Paris (AFP) - We all grow up to accept certain unassailable facts: water is wet, the Earth is round, and to produce a baby you need an egg and sperm.
As it turns out, the last of these may turn out not to be true.
On Tuesday, scientists announced they had produced baby mice by fusing sperm and a type of cell that is not an egg.
The pups that survived this alchemy were healthy, with normal lifespans, and able to procreate the traditional way, researchers from Britain and Germany reported in the journal Nature Communications.
"It had been thought that only an egg cell was capable of reprogramming sperm to allow embryonic development to take place," said the report's senior author Tony Perry of the University of Bath.
"Our work challenges the dogma, held since early embryologists first observed mammalian eggs around 1827 and observed fertilisation 50 years later, that only an egg cell fertilised with a sperm cell can result in a live mammalian birth."
There are two types of cell: "meiotic" reproductive cells (eggs and sperm), and "mitotic" cells which include most of the tissues and organs in our bodies.
Mammal reproduction requires a sperm and egg to fuse, creating an embryo.
But instead of using a meiotic egg cell to produce their mouse pups, the researchers used a type of mitotic cell called a parthenogenote.
These are very early-stage, single-celled embryos that form without fertilisation -- in this case by chemically activating a mouse egg.
Just before the parthenogenotes divided into two cells, they were injected with sperm nuclei to fertilise them.
The resulting pups' survival rate was a quarter that of other mice.
- Reproduction revolution? -
Though these are early days, the feat suggested that other types of mitotic cell, such as skin cells, may one day be used to create offspring.
This could open up the possibility for gay men, older women or infertile couples to have children with both parents' DNA.
"Imagine if any mitotic cell could reprogram a sperm in the same way. Then there would be no need for eggs," Perry told AFP.
"This could revolutionise reproduction."
For now, though, the team still need eggs to create their parthenogenotes.
Parthenogenesis is a technique employed by certain plants and animals -- some lizards, snakes and worms for example -- to fertilise eggs and create embryos without the need for sperm.
It does not happen naturally in mammals, and any parthenogenote created accidentally would not survive.
One day, said Perry, it might be possible to print parthenogenotes instead of having to tease them from egg cells, meaning "the egg feature of this would be history".
Outside experts not involved in the study welcomed the world-first achievement, but stressed a lot still has to happen before the method can be used in humans.
Robin Lovell-Badge of The Francis Crick Institute, a biomedical research unit in London, described the result as a "technical tour de force".
"I am sure it will tell us something important about reprogramming at these early steps of development that are relevant to both fertilisation and SCNT (cloning)," he said.
"It doesn't yet tell us how, but the paper gives a number of clear pointers."
Marie-Helene Verlhac, a scientist at France's CNRS research institute, underlined the finding that one does not need an egg to reprogram sperm to kickstart embryo formation.
"So we can imagine also using parthenogenotes" in human fertility treatment one day, she said -- reducing reliance on eggs which are notoriously difficult to harvest.