Scientists have made a breakthrough that may make it possible to create human organs on demand in the future.
A team of researchers has achieved what one of its leaders described as feeling "a bit like sci-fi" — using CRISPR gene-editing technology to excise the Y chromosome from male mouse embryos, effectively converting them into females.
The resulting animals were female clones genetically identical to their male counterparts, bar the absent Y chromosome. Takashi Ishiuchi, a reproductive biologist at the University of Yamanashi, co-led the research alongside Shogo Matoba from the Riken BioResource Research Center.
The pair believe their technique could prove particularly valuable for conservation programmes, especially in scenarios where an endangered species has been reduced to just a handful of surviving individuals.
Yet this advance sits within a much broader, and sometimes unsettling, landscape of cloning science that spans everything from replicating beloved pets to ambitious schemes aimed at resurrecting species that vanished from the Earth long ago.
Any discussion of cloning inevitably begins with Dolly the sheep, the celebrated animal born in 1996 who became the first mammal ever cloned from an adult cell.
The process involved extracting the nucleus, which contains DNA, from a mammary cell of one sheep and inserting it into an egg cell from which the original nucleus had been removed. That engineered embryo was then implanted into a surrogate, which carried it to term and delivered an animal that was a genetic copy of the cell donor.
The researchers who produced Dolly were motivated by the prospect of genetically altering farm animals. Selective breeding has, of course, served this purpose for millennia, but cloning offered something fundamentally different: the ability to produce exact genetic duplicates of animals possessing particularly sought-after characteristics.
The technology has also found a commercial market in pet replication. Celebrities including Barbra Streisand and Tom Brady are among those who have had dead animals cloned, a service that typically costs tens of thousands of dollars.
The procedure involves taking cells from the original pet and using them to produce a living genetic duplicate. However, it also demands egg cells harvested from a donor animal and a surrogate to carry the pregnancy to term.
Not everyone views this favourably. Given no medical or environmental justification exists for such procedures, the practice has drawn criticism.
Bioethicist Jessica Pierce has characterised the use of other dogs in the cloning process as "the exploitation of the canine underclass".
The ethical calculus shifts somewhat when cloning is deployed for conservation purposes, where proponents argue the environmental benefits provide a more compelling rationale.
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For years, scientists have been banking animal tissues at ultra-low temperatures in so-called frozen zoos.
The San Diego Zoo's facility alone holds cellular material from more than 1,300 species, with some samples collected decades ago.
These preserved collections have already yielded tangible results, enabling researchers to clone animals teetering on the brink of extinction, among them the black-footed ferret and Przewalski's horse.
Efforts to revive species that have already disappeared have proved far more difficult. In 2009, a Spanish research team attempted to resurrect the Pyrenean ibex, an extinct wild goat, using skin cells that had been cryopreserved ten years earlier.
The researchers employed domestic goat egg cells to generate 439 embryos, yet just a single female kid was born alive. She survived only minutes before dying from a lung defect – making the Pyrenean ibex the only known animal to have gone extinct twice.
The biotechnology firm Colossal Biosciences has set its sights on even more ambitious resurrections, aiming to use ancient genetic material to bring back long-vanished creatures such as the woolly mammoth and the thylacine. To date, however, the company's work has primarily centred on editing the genomes of living species.
Human cloning, meanwhile, remains technically feasible but — as far as anyone knows — has never been carried out. That has not stopped some from entertaining the notion.
One biotech startup founder proposed creating human clones engineered without brains, intended solely as repositories of transplantable organs.
The past three decades have seen cloning science advance at a remarkable pace, from Dolly to CRISPR-engineered sex reversal in mice. What the next chapter holds is a question that inspires both anticipation and unease in equal measure.
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