Humanity Saw Its Most Human, Mouse-Human Hybrid Yet

A new mouse-human embryo has been created  and it contained up to 4% human cells. This is the highest human cell content out of any chimera. A chimera by the way is an organism that is composed out of two different DNA sets. Even more surprising is the fact that the human cells adapted to the development speed of the mouse embryo, mimicking their animal counterparts. It was something that the researchers did not foresee at all as a possibility.

While this may seem like the start of a horror movie, human-animal chimaeras help researchers understand how our bodies grow and age, and how certain diseases damage our cells. This research might also one day lead to the efficient use of animal biological scaffolding materials to grow human organs.

In chimeras, the human stem cells and the animal stem cells have to be in the same stage of development in order to communicate and integrate with each other. The magic of stem cells is that they can develop into any other type of cell in the body, making them very useful for creating replacement organs or encouraging their growth.

Human stem cells naturally behave very differently from the stem cells of mice, and this has made any past attempt of combining the two impossible. However, throughout the years, it became clearer and clearer that the problem was not a barrier between species, but rather an issue of maturity. The human stem cells used in past experiments were at a later development stage which is called the “primed” state. This state occurs after the embryo cells have been implanted in the wall of the uterus. The mouse stem cells however were in a “naive” state, which occurs when the embryo is still floating in the fallopian tubes. The main issue here was that researchers could not convert primed human stem cells back to their naive state.

The breakthrough came in the form of a process called the embryonic diapause, which is a process that occurs in many mammals, from mice to bears. This process stops the development of an embryo and returns it to the fallopian tubes of the mother in the case of food shortages, water shortages or very harsh weather. The embryo can then remain in this naive state for several months up to a year, until the environmental conditions improve.

The main advantage of this experiment was the speed at which the human stem cells developed. It takes around 56 days for human red blood cells to develop in a human embryo, but only 17 in a mouse. Similar effects were observed in eye cells. When combined with pig embryos, these human stem cells could grow useful organs that would fit the size requirements for a human body. However, there are obvious ethical issues, especially considering that certain studies indicate the possibility of building a chimera that is 20% human. It is why scientists are currently asking themselves at which point does a chimera develop too many human traits and gains a consciousness or moral status that is similar to ours.