In recent years, there has been excitement about studying human diseases using organoids—small patches of tissue from stem cells that mimic organs. But brain organoids lack connections to specialized brain structures needed to behave "normally." On Wednesday, a research group at Stanford University described a possible way to study brain organoids in a more natural context: They genetically wiped out a large portion of the mouse brain and replaced it with human brain organoid cells.

Organoids provide a better model than disassociated cells, but they still have limitations—no circulatory system or immune cells. This is especially limiting for brain studies, where specialized structures exchange information. One alternative has been to implant human neural stem cells into another species' brains, but surrounding host neurons make it unclear what can be learned.

The obvious solution is to get rid of host cells and have human cells take over. But human neurons mature more slowly than mice's and may not form needed connections quickly enough. The Stanford team deleted a portion of the mouse's brain, choosing the cortex, which handles decision-making and memory. They used a gene active in cortical cells to delete a key gene needed for chromosome separation during cell division.

Despite killing most cells that form the mature cortex and cutting brain volume in half, mice survived. Researchers eliminated most other pups to ensure cortex-free mice got enough nursing, left them with mothers longer, and provided high-calorie food. These steps allowed nearly full survival of mice without m...