Embryos start as a single cell and have to go from there to a complex array of multiple tissues. For organisms such as insects or frogs, the process is fairly easy to study, as development takes place in an egg that is laid in the environment shortly after fertilization. But for mammals, where all development takes place within the reproductive tract, understanding the earliest stages of development is a serious challenge. Performing any experiments on a developing embryo is extremely difficult and at some stages actually impossible.
This week, however, progress has been made with both human and mouse embryos. On the human side, researchers have used induced stem cells to create embryo-like bodies that take the first important step in development, catching up to where research on mice has been for decades. On the mouse side, however, a research team obtained mouse embryos to go outside the uterus for almost a week. While this opens up a world of experimentation that has not been possible before, it does mean that the requirements for getting it to work are unlikely to be widely accepted.
To grow mice, you need rats
The mouse work was technically more interesting, so we’ll get to that first. One of the most important steps in the development of vertebrates is called gastrulation. The process takes a few cells that were set aside during the early embryo, and transforms them into three critical layers that form the embryo: the skin and nerves, the lining of the intestines, and all the others.
Gastrulation occurs between six and seven days after fertilization, and very important developmental events occur shortly thereafter: the formation of nerve cells and their organization, the development of the ordered structures that form the vertebrae, and more. Given the size of the embryo at the moment and its location in the uterus, the process of gastrulation is essentially invisible.
A large team in Israel has decided to figure out how to change that. They first started with embryos that had already passed the critical stages of gastrulation, and determined how to let them survive. It was … not easy. To begin with, their embryos had to be incubated in a bottle that was constantly rotated to ensure that all the nutrients and oxygen around the embryo were thoroughly mixed, instead of the energy needs of the embryo a local ‘dead zone’ could create around there.
The oxygen levels had to be controlled by means of a personal gas supply system which increased the pressure over time to force more oxygen into solution. And fresh glucose also had to be administered regularly in the liquid medium.
About that liquid medium. About a quarter of it was something you could buy from a standard biotech supply catalog. The rest was significantly harder to buy. Half of it is serum obtained from rat blood. And a quarter is serum obtained from human umbilical cord blood. Neither of these two are particularly easy to obtain. They have tested, and you really need the human blood; rat blood alone was not nearly as good. (How often can I write such a sentence?)
It was enough anyway to get the embryos through four days of development. This took them from three layers of unspecialized cells to a site where the spinal cord began to form and the limbs on the side of the embryo began to bud. It contains, in terms of development, a whole host of important events that we would very much like to study.
But at this point, the growing circulatory system had to be integrated with the placenta, to ensure that the entire embryo was well supplied with nutrients and oxygen. The embryos died in a way that indicates that an oxygen supply is probably lacking.
Earlier …
Although it is an achievement, it all happens after gastrulation has taken place. Thus, the researchers backed up a bit further and isolated embryos between four and five days after fertilization. The same medium worked, but here the embryos do not have to be in a rotating bottle to survive. The two incubations can be combined, taking in fact the embryos during an entire week of development outside the uterus.
In addition, the team showed that during the period in culture, they were able to perform a variety of manipulations on the embryos. This included inserting DNA into their cells (using a virus or electric currents) or adding stem cells to see how it develops. So, for anyone who is willing to get enough rats and nail blood to make it all work, there is a lot of developmental biology research that can now be done on mouse embryos.
The one thing that is not automatically accessible by this work is the earliest developmental process, in which a cavity opens up in the uniform ball of cells formed by the first cell divisions of the fertilized egg. This creates the first somewhat specialized populations of cells in the embryo (both the outside and a patch of cells in the cavity). The resulting structure is called a blastocyst.
For mice, we have been able to take a fertilized egg for years and grow it into a blastosis in culture. But this was not done with human cells. And to some extent still not. Instead, two different laboratories started with stem cells, either embryonic stem cells or stem cells caused by adult tissues. Unlike the mouse work, the embryo could get so far with the ingredients in the media in which the cells were cultured.
It opens the earliest stages of human development to study. Although the formation of the blastocyst is interesting, much more goes on in later stages of development. And here, ethical issues can probably be limited to how far we are willing to take human tissue in culture.
The interesting thing here is that we can already make mouse embryos develop into blastocysts, and now we can take blastocysts well on their way through development. So it seems likely that we can probably connect the two processes with a bit of work. That would be enough to get from conception to about two-thirds of the way to birth. It’s pretty impressive.
But at this stage the embryo becomes very three-dimensional, and around all the cells that are supplied with oxygen and nutrients really need a functional blood supply, which is plugged into a source for all the embryos. And it is not clear how we can replace the placenta, which offers a very extensive and specialized interaction between the fetal and maternal bloodstream. In practical terms, however, these results mean that a whole range of experiments is now possible in mice – provided you are willing to bleed enough rats.
Nature, 2021. DOI: 10.1038 / s41586-021-03416-3, 10.1038 / s41586-021-03372-y, 10.1038 / s41586-021-03356-y (About DOIs).