All mixed un

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IN ancient Greek mythology, the Chimaera was an incongruous combination of the front parts of a lion. the mid-parts of a goat and the hind parts of a dragon. Modern chimaeras tend to be a more random mixture of different types of mouse or as recently announced, of sheep and goats. And unlike the Greek Chimaera. thev actuallv exist Most living things start out as a mixing of material from two parents via the sperm and ovum. Ihe parents are usually of the same species, but may be of different but closely related species, in which case the offspring is called a "hybrid".

Chimaeras are a different sort at mixture. Different embryos are combined so that they develop as one. The resulting creature has the normal number of organs but the cells that make it up will have originated from different sets of parents. The chimaera has at least two sets of parents.

Let's now see how the recenty unveiled geep or shoats were created.

The researchers started with embryos at the 4-cell or 8-cell stage. This is when the original ertilised egg has divided two or hree times. At this stage, each 'ell could become any part of the lew organism or could even develop into a complete new organism if separated from the others (identical twins form in his way).

They gently disrupted 3 sheep embryos to separate their cells. Then they added a goat embryo ind allowed the sheep cells to oin up again around the goat Class embryo in the womb of a ewe. Normally. a sheep's body would reject goat cells as being foreign but these ones were surrounded by sheep cells. The placenta would have been made entirely of sheep embryo celis. so the mother's body would have nourished the embryo as usual.

The problem of rejection has held up research into chimaeras of different species. Embryo cells of different species get on quite well as they have not yet devel oped immune defences but the "mother's" body will usually reject foreign embryos causing them to spontaneously abort.

Previous successful chimaeras have been of the same species (mice) or of closely related species (also mice) capable of hybridisation.

Goats and sheep do not pro duce hybrids. All embryos spon taneously abort. The current breakthrough seems to depend on the placenta being made of cells of the same species as the "mother" whatever the rest of the embryo is made of.

in the recently reported experiments, sheep gave birth to eleven lambs, one kid and seven lamb-kids, while goats gave birth to three kids, three lambs (of which two were still-born) and one kid-lamb.

In other words, not all the chimaera embryos were born as chimaeras. Most interesting were the cases where a lamb was born to a goat and vice versa, since the placenta cells must have been completely different from the embryo cells.

The chimaeras usually looked like sheep or goats but would have patches of goat hair mixed randomly with sheep's wool. A sheep-like head might bear goats horns or a goat's head might bear ram's horns.

If the outer layer of the embryo was sheep cells, the skin. brain and nerves would be sheepish. while the guts and other internal organs might be goatish. ira: implications this might have i See the animal I don't *:1T:

FIC 10* slogan if it bit them. make way for those who will'?

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scrap the bosses!