Nov 30, 2010 10:32 GMT  ·  By
Any embryonic short-tailed opossum finishes its development outside the womb,
   Any embryonic short-tailed opossum finishes its development outside the womb,

The tiny, pink marsupial embryo outside its mother's womb has yet another curiosity: it jumps ahead in its development, and mixes the 'natural' order of the growth processes of all vertebrate animals.

Duke University researchers discovered that in the marsupial embryo, the forelimbs develop before the brain or most other organs do.

Anna Keyte, a postdoctoral biology researcher at Duke who did this work as part of her doctoral dissertation, said that “the limbs are at a different place in the entire timeline; they begin development before almost any other structure in the body.”

The underdeveloped embryo needs to be strong enough to drag itself from the birth canal to the teat, across the mothers belly, so a well-developed pair of forelimbs is more than welcome, and this is why its developmental program starts building the forelimbs much sooner than in any other animal.

At first, biologists believed that the development of the limbs was triggered by another organ system coming on line first, but then they found out that such triggers do not exist at all.

Biology professor Kathleen Smith said that the “development is probably more flexible than we might have known otherwise.”

Keyte explained that many of the marsupial embryo's genes “were turned on earlier than you'd see in a mouse or a chick.”

The team also found that the forelimbs received cells from a much wider part of the developing embryo than other vertebrates normally do.

Another amazing thing the researchers did not expect to find, was that the genetic program that creates the hind limbs also started earlier.

The place of each of the four limbs is genetically determined, but it is rather amazing that the embryo gives more of its early cells to the forelimbs, so that they would grow much faster than the hind limbs.

When the embryo gets out out the birth canal, it has strong forearms with formed bones and well developed muscles, while the hind limbs are small and cartilaginous.

Besides its developed forelimbs, a marsupial embryo is blind, hairless and its brain is incomplete, in other words it is terribly underdeveloped to be living somewhere else than in its mother's womb, but this system seems to be working just fine for marsupials.

For this research, the scientists focused on the gray short-tailed opossums (Monodelphis domestica) native to Brazil and Bolivia, but Smith said that things should be the same for any marsupial.

She added that “there are probably 50 explanations for why marsupials develop outside the womb, and none of them are very good.”

The only thing that seems clear is that with the external pregnancy, the female has more control over her reproduction.