Nerve-cell development explained by two theories

For many years, two schools of thought have dominated neurobiologists’ theories about how early nerve cells develop specialties that allow the assembly of a mature brain.

One theory suggests that master regulators trigger the development of the same specialized traits in cells found across wide regions of the brain.

Paul Taghert
Paul Taghert

The other theory attributes the development of specialized traits to interactions between many local factors.

In a new study of developing fruit fly brain cells, scientists at Washington and Harvard universities showed that both models are valid and active. Surprisingly, they both appear to operate within single developing nerve cells.

By learning more about the most basic mechanisms that regulate the creation of the brain, scientists hope to gain new insights into developmental disorders that damage it.

“We really have to consider individual cell properties and the complexity of the mechanisms that underlie these properties,” said Paul H. Taghert, Ph.D., professor of anatomy and of neurobiology.

“Our system in the fruit fly lets us look at these factors at the level of individual cells, but even at that level the harder you push, the more you uncover the complexity that underlies these developmental systems.”

Specialization of nerve cells is essential to normal brain function. All neurons have certain properties in common. But as scientists have focused more closely on individual nerve cells, many variations have emerged.

“Some cell types have arms that are just a simple extension with a few branches, but some of them have quite an elaborate branching pattern,” Taghert said. “The individual chemistries of these different types of nerve cells — the substances known as neurotransmitters that they emit, for example — also vary tremendously.”

Other variations include changes in the cell membrane’s responsiveness to stimulation and in the time period when brain cells are quiet and active. Taghert estimates that the fly brain contains several hundred different subtypes of nerve cells and guesses that the human brain may contain thousands.

Through studies of a fruit fly brain area containing five specialized cell types, Taghert and his colleagues showed that developmental factors could participate in the more interactive model of neurodevelopment.

In this model, many different sets of regulatory compounds interact in a nerve cell’s nucleus to switch specialized traits on and off.

The traits that are turned on and off are determined by which combinations of development factors are present in the cell.

But scientists also found evidence that some of the same developmental factors they studied were producing a different model of neurodevelopment, uniformly dictating the creation of the same specialized traits in many different cells across a wide region of the brain regardless of their interaction with other developmental factors.