https://www.scientificamerican.com/article/organoids-reveal-how-human-forebrain-develops/

Knowing how the human brain develops is critical to understanding how things can go awry in neurodevelopmental disorders, from intellectual disability and epilepsy to schizophrenia and autism. But between the fact that researchers cannot poke around inside growing human brains and the inadequacies of animal models, scientists currently do not fully understand the process. “We know a bit about the early stages because [the situation is] very similar to what happens in rodents,” says psychiatrist Sergiu Paşca of Stanford University. “But everything beyond the second trimester [of pregnancy] and soon after birth is poorly understood.”

Enter the invention of brain “organoids”: cells grown in 3-D clusters in the lab and designed to mimic the composition of the organ’s tissue. The technology recently reached the point where specific brain regions can be modelled for sufficiently long periods to allow researchers to study their development. Paşca and his colleagues have now used organoid models of parts of the human forebrain—the seat of higher cognitive abilities such as complex thought, perception and voluntary movement—to peer into how gene activity drives brain development. “The work brings new understanding of how, as the brain is formed, distinct regulatory regions of the genome are used to execute specific tasks—for example, the generation of specific types of neurons,” says neuroscientist Paola Arlotta of Harvard University, who was not involved in the new study. The researchers used their findings to map genes associated with certain disorders to specific cell types at specific stages, giving insight into the origins of conditions such as autism and schizophrenia.