Stem cells used to create mini-brains in the lab

The organization of the organoid is very reminiscent of that of the developing brain.

Marko Repic and Madeline A. Lancaster

The human brain is a remarkably complex system, yet the vast majority of it forms from a single type of cell, one that also gives rise to the skin. The specification of nerve cells and the gradual generation of many specialized types of nerves take place through an elaborate set of signals, growth, and changes in shape and organization.

It's possible to study many of these processes in other organisms, but the human brain forms specific structures that simply don't exist in lab animals like mice and fish. To make it more difficult, some of these structures form fairly early in development. But now researchers have figured out a way to get human stem cells to form a structure that looks a lot like the cerebral cortex, complete with much of the normal developmental programming. By making stem cells from a patient with microcephaly, they were able to identify the defect involved in the patient's disorder.

The mature cerebral cortex is a brain structure with multiple specialized layers. But during development, it starts off as a layer of undifferentiated and proliferating cells. Over time, these undifferentiated cells give rise to bursts of neurons, which then migrate upward, passing through any existing layers of cells. Once they reach the top of this structure, they form a new layer and begin to establish connections with other nerve cells. This process continues until six distinct layers are formed. Mice do form a cerebral cortex, but the process differs in significant ways from what happens in humans, making them a limited model for studying it.