This cluster of neural cells—that’s a rudimentary eye on the left—is no bigger than an apple seed. Yet it could provide the best model yet of the developing human brain. The clusters, grown from embryonic stem cells, contain a surprising number of structures that resemble early human brains, including retinal tissue, the cerebral cortex (the brain’s outermost layer), and the choroid plexus (the cavity that produces cerebrospinal fluid). A group of scientists is using these structures to study microcephaly, a disorder in which the head and brain are abnormally small. They reprogrammed cells from a person with the disorder into so-called induced pluripotent stem cells and used them to grow organoids. The patient-derived cells produced a shrunken organoid , the group reports online today in Nature. Certain precursor cells matured earlier than normal, bringing tissue growth to a halt prematurely. The resemblance to a real brain only goes so far, however. The organoids do not have any blood vessels, so cells at their core die off. They reach their maximum size—about 3 millimeters in diameter—after 2 to 3 months, and after 4 months they don’t develop any new cell types. For that reason, the organoids are not yet useful for studying more complex neurodevelopmental conditions such as autism or schizophrenia. The researchers are working on ways to grow larger organoids, but they say that technical hurdles make it very unlikely these mini-brains will ever be capable of higher-level brain function. See tomorrow’s issue of Science online for more on the new technology .