Making eggs out of human skin cells

Scientists have successfully created human egg cells from skin cells and fertilised them to form early-stage embryos—a major breakthrough in reproductive medicine and fertility treatment. The technique, called somatic cell nuclear transfer, involves transferring the nucleus from a skin cell into a donor egg, which is then induced to discard extra chromosomes through a new process called "mitomeiosis". This process enables the cells to fertilise and develop into embryos, offering hope for people who cannot use their own eggs. 

The process, fraught with safety concerns, involves removing the nucleus from a woman's skin cell and inserting it into an egg, or oocyte, from which the nucleus has been removed, scientists detailed in Nature Communications.

The work draws on cloning pioneered in the 1990s at the Roslin Institute in Scotland. A team led by the late Ian Wilmut used somatic cell nuclear transfer to create Dolly, the sheep. The process involved plucking the nucleus from an adult sheep cell and placing it into a sheep egg that had its own nucleus removed. The resulting egg was carried to term in Dolly’s surrogate mother.

Researchers from Oregon Health & Science University took a similar approach by collecting skin cells from women and removing the nucleus. The nucleus contains the 46 chromosomes that carry approximately 20,000 genes. Each skin cell nucleus was placed in a healthy donor egg that had its own nucleus removed.

Eggs contain 23 chromosomes needed for human development, which is half the usual number, and the sperm that fertilises the egg will contribute the other 23 chromosomes. But skin cells and other non-reproductive cells—and any cells generated from them—contain two sets of human chromosomes, a total of 46.

Researchers claim to have solved the problem of the extra set of chromosomes by inducing a process they call mitomeiosis, which mimics natural cell division and causes one set of chromosomes to be discarded, leaving a functional egg.

In one experiment, the researchers fertilised 82 functional modified eggs in test tubes using sperm. Only about 9% of the fertilised eggs developed to the blastocyst stage of embryo development, the point at which embryos consisting of 70 to 200 cells are transferred to the uterus during in-vitro fertilisation treatments. None of the blastocysts were cultured beyond this point.

Most of the eggs created via mitomeiosis did not progress beyond the 4- to 8-cell stage after fertilisation and displayed chromosomal abnormalities. As the success rate in the study was low, the prospect of putting all this to clinical use remains distant. The researchers say that at least a decade of research is needed before safe clinical trials can be conducted.