On Thursday, the National Institutes of Health announced that it was revising the rules that govern its funding of stem cell research. The rules focus on cases where human stem cells are introduced into embryonic animals, creating an embryo that's a mixture of human and animal tissues. While the rules would lift a blanket moratorium on funding for this research, they'd also tighten the regulations that were in place prior to the moratorium.

An animal that's a mixture of two different organisms is called a chimaera. While they're named after mythological beasts, creating them is rather run-of-the-mill in modern research, where chimaeras between different mouse strains are an essential part of knocking out genes to study the effects. Human-mouse chimaeras are also quite common, as we inject human tumor cells into mice to study cancer and replace the mouse immune system with a human one in order to study diseases like AIDS.

Some stem cell research would be similar in nature. For example, if you wanted to determine if it is safe and effective to use stem cells to repair cardiac injury in humans, a reasonable first step would be to see what happens when you inject human stem cells into an adult mouse that has a damaged heart.

But stem cells also allow an entirely different class of experiments, one where human stem cells are injected early in the development of a different species and go on to contribute to a variety of tissues. Why would anyone want to do this? With the right manipulations, it could be possible to produce humanized organs in other animals that would then be used for transplants. This approach would also allow us to study human genetic diseases that affected specific tissues, such as muscle cells.

But the work raised some obvious concerns, such as cases where human cells contribute significantly to the brain or germ cells (meaning sperm and egg) of a non-human species.

In 2009, the NIH set rules that prohibited injecting human cells at the earliest stages of embryonic development—called the blastocyst—and prohibiting the breeding of animals that could have human germ line cells. Last September, however, the NIH determined that these restrictions might not be sufficient, so it put a blanket moratorium on funding work on chimaeras produced early in development. It now proposes to lift the moratorium, while tightening up the rules a bit.

The new rules would see the 2009 ban on funding apply earlier in development, when the embryo is little more than a ball of disorganized cells. Beyond that, the NIH is establishing an internal steering committee that would add a layer of review to grants that propose doing specific types of stem cell work. This would look at any grants that propose to use human stem cells in the period after the blastocyst. The next developmental process, called gastrulation, is when the three main types of tissues are specified (you can think of these as roughly body, brain and skin, and guts).

The committee will also examine cases where a later addition of stem cells can result in a substantial contribution of human cells to the brain. This recognizes that there are genetic manipulations that can skew the contributions of stem cells to specific tissues. For example, mutations exist that allow development of specific organs to start, but cause them to stall out. If you put stem cells without this mutation into an embryo that has it, the stem cells can rescue organ development. As a result, the organ will be primarily composed of stem cells.

This approach can be highly valuable—it's how you'd engineer something like a liver made of human cells to develop inside a pig, for example. But it could just as easily be used to create a brain that's primarily populated by human cells inside other animals.

The new rules have been published in the Federal Register and are open to comment until early September.