight cell stages, the compacting morula and the blastocyst all describe particular stages of preimplantation embryonic development.

  Prior to implantation, the embryo represents a simple cellular structure with minimal cellular specialization, but soon after implantation a defined structure called the primitive streak begins to form and marks the future posterior region of the embryo. After this time twinning of the embryo can no longer occur as there is irreversible commitment to the development of more complex and specialized tissues and organs.

  Classical embryology used the term embryo to connote different stages of post-implantation development (for example, the primitive streak and onwards to fetal stages). Indeed, Dorland’s Illustrated Medical Dictionary (27th edition,1988 edition, W. B. Saunders Company) provides the definition “in animals, those derivatives of the fertilized ovum that eventually become the offspring, during their period of most rapid development, i.e., after the long axis appears until all major structures are represented. In man, the developing organism is an embryo from about two weeks after fertilization to the end of seventh or eighth week.” An entry in Random House Webster’s College Dictionary reads “in humans, the stage approximately from attachment of the fertilized ovum (egg or MII oocyte) to the uterine wall until about the eighth week of pregnancy.” However, the nomenclature is often extended by modern embryologists for the human to include the stages from first cleavage of the fertilized ovum onwards to seven to nine weeks post fertilization, after which the term fetus is used.

  Fetus: In this document, the term “fetus” is used to describe post-embryonic stages of human prenatal development, after major structures have formed. In humans, this period is from eight to nine weeks after fertilization until birth. The term is often not used in animals where “embryo” is used for any stage from fertilization to term.

  Stem cell-based embryo models: Advances in cellular engineering make possible the assembly, differentiation, aggregation, or re-association of cell populations in a manner that models or recapitulates key stages of embryonic development. Such experimental systems can provide essential insights into embryo and tissue development but raise concerns when such structures achieve complexity to the point where they might realistically manifest the ability to undergo further integrated development if cultured for additional time in vitro. There are two types of stem cell-based embryo models.

  • Non-integrated stem cell-based embryo models: These stem cell-based embryo models will experimentally recapitulate some, but not all aspects of the peri-implantation embryo, for example differentiation of the embryonic sac or embryonic disc in the absence of extraembryonic cells. These stem cell-based embryo models do not have any reasonable expectations of specifying additional cell types that would result in formation of an integrated embryo model. Gastruloids are an example of a non-integrated stem cell-based embryo model.

  • Integrated stem cell-based embryo models: These stem cell-based embryo models contain the relevant embryonic and extra-embryonic structures and could potentially achieve the complexity where they might realistically manifest the ability to undergo further integrated development if cultured for additional time in vitro. Integrated stem cell-based embryo models could be generated from a single source of cells, for example expanded potential human pluripotent stem cells capable of coordinately differentiating into embryonic and extraembryonic structures. Alternatively, integrated stem cell-based embryo models could also be generated through the formation of cellular aggregates where extraembryonic/embryonic cells from one source are combined with embryonic/extraembryonic cells from different sources to achieve integrated human development. This might include using non-human primate cells as one of the sources. Previous restrictions on preimplantation human embryo culture (the “14-day/primitive streak rule”) were not written to apply to integrated stem cell-based embryo models. Thus, these guidelines specify the imperative for specialized review when such research is designed to model the integrated development of the entire embryo including its extraembryonic membranes. A guiding principle of review should be that the integrated stem cell-based embryo models should be used to address a scientific question deemed highly meritorious by a rigorous review process. Blastoids are an example of an integrated stem cell model.

  Morula: The compacting grape-like cluster of 16 cells, typically formed by the human embryo four days after fertilization.

  Nuclear Transfer: This process involves the insertion of a nucleus of a cell into an ovum from which the nuclear material (chromosomes) has been removed. The ovum will reprogram (incompletely) the cell nucleus to begin development again. Embryos created by nuclear transfer are typically abnormal and often die during development, but rarely are capable of development to term. ICMs from blastocysts derived by nuclear transfer can form apparently normal embryonic stem cells.

  Organoid: A tissue culture-derived structure growing in 3D and derived from stem cells that recapitulate the cell composition and a subset of the physiological functions of an organ through principles of self-organization.

