Pacific Grove, California--Pop a few human stem cells into culture, provide the right molecular signals, and before long a mock cerebral cortex or a cerebellum knockoff could be floating in the medium. These neural, or brain, organoids, typically just a few millimeters across, are not "brains in a dish," as some journalists have described them. But they are becoming ever more sophisticated and true to life, capturing more of the brain's cellular and structural intricacy. "It's surprising how far this [area] has advanced in the last year," says John Evans, a sociologist at the University of California San Diego who follows the research and public opinions on it. That progress has allowed researchers to delve deeper into how the human brain develops, functions, and goes awry in diseases, but it has also sharpened ethical questions.

It's not often that a twitching, snowman-shaped blob of 3D human tissue makes someone's day. But when Dr. Sergiu Pasca at Stanford University witnessed the tiny movement, he knew his lab had achieved something special. You see, the blob was evolved from three lab-grown chunks of human tissue: a mini-brain, mini-spinal cord, and mini-muscle. Each individual component, churned to eerie humanoid perfection inside bubbling incubators, is already a work of scientific genius. But Pasca took the extra step, marinating the three components together inside a soup of nutrients.

Brain development is a remarkable self-organization process in which cells proliferate, differentiate, migrate, and wire to form functional neural circuits. In humans, this process takes place over a long fetal phase and continues into the postnatal period, but it is largely inaccessible for direct, functional investigation at a cellular level. Therefore, the features that make the human central nervous system unique and the sequence of molecular and cellular events underlying brain disorders remain largely uncharted. Human pluripotent stem (hPS) cells, including those obtained by reprogramming somatic cells, have the ability to self-organize and differentiate when grown in three-dimensional (3D) aggregates rather than in direct contact with a flat plastic surface (1). Such 3D neural cultures, also known as organoids and organ spheroids, recapitulate many aspects of human brain development in vitro (1) and have the potential to accelerate progress in human neurobiology.