To make them, the scientists reprogrammed adult skin cells into embryonic stem cells, which were then grown for a couple of months. That was long enough for them to develop four types of neurons and two support cells (astrocytes and oligodendrocytes). The team then tested the mini-brains by placing them on an electrode array and listening to the neurons' electrical communication as the drugs were added.

Since they're so small, researchers can grow a hundred of test subjects in a single petri dish, and each batch can produce hundreds to thousands of identical copies. Other institutions like Brown University developed in-vitro brain cell clumps in the past, but Hartung says these ensure accurate results:

We don't have the first brain model nor are we claiming to have the best one. But this is the most standardized one. And when testing drugs, it is imperative that the cells being studied are as similar as possible to ensure the most comparable and accurate results.

Hartung is in the midst of building a company called ORGANOME in hopes of starting the mass production of his mini-brains by this year. He's also hoping that labs across the world adopt the practice of making them, because we can only "replace animal testing on a large scale" -- for brain-related research anyway -- if using brain models becomes the norm. His creation has other potential applications, as well. By using skin cells from people afflicted with certain diseases, scientists can also grow tiny brains to study Alzheimer's, Parkinson's, multiple sclerosis and autism.