June 27, 2013
Donor Mouse Cloned Using Just A Drop Of Blood From The Tail
Alan McStravick for redOrbit.com - Your Universe Online
In one of the more popular science fiction novels of the 1990s and its Oscar-winning silver screen adaptation, the farfetched premise hinged on a newly discovered technique that allowed scientists to extract dinosaur DNA from blood thirsty mosquitoes trapped and fossilized in amber. But we all know the impossibility of cloning an organism from just a blood sample, right? Japanese scientists are urging us to think again.
Scientific research relies on animal subjects that can be modified at the genetic level. Mice and rats are considered excellent models for studying human diseases from obesity to anxiety disorder and cancer. In order to arrive at a well-suited strain, scientists often spend years genetically manipulating the animal in the hopes of arriving at a particular human disorder.
As there is much time and effort put towards the creation of each strain, a disappointment (if not a danger) arises when one of the mutated animals is unable to reproduce due to unforeseen infertility. In vitro fertilization was a propagative option if the germ cells of the male were healthy and there was an eligible female.
Another option relies on somatic-cell nuclear transfer (SCNT) which creates a cloned animal by replacing an oocyte's nucleus with an adult somatic cell. This was the method employed in the creation of Dolly, the famous cloned sheep.
In the time since the world met Dolly, SCNT techniques have undergone vast improvements. In fact, earlier this year researchers at the RIKEN Center for Developmental Biology in Kobe, Japan developed a technique for avoiding the diminishing returns often experienced as a result of recloning the same cell. They have seen their cloning success rate increase from three percent to ten percent for a mutation's first generation. For successive generations, the team has increased viability rates to 14 percent.
Of course, somatic cells typically used for this process are critical, and their use depends largely on their efficiency in producing live clones. Cumulus cells found around oocytes in the ovarian follicle are generally the preferred cell type for cloning efforts. However, researchers Satoshi Kamimura, Atsuo Ogura and colleagues at the RIKEN BioResource Center in Tsukuba, Japan wanted to explore the efficacy of white blood cells, or leukocytes, as donor cells. Leukocytes would be much easier to extract. In the case of mice, for example, one easily accessible site would be the tail. Scientists would be able to perform repeated sampling from the site while posing little to no risk to the donor mouse.
Of the five different types of white blood cells, the team found that lymphocytes were the lowest performing type. Lymphocytes are only able to yield offspring from 1.7 percent of the embryos. Granulocytes and monocytes, the physically largest of the white blood cell types, had a higher success rate, yielding viable embryos about 2.1 percent of the time. The cumulus cell, which is the most preferred cell type, has a 2.7 percent yield.
As the team notes, the granulocytes' performance was poorer than expected due to a much higher rate of fragmentation in early embryos. This fragmentation was a full twofold higher than cloning with lymphocytes. Additionally, the observed fragmentation was fivefold higher than that which occurs as a result of cumulus cell cloning. At this time, the team does not yet understand the precise causes of the fragmentation. They have, however, mentioned that their future studies will focus not on the cause but rather on improving the future performance of granulocyte donor cells.
The original oocyte cumulus cell-based approach is still the most effective method of producing viable cloned offspring. What the researchers have demonstrated for the first time is that the effective cloning of mice can be achieved using the nuclei of peripheral blood cells. These cells are immediately available for use following collection. This method inevitably becomes very important in helping to preserve genetically engineered strains that cannot be preserved via the other more widely used reproductive techniques.