SciBar: Embryo Development and Animal Models

We know how fertilization occurs in humans – the meeting of a sperm and an egg. We are also familiar with embryos from the stage that we are able to see them via ultrasound scans. What happens in-between? It's impossible to examine this development in humans as there are obvious moral and ethical problems with cutting open expectant mothers. Since this all happens inside someone else, we can only infer what goes on and so we use animal models. People have been using animal models since the nineteenth century.

What animal should we use? Chimpanzee? Mouse? Chicken? Frog? We need to consider their relationship to humans e.g insects vs fish vs reptiles vs mammals. Mammals are very closely related to us, but they have internal fertilization and internal development. Reptiles and birds are quite closely related to us but they lay eggs. The fertilization still occurs internally though and so the really early steps of development have already happened by the time the egg is laid.

Amphibians lay their eggs in water and they are fertilized externally. They also have quite large eggs, for example frogspawn. Fish have a lot of eggs so there a lot of chances to experiment but we are getting pretty far away from humans at this point. Chordates are really dissimilar to mammals – they have no backbone – but it is still interesting to look at some of their internal structures. Insects are far too different, with their exoskeletons and their nervous system down the front of their bodies. So, they are not a good model.

Of all these, amphibians are the best compromise. There are three orders of amphibians, caecilians, salamanders and frogs/toads. Of these, we're mainly interested in axolotl (a salamander) and xenopus (a frog). The axolotl can grow back limbs if it loses them. So, if its hand is cut off, it grows a new hand. If its whole arm is cut off, it grows a new one, elbow, arm, hand, everything. Mashed-up axolotl eggs have also been put onto cancer and kill it, so could potentially offer ways of fighting cancer in the future. While this is incredibly interesting, it's not really that useful for looking at embryo development.

Since 1970, xenopus has been the go-to choice for an animal model, it lays more eggs and those eggs develop faster, allowing more investigations to take place. Until the fifties, xenopus was used as a pregnancy test – a woman's urine was injected into the frog and if it laid eggs the next day then the woman was pregnant. So, the xenopus was absolutely everywhere and you have an easy way of forcing it to lay eggs when you're ready to start your experiments.

The xenopus eggs are a two-tone colour – a so-called animal hemisphere and a vegetable hemisphere. In the egg, the single nucleus, formed at fertilization, separates into two. This happens first in the animal hemisphere. It then splits into four, into eight, etc. At this point, it is still a ball of cells.

Then gastrulation happens – the ball starts to fold in on itself to form a tube. The first hole that is formed is your arse. All vertebrates are deuterostomes, from the Greek for "mouth second". This is because the mouth is formed second. In insects the mouth is formed first. Does this same thing happen in humans? Is development conserved? In the nineteenth century, Ernst Haeckel released illustrations showing the similarities of development in a number of species including dogs, humans and turtles. While these drawings are now considered to be inaccurate, the consensus is that development is indeed conserved and similar across species.

What about germ cells? These are the cells that give rise to the gametes of an organism. When you're pregnant, you're not just making your children, you're laying down the cells that will become your grandchildren too. As well as the axolotl and the xenopus, the zebrafish is used for research here, as it has eggs that are see-through.

It is germ plasm that determines cell fate – it only ends up in a small number of cells. The germ plasm is already in the egg when it meets the sperm. The germ plasm process seems to happen in a lot of species and so it was thought that germ plasm was conserved too. However, a surprise came in the nineties, when it was discovered that there was no germ plasm at all in mice. In mice, the germ cells seem to just appear out of nowhere – they see, to be induced by the embryo itself rather than found in the egg.

So, does this mean that mammals are unique? In the late nineties, people went back to amphibian germ cells and started looking at axolotl again. These didn't have germ plasm either, and the germs cells were formed by similar signals to those seen in the mice. Further research showed that you can take a cell and turn it into a germ cell just by adding two proteins.

Clearly mammals aren't unique then and in fact, there are a lot of species that do not have germ plasm. While chickens do, lizards don't, and while zebrafish also have it, sturgeons do not. Hence germ plasm is not conserved, it has evolved independently in a number of species. Having germ plasm or not has a huge impact on early development as it's related to changes in the genome.

Everything that we thought we knew is actually only true for some species and as a result, textbooks are having to be rewritten. In the last two years, we have worked out how to turn embryonic stem cells in humans into germ cells. The process is actually more similar to what happens in axolotl rather than what happens in mice. We have also discovered that species with germ plasm seem to evolve much quicker than those without. However, these differences in germ plasm in different species does serve to highlight the limitations in using animal models.

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