  Parthenogenetic embryo: Activation of the unfertilized mammalian ovum (usually accompanied by duplication of the haploid genome) can result in embryonic development, and embryonic stem cells can be derived from the ICMs of parthenogenetic blastocysts. After uterine transfer, parthenogenetic embryos of non-human animals have been observed to progress through early post-implantation development but further development is compromised by an underdeveloped placental system that prevents normal gestation. Gynogenesis is a particular form of parthenogenesis in which an embryo is created from the genetic contributions (female pronuclei) of two different zygotes. Androgenesis entails creation of an embryo that incorporates the male pronuclei from two different zygotes.

  Zygote: The fertilized single cell pronuclear ovum (egg), typically observed in humans between 20-35 hours after insemination with sperm.

• G.2 Terminology relating to developmental potential

  Pluripotent: The state of a single cell that is capable of differentiating into all tissues of an organism, with exception of the extraembryonic cell types.

  Multipotent: The state of single cells that are capable of differentiating into multiple cell types, but not all of the cells of an organism. Multipotent cells, exemplified by the hematopoietic stem cell, give rise to a range of cells within a specific tissue. Within the developing organism multipotent cells may give rise to derivatives of more than one embryonic germ layer, as for mesendodermal progenitors. In the adult, multipotent cells are typically restricted to becoming derivatives of a specific germ layer (endoderm, ectoderm, mesoderm), organ, or tissue.

  Teratoma: A benign, encapsulated mass of complex differentiated tissues comprising elements of all three embryonic germ layers: ectoderm, endoderm, and mesoderm. In the context of stem cell research, the teratoma assay entails injection of cell populations into immune-deficient murine hosts to assess their pluripotency (their capacity to produce derivatives from all three germ layers). These structures are distinct from teratocarcinomas in which, in addition to the differentiated derivatives, undifferentiated stem cells persist.

  Totipotent: The state of a cell that is capable of giving rise to all types of differentiated cells found in an organism, as well as the supporting extra-embryonic structures of the placenta. A single totipotent cell might, by division in utero, reproduce the whole organism, but to date this has only been demonstrated by zygotes or blastomeres of early cleavage stage embryos.

  Unipotent: The state of single cells that are capable of differentiating only along a specific cell lineage and are exemplified by lineage-committed progenitors of the hematopoietic system (for example, erythroblasts). Unipotent stem cells undergo self-renewal and differentiation along a single lineage, as exemplified by the spermatogonial stem cell.

• G.3 "Chimeras” in stem cell research

  Chimera: An organism carrying cell populations derived from two or more (genetically distinct) sources, where the latter include zygotes, later stage embryos, liveborn animals, or cells grown in culture. N.B. While rare, some humans are natural chimeras due to the aggregation of two preimplantation stage embryos. More commonly, cells may cross the placental barrier from mother to fetus or vice versa and persist in the ‘host’ for life (Madam 2020). The word chimera should, therefore, be used as a neutral scientific term, in contrast to its mythological origins.

  Interspecies chimeras: Interspecies chimeras are animals containing integrated cellular contributions from another species. The degree of contribution can range from minor to extensive. For example, chimeras can be derived when human stem cells are transferred into non-human embryos. There are three types of true human-animal chimeras bearing special concern: (a) those that have the capacity for widespread chimerism, and (b) those that have a significant degree of chimerism to the central nervous system and (c) those that have chimerism of the germline. Human-to-non-human primate chimeras or cellular xenotransplants formed at any stage of development warrant particular attention. For additional guidance on the review of human-animal chimeras, please consult the ISSCR white paper on chimeras (Hyun et al., 2020).

  Cellular transplants into postnatal animal hosts: Although formally the resulting animal can still be classed as chimera, where human cells with limited fate, in terms of cell type or tissue distribution are introduced into defined positions in postnatal animals (or late embryonic stages), this is usually referred to as a transplant or graft into a host. The graft can be homotopic, where it can integrate into the host tissue, or ectopic, where it may develop as a defined structure. Unless the methods involve transplanting human germ cells into animal gonads, these types of experiment will generally be of little concern, although they should be subject to review by an animal ethics committee.

