Clusters of human brain cells can integrate into rat brains, and that’s raising concerns about giving animals some form of human consciousness.

Researchers can grow stem cells into tiny clumps of cells, called organoids, that display similar activity and structure to human brains. To find out more about how exactly that works, read our primer from when we made the technique one of our Ten Breakthrough Technologies of 2015.

Now, though, reports Stat, several labs have inserted those organoids into rat brains and connected them to blood vessels; some of the organoids have even grown physical links with the rat brains. From Stat’s report:

Some of the axons grew as much as 1.5 millimeters, connecting to the corpus callosum, a bundle of neurons connecting the left and right cerebral hemispheres. When the scientists shined light on a rat’s eye, or stimulated brain regions involved in vision, neurons in the implanted organoid fired. That suggested the human brain tissue had become functionally integrated with the rat’s.

The aim of this kind of research is noble: to work out how lab-grown clusters of brain cells could be used to understand or even treat brain diseases or injuries. But while a handful of cells in a rat brain may not be a problem now, and the idea of imbuing animals with human characteristics or consciousness seems distant, the integration reported by the labs in Stat’s report is giving some ethicists cause for concern.

That’s especially the case as the number of organoids placed inside a rat’s head increases. “People are talking about connecting three or four,” says Stanford bioethicist Hank Greely to Stat. “But what if you could connect 1,000? That would be getting close to the number of cells in a mouse brain … At some future point it could be that what you’ve built is entitled to some kind of respect.”