Nobody has counted them, but the best estimates put the number of human brain cells in the trillions. The best known among them, called neurons, do the heavy thinking and remembering. Each of these cells can connect to 10 or more others, forming a vast network of feelings, thoughts, memories, prejudices, and PINS.

But neurons don’t do their jobs alone. They are supported and regulated by an immense system of star cells, called astrocytes, because of their shape. New research has discovered how these stars are born. The discovery also hints at how defective astrocytes may contribute to Alzheimer’s disease.

It has been known for years that both neurons and astrocytes come from the same brain stem cells. But how do these cells know whether and when to make one or the other?

The distinction is important because astrocytes do many vital things. They promote the formation of synapses, the connections between neurons. Without the right connections and exchange of messages, there would be no memories. The star-shaped cells also regulate how synapses function. For example, they remove excitatory chemicals from the connections, preventing overstimulation that can lead to seizures. They also control the movement of newborn brain cells, which must migrate to the right places for the brain to operate at high efficiency. Finally, astrocytes form the well-known blood-brain barrier that prevents many toxic substances from being carried into your brain by your blood.

Despite these vital functions, little has been known about how stem cells make the right number of each cell type at the right time. Neurons can’t make a brain think right without astrocytes, and too many astrocytes would leave some of them with nothing to do. So how is the fate of a brain determined?

After long years of working on the problem, Gabriel Corfas, an associate professor of neurology at Harvard Medical School, and his colleagues at the Neurobiology Program at Children’s Hospital Boston, believe they have solved the mystery. In doing so, they found that the process is regulated by a molecule involved in Alzheimer’s disease.