Textbooks have long described the human brain as a single, unified organ. New research from Stanford Medicine suggests it is better understood as two ancient nervous systems that evolved separately and were later packaged into one structure. The study, published September 18 in Nature Neuroscience, overturns a model of brain development that researchers have relied on for decades.

The old model held that one progenitor cell early in embryonic development gives rise to the entire brain, meaning every region shared a common developmental origin. The new findings show something different. Two progenitor cell populations form at the earliest stages of development and run in parallel, never overlapping. One population, which expresses a gene called Otx2, is destined to become the forebrain and midbrain. The other, which expresses a gene called Gbx2, is committed to forming the hindbrain. According to the researchers, these populations are mutually exclusive from the start.

A split at the earliest stage of development

The breakthrough came from studying gastrulation, the stage of embryonic development when the body first takes shape. Graduate students Rayyan Jokhai and Carolyn Dundes found that the hindbrain follows a developmental path that runs alongside the pathway creating the forebrain and midbrain, rather than branching off from it.

The team confirmed the split by examining chromatin, the DNA packaging that determines which genes a cell can access and which remain bundled away. The anterior neural ectoderm, which becomes the forebrain and midbrain, and the posterior neural ectoderm, which becomes the hindbrain, carry fundamentally different chromatin configurations. Those differences effectively lock each progenitor cell into its fate. Jokhai said the finding explains decades of frustration in the lab. Previous efforts to grow hindbrain neurons had tried to coax forebrain and midbrain progenitors into becoming hindbrain cells, something the study shows is simply not possible.

Growing hindbrain neurons in a dish

Armed with this knowledge, the researchers accomplished something no lab had managed before. They coaxed human pluripotent stem cells, the versatile cells that can become any cell type in the body, into functional hindbrain motor neurons. The lab-grown neurons displayed the hallmarks of genuine hindbrain cells. They produced waves of electrical activity known as action potentials and made proteins that identify the hindbrain segments controlling facial and swallowing muscles.

Senior author Kyle Loh, an associate professor of developmental biology at Stanford Medicine, described the result as the first demonstration that the front of the human brain arises from a totally different progenitor cell than the back. The practical payoff could be large. The hindbrain, often called the brain stem, runs the automatic functions that keep people alive: breathing, sleep, heartbeat, and hunger. Studying diseases that attack this region has been nearly impossible because scientists cannot obtain brain stem tissue from living patients.

That gap has slowed research into spinal muscular atrophy and amyotrophic lateral sclerosis. SMA is a leading genetic cause of death in children under one year of age, while ALS is typically diagnosed between ages 40 and 70. In both disorders, certain hindbrain neurons gradually stop working, and patients lose the ability to swallow, which can cause pneumonia when food or liquid enters the lungs, and eventually the ability to breathe. A dish full of functioning hindbrain neurons gives researchers a working model to study what goes wrong. The study reports an unexpected connection to obesity treatment as well: the hindbrain contains the hunger-regulating circuits that weight-loss drugs like semaglutide act on.

An ancient arrangement

The researchers also looked far back in evolutionary time. They found the same two-origin brain pattern in chickens, in zebrafish, and in acorn worms, small creatures on the ocean floor that share a distant common ancestor with humans, across more than half a billion years of evolutionary history. Jellyfish, whose lineage diverged from ours roughly 600 to 700 million years ago, carry two nervous systems at opposite ends of their bodies. Loh said the evidence points to evolution taking two existing neural systems and pushing them together spatially.

The next step is to pin down the developmental origins of the spinal cord and learn exactly how SMA and ALS compromise hindbrain neurons. Researchers from the California Institute of Technology and the University of California, San Francisco also contributed to the work. Full details appear in the paper by Jokhai and colleagues, "Two parallel neural ectoderm progenitors contribute to the developing brain," available at Nature Neuroscience.