New research suggests that the forebrain and hindbrain develop from different early progenitor cells, pointing to two ancient developmental systems that have remained together within the human brain.
For decades, scientists have generally worked from the idea that the brain develops from a single early progenitor population. That model treats the major regions of the brain as having a common developmental origin.
The new research points to a different process. The forebrain and midbrain arise from one type of progenitor cell, while the hindbrain develops from another. Rather than one pathway branching into two, the two pathways develop alongside each other from the earliest stages examined.
The finding emerged from experiments on developing mouse embryos during gastrulation, a stage when the basic body plan begins to form.
The researchers identified two populations of brain progenitor cells. Cells expressing the gene Otx2 were destined to form the forebrain and midbrain. Cells expressing Gbx2 were committed to forming the hindbrain. The two populations did not overlap.
The team also examined chromatin, the DNA-protein structure that helps control which genes cells can access. The future forebrain and midbrain cells had a fundamentally different chromatin configuration from the cells destined to become the hindbrain. Those differences helped keep the two groups on separate developmental paths.
“Our discovery means that we can now grow neurons from the back of the brain, the hindbrain, in a Petri dish and study their functions,” said Kyle Loh, associate professor of developmental biology at Stanford Medicine and senior author of the study.
The hindbrain has been difficult to grow in the lab
The hindbrain, located toward the back of the skull, is responsible for many automatic functions, including breathing and regulation of the heartbeat, sleep and hunger. Its neurons also control muscles involved in facial movement, speech and swallowing.
Yet producing human hindbrain neurons in the laboratory has been difficult. The researchers say the new developmental finding helps explain why.
Earlier efforts may have started with forebrain and midbrain progenitors and attempted to turn them into hindbrain cells. But the new work indicates that these progenitor populations are fundamentally different from the outset.
“Previous attempts to make hindbrain neurons likely tried to coax forebrain and midbrain progenitors into hindbrain cells, which our study shows is not possible,” said graduate researcher Rayyan Jokhai, a co-first author.
The researchers instead used their understanding of the early developmental split to guide human pluripotent stem cells toward becoming hindbrain motor neurons.
The resulting cells displayed electrical activity known as action potentials and produced proteins associated with hindbrain regions that control facial and swallowing muscles. These characteristics indicated that the cells had the features of functional hindbrain neurons.
The developmental pattern extends deep into evolution
The researchers then looked beyond human and mouse development to organisms separated from humans by hundreds of millions of years of evolution.
They found the same two-origin pattern in chickens and zebrafish. They also found it in acorn worms, small ocean-floor animals that share a distant common ancestor with humans.
The researchers traced the pattern back more than 550 million years. They also noted that jellyfish, which diverged from humans about 600 million to 700 million years ago, have two nervous systems at different ends of their bodies.
“Our research suggests that evolution took two existing neural systems and pushed them together spatially,” Loh said.
The researchers propose that the two systems eventually came to function within what is now treated as one brain, while retaining their distinct developmental origins.
The lab-grown cells could help study brain stem diseases
The ability to produce hindbrain neurons in the laboratory could also provide a way to investigate diseases that damage these cells.
In spinal muscular atrophy, or SMA, and amyotrophic lateral sclerosis, or ALS, certain hindbrain neurons gradually stop functioning. Patients can lose the ability to swallow, and eventually the ability to breathe.
SMA is described in the research as a leading genetic cause of death in children under 1 year of age. ALS is often diagnosed between ages 40 and 70 and can affect both the forebrain and hindbrain.
Scientists have not been able to obtain brain stem tissue from living patients, making these diseases difficult to study directly. The newly developed laboratory-grown neurons provide another way to examine what happens to hindbrain cells in these conditions.
The researchers also point to the hindbrain’s role in regulating hunger as a possible connection to studies of obesity treatments, including drugs such as semaglutide.
Their next work will examine the developmental origins of the spinal cord and investigate how SMA and ALS impair hindbrain neurons.
The study was published in Nature Neuroscience.
