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From cells to embryos: How does the mysterious process of SCNT work?
In genetics and developmental biology, somatic cell nuclear transfer (SCNT) is a technique for creating viable embryos in the laboratory from somatic cells and egg cells. The technique first requires the removal of an egg cell, which has been enucleated, and then the implantation of a donor's nucleus. SCNT is used not only for therapeutic cloning but also for reproductive cloning. In 1996, a sheep named Dolly became famous for being the first mammal to be successfully reproductively cloned. In January 2018, a group of scientists in Shanghai announced the successful cloning of two female crab-eating macaques named Zhong Zhong and Hua Hua, which once again attracted people's attention to SCNT technology.
“Therapeutic cloning” refers to the potential application of SCNT in regenerative medicine, where the technique is seen as the answer to problems with the use of embryonic stem cells (ESCs).
The basic principle and process of SCNT
The process of somatic cell nuclear transfer requires two different types of cells. The first is the female reproductive cell, the egg cell (ovum/follicle). In human SCNT experiments, these eggs are usually provided free of charge and obtained through ovarian stimulation techniques. The second type is somatic cells, which refer to various cells in the human body, such as skin cells, fat cells, and liver cells.
During the SCNT process, the genetic material of the donor egg cell is removed, leaving behind a deprogrammed egg cell. These enucleated eggs are then fused with somatic cells, and the resulting embryo begins to divide, eventually forming an individual with a single source of complete genetic information. During this process, cytoplasmic factors in the egg reprogram the inserted somatic cell, ultimately causing the egg to begin dividing and form an early embryo.
The key to SCNT is how to effectively activate the genes in the nucleus continuously, which is crucial for the normal development of the embryo.
Applications and Challenges of SCNT
Somatic cell nuclear transfer technology has received widespread attention in stem cell research. Its main purpose is to obtain pluripotent cells from cloned embryos. These cells are completely genetically matched to the individuals from which they originated, and therefore have the potential to be used for the treatment of specific patients.
However, the SCNT process faces many challenges, including low efficiency and high mortality. In 1996, Dolly's birth was actually based on 277 eggs, and only 29 viable embryos were successfully formed, which reflects the very low success rate of SCNT. In addition, the remodeling and development process of the embryo involves complex biochemical reactions, about which there are still many unknowns.
Many scientists are beginning to explore new approaches, such as induced pluripotent stem cells (iPSCs), which may reduce reliance on SCNT.
Moral and Ethical Issues
SCNT technology has caused many ethical controversies. First, many people worry that SCNT will lead to the realization of human cloning, which is closely related to the feasibility of reproductive cloning. Secondly, SCNT requires a large number of human oocytes, and the acquisition of these oocytes often needs to be done through donations from women, which may bring certain health risks. In this context, how should we balance ethics and scientific progress?
Although SCNT offers many potential biomedical applications, the ethical considerations involved still need to be taken seriously. In the future, will humans be able to use this technology to conduct more research without violating ethics?