• G.4 Terms used in transplantation

  Allogeneic transplantation: Refers to the transplantation of cells from a donor to another person, either related (as when from a sibling or parent) or from an unrelated individual. In hematopoietic stem cell transplantation, unrelated donors may be identified from large donor registries as being histocompatible or matched to the transplant recipient at a series of human leukocyte antigens known to mediate transplant rejection. Allogeneic hematopoietic stem cell transplantation carries with it the potential for the donor’s transplanted cells to mount an immune attack against the recipient (graft versus host disease), while solid organ transplant carries the risk of the recipient’s immune system rejecting the allograft. Both clinical settings require the use of immunosuppressive drugs, which in the case of solid organ transplant recipients must be taken lifelong, placing them at risk of infectious complications.

  Autologous transplantation: Refers to the transplantation to an animal or human patient of his/her own cells. Because the cells are recognized by the patient’s immune system as “self,” no rejection or immune incompatibility is observed. Consequently, autologous transplantation of cells typically carries fewer risks than allogeneic transplantation. Generation of embryonic stem cells by somatic cell nuclear transfer or derivation of induced pluripotent stem cells by reprogramming offers a potential source of autologous cells for many different transplantation studies, offering the theoretical advantage of immune compatibility.

  Homologous use: Refers to intended therapeutic use of cells within their native physiological context, for example, the transplantation of hematopoietic stem cells to regenerate the blood, or the use of adipose tissue to reconstruct a breast.

  Non-homologous use: Refers to intended therapeutic use of cells outside their native physiological context, for example, the transplantation of hematopoietic cells or mesenchymal stromal cells into the heart or brain.

  Tumorigenicity: The property of cells that describes their potential for forming tumors, or an abnormal growth of cells.

• G.5 General terms pertaining to research and research participants

  Assent: In the context of clinical research, assent means the participant agrees to take part. To give assent means that the participant is engaged in research decision-making in accordance with his or her capacities. Children and adolescents who are legal minors cannot give legally valid informed consent, but they may be able to give assent. Assent demands that the legal minor provide affirmative agreement to participate in research.

  Clinical research: Any systematic research conducted with human subjects or groups of human subjects or on materials from humans, such as tissue samples.

  Clinical trials: Any research study that prospectively assigns human subjects or groups of human subjects to one or more health-related interventions to evaluate the effects on health outcomes. Interventions include but are not restricted to drugs, cells and other biological products, surgical procedures, radiological procedures, diagnostics, devices, behavioral treatments, process-of-care changes, preventive care.

  Compensation: Payment for research subjects’ non-financial burdens incurred during the course of their research participation, most commonly their time, effort, and inconvenience.

  Correlative studies: Studies, typically occurring within clinical trials, that explore the cause and effects of an intervention on biological targets involved in a disease process or linkages among groups or different elements of a group.

  Incidental finding: A discovery concerning an individual research participant or tissue donor that does not relate directly to the aims of a study but that has potential health or reproductive importance for the individual.

  Minimal risk: Risk from procedures to human subjects or tissue donors that is comparable to the probability and the magnitude of harms that are ordinarily encountered in daily life or during the performance of routine physical or psychological examinations or tests.

  Minor increase over minimal risk: An increment in risk that is only a fraction above the minimal risk threshold and considered acceptable by a reasonable person.

  Observational studies: A type of clinical research where investigators observe human subjects or groups of human subjects to measure variables of interest; the assignment of subjects into a treated group versus a control group is not controlled by the investigator.

  Reimbursement: Repayment for research subjects’ out-of-pocket expenses incurred during their participation in research.

  Sham procedures: Procedures used as controls in clinical trials that mimic experimental procedures for research subjects in the “treatment” arm. These are performed to prevent research subjects and physicians assessing their outcomes and from knowing which arm of the trial the subject has been enrolled in. They are also sometimes performed to control for the effects that treatment delivery (rather than the treatment per se) has on a disease process. Sham procedures vary in their invasiveness. Examples include saline injections (where research subjects are injected with saline instead of cells), sham cardiac catheterization (where research subjects receive cardiac catheterization but are not injected with cells), and partial burr holes to the cranium (where researchers imitate the experience of receiving brain surgery by drilling a depression in the skull).

  Undue inducement: An offer or reward so attractive that it threatens to impair the ability of prospective research subjects or donors to exercise proper judgment, or it encourages them to agree to procedures for which they are strongly averse.

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