Originally created 01/19/98

Cross-Species combinations represent surprising twist



It sounds like a recipe for a witch's brew: Scientists in Wisconsin have mixed ear of pig and egg of cow -- also ear of rat and egg of cow and a variety of other cross-species combinations -- to clone living embryos of rats, pigs, sheep and monkeys. The series of experiments, being described Monday at a meeting in Boston, represents a new and surprising twist on the cloning techniques that led to the birth of Dolly the sheep.

The work suggests that cow eggs, available by the thousands from slaughterhouses, contain all the ingredients and molecular machinery needed to grow embryos of many species. All that's required is genetic material from the species to be cloned -- taken from, say, a bit of an animal's ear -- mixed with a cow egg that's had its own genes removed.

Natural chemicals in the cow egg can apparently activate the foreign species' genes, and soon an embryo of that species is growing in a laboratory dish, ready to be transferred to a surrogate mother animal.

So far, the researchers have grown apparently healthy sheep, monkey, rat, pig and cow embryos from cow eggs. But none of the embryos have grown into mature animals, and it's possible that the technique will never lead to live births.

Already, however, the work is shedding light on the cellular mechanisms that coordinate the earliest stages of embryo development. And eventually, researchers said, the technique may allow conservationists to save threatened species with just a scraping of cells from a rare animal's skin and a batch of cow eggs.

"It's potentially very useful for endangered species," said Tanja Dominko, who led the work with Neal L. First, a pioneer cattle cloner, at the University of Wisconsin in Madison. "If you want to make a rhino, you just take an ear punch from the rhino," which contains hundreds of skin cells, and fuse each of those cells to its own cow egg.

"I'm intrigued by this and by where this may go," said Carl Pinkert, a developmental biologist at the University of Alabama in Birmingham. Like others, Pinkert expressed some surprise that the developmental machinery inside cow eggs is similar enough to those in other species to drive embryo development in those species. "It's preliminary," he said, "but exciting."

Researchers have known for years that chemicals inside fertilized eggs coordinate the early stages of embryo development -- the splitting of that one cell into two, and two cells into four -- by turning various genes on and off in the proper sequence. Once an embryo gets big enough, it produces new chemicals that coordinate its further development.

Little is known about the chemicals in eggs, called "maternal factors," that flip those earliest genetic switches. And the main thing that researchers thought they knew was proved false last year after researchers in Scotland announced they had cloned an adult sheep.

Scientists had thought that adult cells are irreversibly adult, and that even if they were drenched with maternal factors from an egg they could not be "reprogrammed" to become embryo cells again. Dolly's birth proved that under certain laboratory conditions, an adult cell exposed to an egg's maternal factors can be transformed into an embryo cell.

Researchers had also presumed that maternal factors varied significantly from species to species. But the new work suggests that for many mammalian species, maternal factors are largely interchangeable. That is, maternal factors inside cow eggs apparently can reprogram the genetic machinery of cells from a variety of species.

Given the easy availability of cow eggs and the significant amount that is already known about them from previous research, it appears they will become a valuable tool for studying embryo development and for facilitating the cloning of other species, said First, who, with Dominko and co-workers, is presenting their results Monday at a meeting of the International Embryo Transfer Society in Boston.

Scientists said they are intrigued by the idea of using cow eggs to help clone endangered species. Previous speculation about cloning such species had been tempered by the assumption that scientists would have to use egg cells from the species they wanted to clone. It is difficult to get egg cells without harming an animal. But if cow eggs could be used instead, a single endangered animal could provide enough skin cells to produce an entire clutch, herd, flock or swarm of individuals.

The plan is not perfect. Because clones are genetically identical, a population of them could suffer from inbreeding. And each embryo would have to be implanted into a surrogate mother of the same species, of which there may be few.

But the fact that every Wisconsin embryo so far transferred to a surrogate mother has been lost to miscarriage suggests that the embryos may not be as normal as they appear. First said he suspects that some bovine genes relating to metabolism are left behind in the eggs when the other cow genes are removed, and these genes may be clashing with the embryo's own energy genes. "Energy metabolism is very different in a ruminant than it is in a pig," he said.

The problem might be solved, he said, by doing a better job of removing every cow gene while leaving the maternal factors behind.

Colin Stewart, an embryologist working at the National Cancer Institute's research facility in Frederick, Md., said he had doubts about the technique's ability to generate live animals. But he said the work could help answer the basic question of how cloning works. "It could be a useful approach to trying to understand the factors that do the reprogramming," he said.