Scientists at Oregon Health & Science University have successfully cloned human embryonic stem cells using a process similar to the one used to clone Dolly the Sheep.
Ultimately, researchers and clinicians hope that stem cells will serve as a cure for serious diseases including multiple sclerosis, Parkinson’s and diabetes. But the field first needs to establish methods to obtain reliable and healthy pools of stem cells, generated using a patient’s own DNA, something this research illustrates is feasible.
By methodically working first with rhesus monkey cells, scientist Shoukhrat Mitalipov and his team identified critical components and characteristics of mammalian egg cells that allowed them to then succeed with human cells, where others have failed.
Using donated skin cells for the genetic material, the Oregon group reports in the journal Cell that they have grown and harvested stem cells from six human embryos bred from donated eggs. Two embryos were created from the DNA of a child with a genetic disorder, and the other four were raised with DNA from fetal skin cells. The embryos were then teased apart and the cell clusters were treated to create individual pools of embryonic stem cells. Those pools were then differentiated into various tissues, demonstrating a key characteristic of stem cells, pluripotency, or the ability to become any tissue.
The cloning process used is called somatic cell nuclear transfer, and it means that the nuclear DNA of any cell–excluding egg or sperm cell–is transferred into a germ cell, or egg, that has previously had its own nucleus removed.
To overcome previous technical hurdles faced by other researchers, Mitalipov’s team focused on the recipient egg, and in particular the condition of the cytoplasm of the egg, after the nucleus was removed. Using the skin cells as the somatic cell–the source of the nucleus for transfer, the group found that successful growth of the de-nucleated egg, or cytoplast, and its division into a cluster of stem cells following somatic cell nuclear transfer was highly dependent upon several factors.
The first was complete removal of the egg’s original spindle–fibers that pull chromosomes apart during cell division. Incomplete removal of these fibers seemed to preclude cell division beyond the eight-cell stage. The second important factor was the activation or stimulatory state of the cytoplast. The authors found caffeine to be helpful in this regard. Third, the researchers emphasized the importance of exceptional oocyte quality. In this study, eight eggs from one donor generated five separate healthy cell clusters, subsequently generating four of the five embryonic stem cell lines reported. Prior egg donation by the same donor was also associated with exceptional outcomes that supported derivation of four additional embryonic stem cell lines in another study.
The researchers discuss in their Cell paper that somatic cell nuclear transfer embryos typically fail to progress beyond the eight-cell stage due to an inability to activate critical embryonic genes from the somatic donor cell nucleus. Though the cause of this early developmental arrest remains unclear, they speculate that most of the previous studies with human egg cells applied somatic cell nuclear transfer protocols developed for nonprimate species.
May 17, 2013