The brain may not spring from a single initial pool of cells after all. A Nature Neuroscience study argues it arises from two parallel neural ectoderm progenitors, one destined to form the forebrain and midbrain and another to build the hindbrain. The work, led by Rayyan T. Jokhai, Carolyn E. Dundes and Kyle M. Loh, tracked early development in mouse embryos and human stem cells, revealing two distinct populations shaping the organ long before birth.
In early development, during gastrulation, some stem cells form neural ectoderm that will become the brain, while others contribute to other structures such as the spinal cord. The researchers used genetic lineage tracing in mouse embryos, tagging early neural cells as gastrulation ended. The tagged cells expressing a specific gene called Gbx2 were almost exclusively found in the hindbrain and largely absent from the forebrain and midbrain, suggesting an early, dedicated hindbrain path.
To test whether these cells could switch fate, they turned to human pluripotent stem cells in dishes. With different chemical signals, they produced two early brain tissues in two days: anterior neural ectoderm, forming the forebrain and midbrain, and posterior neural ectoderm, forming the hindbrain. When these two populations were exposed to the opposite developmental signals, they largely resisted changes in fate, suggesting a strong lineage memory.
Chromatin analysis showed the two cell types have different landscapes: forebrain genes are accessible in anterior cells, whereas hindbrain genes are largely locked away in posterior cells.
Eduardo Sequerra, of the Brain Institute at the Universidade Federal do Rio Grande do Norte, said the study supports the idea that the neuroectoderm divides into two progenitor populations that diverge in their potential “much earlier than the neural tube formation.” He noted past work hinting at multiple brain origins and said the findings align with that possibility. “During the ’90s Nicole Le Douarin’s group performed a series of transplantation studies showing that the folding neural tube is divided into two parts,” Sequerra explained, adding one part sits above the notochord and “cannot differentiate into anterior structures,” while the other differentiates into the midbrain and forebrain.
He further commented that the split likely occurs when future notochord-mesoderm cells are still migrating, before neural folds stage. “This split possibly occurs when the future notochord-mesoderm cells are still migrating,” he said.
Examining embryos from other species—macaques, chickens, zebrafish and acorn worms—the researchers found distinct anterior and posterior neural cell groups across all, suggesting the two-origin mechanism has been conserved for roughly 550 million years. Loh described the broader implication: while in humans, mice and other animals the two parts develop alongside one another, they originated from different sources and later converge to form a single organ. He even compared the concept to the way a smartphone is assembled from parts made in different places before becoming a unified device. “We hope to learn more about how the brain was built during evolution,” Loh said, noting that jellyfish, which diverged from us about 600 million years ago, have two separate nerve rings at different ends of the body.
The team succeeded in directing posterior neural ectoderm cells to become specialized hindbrain motor neurons in culture. The neurons showed spontaneous calcium activity and fired electrical impulses when stimulated, indicating functionality in a laboratory dish. Loh described the result as a significant step, saying, “The hindbrain—the back part of the brain, otherwise known as the brainstem—controls life-sustaining functions such as sleep, wakefulness, consciousness, hunger, and so on.” He added that producing hindbrain neurons in a dish could help researchers understand these fundamental processes and diseases that impact them.
The work has potential relevance for neurological diseases that target hindbrain motor neurons, such as spinal muscular atrophy and amyotrophic lateral sclerosis (ALS). Loh noted that SMA is “the #1 genetic cause of death in babies under 1 year of age,” and that both SMA and ALS kill hindbrain motor neurons that control muscles involved in eating, swallowing and speech. He stressed that the dish-based model offers a means to study these diseases in patient-derived cells, though not to replace damaged tissue in the body.
Experts also cautioned against reading the findings as meaning the brain lacks unity. Loh urged that the brain remains one organ built from two sources. “Some people reading our paper sometimes think it suggests the brain is two separate organs. More precisely, the brain is one organ, but it is built from two different sources,” Loh explained. “By the analogy, the heart is likewise built from two different sources: one source creates the right ventricle, and another source creates the three other chambers of the heart. But ultimately, all four chambers of the heart work together, similar to how different parts of the brain all work together to give rise to all the amazing things that the brain can do.”
Growing human cells in a dish does not perfectly replicate the embryo’s complex three-dimensional environment, and undiscovered chemical combinations might still alter cell identity. Testing these ideas in living mammals would be technically challenging and would require placing labeled progenitor cells into the wrong location in a developing embryo. Nonetheless, the ability to predictably grow specialised hindbrain neurons offers a useful tool for studying fatal neurodegenerative diseases in the lab.
The study, “Two parallel neural ectoderm progenitors contribute to the developing brain,” was led by Rayyan T. Jokhai, Carolyn E. Dundes, Hadia S. Ahsan, Rachel S. Kang, and others, with Kyle M. Loh as senior author.
