Why do all animal embryos look the same?

An international team of biologists has shed light on why all vertebrate animals look alike during the phylotypic stage of embryo development.

Led by The University of Western Australia (UWA) in collaboration with the Spanish National Research Council and Radboud University, the team’s discovery has been published in the journal Nature Genetics.

During the phylotypic stage, embryos of birds, fish and even humans start to look the same — before they diverge again and become very different looking animals. The similarity was first described by 19th-century embryologist Karl Ernst von Baer, who accidentally mixed up phylotypic-stage embryos of different vertebrate species and was unable to differentiate between them.

“If you were to put a human embryo next to a fish, a toad and a mouse at that stage, the human embryo would look very much like the others,” said lead author Dr Ozren Bogdanovic, from the ARC Centre of Excellence in Plant Energy Biology at UWA. This is despite the time taken to reach the phylotypic stage varying between species, occurring 1–2 days after fertilisation in fish and toads, 9.5 days after conception in mice and four weeks after conception in humans.

Examining mice from Madrid, fish from Seville and toads from Nijmegen, the team studied the epigenome — an extra layer of information present in cells that is made up of millions of miniscule chemical tags attached to the DNA — which can switch genes on or off to trigger the correct development of early embryonic structures.

The researchers used powerful genome analysis technologies to precisely map the location of these chemical signposts in order to better understand the epigenetic process of development. Dr Bogdanovic explained, “By looking at early-stage embryos of different species, we were able to find the existence of multiple epigenetic switches that appear to be critical for limb formation or brain development.

“The switches change similarly in all these different organisms, even though they’re separated by hundreds of millions of years of evolution.”

The researchers believe vertebrates have a similar type of epigenetic control during this period because that is when the fundamental structure of the body is being set up. According to Professor Ryan Lister, also from the ARC Centre, “Correct establishment of the body plan and organ formation at that early stage is so critical to life that the molecular processes underlying it have remained very similar despite millions of years of divergence between these species.”

The discovery is especially significant, noted Professor Lister, as it opens “a window onto the processes that likely occur during human embryo development” — processes that cannot be studied directly during such development.

Professor Lister added that the research could eventually be used to screen for potential epigenetic aberrances associated with early development.

“Investigating these processes is essential in order to understand the potential repercussions when they don’t take place correctly,” he said.

Image caption: Dr Ozren Bogdanovic and Professor Ryan Lister. Image credit: ARC CoE Plant Energy Biology.