This is a huge discovery that brings on possibilities for studying the early development of brain disorders.
It also has left some scientists feeling hesitant.
These lab-grown brains are known as organoids - three-dimensional, miniature, simplified versions of organs grown in a lab for research purposes, such as testing drug responses, or cell development under certain adverse conditions.
"The level of neural activity we are seeing is unprecedented in vitro," says neuroscientist Alysson Muotri from the University of California, San Diego.
"We are one step closer to have a model that can actually generate these early stages of a sophisticated neural network."