A useful guide explaining key terms used in embryo research. Provides definitions of the main terms used, including biological definitions and therapy explanations.
Table of contents
Human embryo is defined as ‘a discrete entity that has arisen from either:
- the first mitotic division when fertilisation of a human oocyte by a human sperm is complete; or
- any other process that initiates organised development of a biological entity with a human nuclear genome or altered human nuclear genome that has the potential to develop up to, or beyond, the stage at which the primitive streak appears; and has not yet reached 8 weeks of development since the first mitotic division.
(Research Involving Human Embryos Act 2002 (RIHE Act) section 7)
‘Human Embryo – A biological definition’
NHMRC Council asked the NHMRC Licensing Committee to develop a discussion paper that defined a ‘human embryo’ from a purely biological standpoint. The paper was presented to Council in December 2005 and then released as a discussion paper so that a wider audience could comment on the biological definition of 'human embryo'.
The definition proposed in the paper was subsequently adopted by the Australian Parliament in the Prohibition of Human Cloning for Reproduction and Regulation of Human Embryo Research Amendment Act 2006 to replace the previously used definition.
An edited version of the discussion paper was published in Human Reproduction.
Human Embryo: a biological definition. Human Reproduction (2007) 22:905-911, (Findlay, JK, Gear, ML, Illingworth, PJ, Junk, SM, Kay G, Mackerras AH, Pope A, Rothenfluh, HS, Wilton L.)
Excess assisted reproductive technology (ART) embryo
An excess ART embryo is a human embryo that:
(a) was created by ART, for use in the ART treatment of a woman; and
(b) is excess to the needs of:
(i) the woman for whom it was created; and
(ii) her spouse (if any) at the time that the embryo was created.
For the purposes of paragraph (b), a human embryo is excess to the needs of the persons mentioned in that paragraph at a particular time if:
(a) each such person has given written authority for the use of the embryo for a purpose other than a purpose relating to the ART treatment of the woman concerned, and the authority is in force at the time; or
(b) each such person has determined in writing that the embryo is excess to their needs, and the determination is in force at that time.
(RIHE Act section 9)
To assist ART clinics and researchers to determine how their activities are affected by Commonwealth legislation, the NHMRC Licensing Committee has provided the following explanation in relation to living embryos:
Subsection 7 (2) of the RIHE Act, states that a reference to an embryo (including a human embryo) is a reference to a living embryo.
An embryo is considered to be a living embryo unless:
- when maintained in suitable culture conditions, the embryo has not undergone cell division between successive observations not less than 24 hours apart,
- the embryo has been allowed to succumb by standing at room temperature for a period of not less than 24 hours.
Once an embryo has more than 12 cells it is not possible to determine whether any individual cell has divided within a 24-hour period. Therefore, such embryos can be considered to have succumbed only after a 24-hour period at room temperature.
Disclaimer: this information is for guidance only. It is not intended to be taken as legal advice. If you are in any doubt about provisions of, or consequences arising from the operation of, the legislation or the issuing of licences, you should seek your own independent legal advice.
Mitochondrial Replacement Therapy
Mitochondrial replacement therapy (MRT) is also known as mitochondrial transfer or mitochondrial donation. It is a form of ART that may help a woman whose mitochondrial disease is caused by mutations in her mitochondrial DNA to avoid passing that mutation to her children. It involves taking the nuclear DNA from the intended mother and father and placing it in a donated egg which has had its own nuclear DNA removed. The reconstructed embryo has nuclear DNA from the mother and father and healthy mitochondrial DNA from the egg donor. It is transferred to the uterus of the intended mother to attempt to achieve pregnancy.
MRT is legal in the United Kingdom following legislation changes. It cannot be used in Australia for reproduction but some research may be legal (Prohibition of Human Cloning for Reproduction Act 2002 (PHCR Act), sections 13, 15 and 20; RIHE Act, section 20). If you require more information see the information for applicants page.
More information about mitochondrial disease is available from the Mito Foundation Australian Mitochondrial Disease Foundation and information about mitochondrial replacement therapy is available from the Human Fertility and Embryology Authority of the United Kingdom
Gene editing in human embryos
The use of CRISPR/Cas-9 and related technologies may offer the ability to make precise changes to genes in the one- or two-cell embryo. This gives the possibility of correcting specific genetic mutations causing disease in the people born from the embryos. Some research in this area may be legal in Australia. However, using edited embryos for reproduction is prohibited under current legislation (Sections 15 and 20, PHCR Act)
- Stem cells are ‘unspecialised’ cells that have the potential to develop into ‘specialised’ cell types in the body (for example blood cells, muscle cells or nerve cells). This can be either for growth and development, or for replenishment and repair.
- Stem cells occur at all stages of human development, from embryo to adult but their versatility and numbers tend to decrease with age.
- Given the right conditions in the body or the laboratory, stem cells (unlike muscle cells, nerve cells and or blood cells) can replicate themselves many times over.
- When a stem cell replicates, the resulting cells can either remain as stem cells or can become specialised cells.
Embryonic stem cells
- Embryonic stem cells are derived from the early embryo. They have the potential to develop into all cell types in the body.
- In Australia, human embryonic stem cells can be derived from human embryos that are excess to the needs of patients undergoing ART and have been donated to a particular research project by the people for whom they were created. They are not derived from eggs fertilised in a woman’s body.
- Embryonic stem cells can also be derived from embryos created by somatic cell nuclear transfer (see 'Cloning' section below).
- "Cloning" is an umbrella term traditionally used by scientists to describe different processes for duplicating biological material.
- The possibility of human cloning arose when Scottish scientists at the Roslin Institute created 'Dolly' the sheep (Nature 385, 810-13, 1997)
- Reproductive cloning is banned in Australia under the PHCR Act. Reproductive human cloning aims to recreate an existing living or dead person by creating a cloned embryo and placing it in a woman’s uterus to attempt to achieve pregnancy.
- Therapeutic cloning, also known as somatic cell nuclear transfer (SCNT) is permitted in Australia under a licence issued by the NHMRC Embryo Research Licensing Committee. The aim is to create a cloned embryo and then to derive an embryonic stem cell line that could be used for research.
- SCNT was the technique used to create the first cloned mammal, 'Dolly' the sheep. SCNT involves isolating a somatic cell from an adult body, often a skin cell, and transferring the nucleus from that cell to an egg from which the nucleus has been removed. This new cell is then stimulated to begin embryonic growth. The embryo was transferred to a sheep and the lamb that resulted was genetically identical the sheep whose skin cell was used.
- The first human embryonic stem cell lines derived from SCNT research were announced in May 2013 by Mitalipov and coworkers from Oregon Health and Science University.
- Two other groups reported successful stem cell line derivations in 2014 – Yamada, M et al Human oocytes reprogram adult somatic nuclei of a type 1 diabetic to diploid pluripotent stem cells. Nature (2014) 510: 533-6; Chung YG et al Human somatic cell nuclear transfer using adult cells Cell Stem Cell (2014) 14: 777-780.
- Parthenogenesis is a process where an unfertilised egg is induced to divide and develop as though it had been fertilised.
- Parthenogenetic embryos cannot be used for reproduction as they will not develop past an early stage of pregnancy. In addition their use for pregnancy is prohibited (PHCR Act, section 20).
- Creation of embryos by parthenogenesis and their use for research is permitted in Australia under a licence issued by the NHMRC Licensing Committee.
- Embryonic stem cell lines derived from a parthenogenetic embryo are genetically matched to the woman who provided the egg.