Why do these lab-grown mini-brains emit brain waves?

In what could be a significant finding to understand the development of the human brain, a group of scientists has created miniature brains from stem cells that developed a functional neural network. These lab-grown brains are the first observed to produce brain waves that resemble those of preterm babies, despite being a million times smaller than human brains, according to the study, published in the journal Cell Stem Cell. "The level of neural activity we are seeing is unprecedented in vitro," says Alysson Muotri, a biologist at 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." The pea-sized brains, called organoids, are derived from human pluripotent stem cells.