To grow the brain structures, called organoids, the scientists used stem cells, which can develop into any other kind of cell in the body. They put the stem cells into a special solution designed to promote the growth of neural cells. Bits of gel interspersed throughout the solution gave the cells a three-dimensional structure to grow upon. In eight to 10 days, the stem cells turned into brain cells. After 20 days to a month, the cells matured into a size between three and four millimeters, representing specific brain regions such as the cortex and the hindbrain. Growing brain tissue this way marks a major advancement because the lab-grown brain cells self-organized and took on growth patterns seen in a developing, fetal brain. (via Scientists Grow Human Brain From Stem Cells)