The Complexity of Brain Development
The brain is arguably the most intricate organ within the human body, and many of its mechanisms and structures remain shrouded in mystery. A significant part of this enigma lies in our limited understanding of its formation. In fact, the questions concerning how and when the various regions of the brain diversify remain unanswered.
For decades, the scientific community has maintained that the brain develops from a single population of early progenitor cells, from which all its regions emerge. This model suggested that different parts of the brain share a common origin during embryonic development. However, recent research spearheaded by scientists at Stanford University’s School of Medicine challenges this widely accepted notion. Is it truly the case that the entire brain originates from one group of precursor cells, or do distinct populations exist from very early stages, each destined to form specific brain regions?
To investigate this question, researchers designed a phased study to analyse and compare the mechanisms that give rise to the anterior and posterior regions of the brain at various points in embryonic development.
Insights from Mouse Embryos
As detailed in a publication in Nature Neuroscience, the research commenced with the examination of developing mouse embryos. The focus was on brain development during a very early stage known as gastrulation, where the cells forming the embryo are organised into layers, ultimately leading to the formation of different tissues and organs.
During this phase, the researchers identified two distinct populations of progenitor cells that remain separate in the initial stages of brain development. One population expresses the OTX2 gene and is destined to generate the forebrain and midbrain, while the other expresses GBX2 and gives rise to the hindbrain. The study clarifies that these two groups are mutually exclusive, meaning they do not mix or overlap during the early stages of embryonic development.
Human Cellular Models in Research
The scientists expanded their analysis to determine if these same pathways are present in humans. They modified human pluripotent stem cells in the laboratory to adopt two distinct states that mimic very early cellular populations involved in nervous system development. Some cells acquired characteristics of anterior neural ectoderm (aNE), linked to regions that will later form the forebrain and midbrain, while others took on traits of posterior neural ectoderm (pNE), associated with hindbrain development.
To differentiate the two groups, the pNE cells were marked with fluorescence, while the aNE cells remained unmarked. The two populations were then mixed in a single culture and exposed to various molecular signals simulating the instructions cells receive during embryonic development. These signals promoted the formation of characteristics associated with the forebrain, midbrain, and hindbrain—three embryonic regions that give rise to distinct structures within the human brain.
Implications for Broader Vertebrate Development
The cultivated and artificially activated cells were analysed through immunostaining, a technique that identifies specific proteins to determine the type of cells formed. The results indicated that, in both cellular populations, the initial state and the signals they received significantly influenced the type of brain tissue they could generate. With this understanding, the researchers successfully induced human cells to develop into motor neurons of the hindbrain. They confirmed that these neurons exhibited the characteristic proteins and electrical activity, indicating they had acquired functional properties typical of this neuronal type.
Ultimately, the scientists utilised other animal models to ascertain whether this mechanism could apply to the development of vertebrates in general. They discovered that the process was also observable in macaques, chickens, zebrafish, and even acorn worms. The comparison across species with various evolutionary histories and the similarities found suggest that this mechanism may be quite ancient.
Revolutionising Understanding of Brain Regions
Collectively, the experiment demonstrated that the mechanism initially observed in mouse embryos is also evident in human cells and other animals. The authors concluded that the development of different brain regions begins very early and relies on progenitor populations that follow distinct pathways, rather than emanating from a single progenitor population.
Dr. Kyle Loh, a developmental biology expert and associate professor at Stanford, remarked, “We have demonstrated for the first time that the front of the brain originates from a progenitor cell that is entirely different from that of the back. Our discovery means we can now cultivate hindbrain neurons in a Petri dish and study their functions.”
New Perspectives for Neurological Diseases
The findings provide a pathway for understanding why the scientific community has struggled for decades to cultivate certain types of brain cells in the laboratory. Rayyan Jokhai, a graduate student and co-author of the study, pointed out, “Previous attempts to create hindbrain neurons likely tried to induce differentiation from forebrain and midbrain progenitors into hindbrain cells, which our study shows is not feasible.”
Moreover, this research offers a fresh perspective for studying diseases affecting the brainstem, such as spinal muscular atrophy and amyotrophic lateral sclerosis. “Now we have a model to better understand these devastating diseases and to work on developing regenerative therapies to combat them. This represents a new and promising frontier in brain research,” concluded Jokhai.