The human brain assembles itself through a cascade of tightly choreographed events that begin in the third week after conception and, in some respects, do not finish until the third decade of life. A flat sheet of embryonic cells folds into a tube, stem cells inside that tube churn out billions of neurons, those neurons migrate to precise locations, and then the real complexity begins: axons extend, synapses form, unused connections get stripped away, and insulating sheaths slowly wrap the wiring. Each stage depends on the one before it, and disruptions at any point can ripple forward in ways that shape cognition, behavior, and disease risk for a lifetime.
From Flat Sheet to Neural Tube
Around three weeks after fertilization, a strip of cells on the surface of the embryo thickens into the neural plate, then folds inward and fuses to form the neural tube. This tube is the precursor to the entire central nervous system: the front end balloons into what will become the brain, while the trailing portion becomes the spinal cord. The process of closure is not one smooth zipper but several separate closure points that must seal in the right sequence. When any of those closures fails, the result is a neural tube defect. Which defect depends on which segment stays open: failure at the head end produces anencephaly, while failure lower down produces spina bifida. Key signaling pathways, including hedgehog signaling and the planar cell polarity pathway, must be precisely calibrated for closure to succeed.
1PubMed Central. Genetics and development of neural tube defectsThe Stem Cells That Expand the Cortex
Once the neural tube closes, a layer of stem cells lining its inner surface begins dividing furiously. These cells, called radial glia, serve a dual role: they are the primary progenitor cells that generate neurons, and they also act as physical scaffolding that guides newborn neurons to their destinations.
2PubMed Central. Coevolution of radial glial cells and the cerebral cortexWhat makes the human cortex so large compared to that of other mammals comes down to a particular class of these progenitors. Outer radial glia, which reside in a zone farther from the inner wall of the tube, are far more abundant in humans than in most other species. Their abundance and proliferative capacity are closely linked to cortical expansion.
3PubMed. Neural progenitor cells and their role in the development and evolutionary expansion of the neocortexSingle-cell studies have revealed that outer radial glia are molecularly distinct from their counterparts closer to the ventricle. They activate a self-renewal pathway involving STAT3 signaling that the inner radial glia do not use, and they produce extracellular matrix proteins and growth factors that sustain their own niche. A single outer radial glia cell can generate hundreds of neurons destined for both deep and upper layers of the cortex.
4Cell. Single-Cell Analysis Reveals Molecular Characteristics of Human Outer Radial GliaHow Neurons Find Their Place
Newborn neurons do not stay where they are born. They migrate, sometimes over considerable distances relative to their size, to reach their final positions. In the developing cortex, two broad migration routes operate simultaneously. A radial pathway carries neurons outward from the proliferative zone near the ventricle along the radial glia scaffolding, and a tangential pathway brings a separate population of neurons laterally from a structure called the ganglionic eminence. Diffusion imaging of the human fetal brain has captured both of these three-dimensional migration streams in action.
5PubMed Central. Radial and tangential neuronal migration pathways in the human fetal brain: anatomically distinct patterns of diffusion MRI coherenceThe cortex ends up with six layers, and those layers form in an inside-out sequence. The earliest-born neurons settle in the deepest layers. Later-born neurons migrate past them to form progressively more superficial layers. This means the outermost layer of the cortex is actually the youngest.
6PubMed. Cerebral cortex development: an outside-in perspectiveWiring the Network
Once a neuron reaches its destination, it needs to connect. This happens in stages. First, the neuron extends an axon, a long thin projection that will carry outgoing signals to distant targets. The tip of the growing axon, called the growth cone, navigates through the developing brain by reading molecular cues in its environment. Families of guidance molecules either attract or repel the growth cone, steering it toward the correct target. How a relatively small number of these guidance cues can set up the patterning of an extraordinarily complex nervous system remains one of the open puzzles of developmental neuroscience.
7PubMed Central. Axon growth and guidance: receptor regulation and signal transduction 8PubMed Central. The growth cone: an integrator of unique cues into refined axon guidance
While the axon grows outward, the neuron also sprouts dendrites, the branching input antennae that receive signals from other cells. The shape and branching pattern of a neuron’s dendritic tree are not random. They are cell-type specific, meaning a Purkinje cell in the cerebellum and a pyramidal cell in the cortex build very different arbors, and those shapes are critical to how the neuron processes information. Tiny protrusions called dendritic spines form along the branches and serve as the main sites for receiving excitatory input.
9PubMed Central. Mechanisms regulating dendritic arbor patterningBuilding Synapses, With Help From Astrocytes
The junction between an axon terminal and a dendritic spine (or another target) is a synapse, and building synapses turns out not to be a purely neuronal affair. Astrocytes, star-shaped support cells that outnumber neurons in some brain regions, play an active role. At what researchers call the tripartite synapse, an astrocyte process wraps around the connection point and engages in two-way signaling with both the sending and receiving neuron. This interaction helps shape both the astrocyte’s own maturation and the synapse’s development.
10PubMed Central. Cell adhesion molecules regulating astrocyte-neuron interactionsOne concrete example: astrocytes secrete a protein called hevin, which physically bridges two adhesion molecules on opposite sides of the synapse that cannot bind each other on their own. In the developing visual cortex, this astrocyte-mediated bridge is critical for the formation and later plasticity of connections between the thalamus and the cortex.
11PubMed Central. Astrocytes Assemble Thalamocortical Synapses by Bridging NRX1α and NL1 via HevinThe adhesion molecules involved in synapse assembly, particularly the neurexin and neuroligin families, come in multiple forms that pair selectively. Different isoforms of neurexin interact with different neuroligins at excitatory versus inhibitory synapses, which is one way the brain establishes the correct balance between excitation and inhibition.
12PubMed Central. Analysis of neurexin-neuroligin complexes supports an isoform-specific role for beta-neurexin-1 dysfunction in a mouse model of autismSculpting by Subtraction
The developing brain deliberately overproduces both neurons and synaptic connections, then prunes the excess. During a wave of programmed cell death that peaks in the fetal period and early infancy, neurons that fail to make appropriate connections are eliminated. The survival signals come from neurotrophins, proteins released in limited quantities by the cells a neuron is supposed to connect with. Only neurons that successfully innervate their targets receive enough of these survival factors to stay alive. The result is a rough matching of neuron numbers to the wiring demands of each region.
13PubMed Central. Neurotrophins: roles in neuronal development and functionA second, longer phase of subtraction targets synapses rather than whole cells. In this process, the immune system’s complement cascade plays a surprising role. Complement proteins, particularly C1q and C3, tag certain synapses for removal, and microglia, the brain’s resident immune cells, engulf the tagged connections. The molecular machinery that microglia use to dismantle a synapse resembles, at a local scale, the same signaling that cells use when undergoing apoptosis. This pruning refines circuits throughout childhood and adolescence, sharpening the connections that are used frequently while eliminating those that are not.
14PubMed Central. Local apoptotic-like mechanisms underlie complement-mediated synaptic pruning 15PubMed Central. The Role of Complement in Synaptic Pruning and Neurodegeneration
The Slow Wrap of Myelination
For neural circuits to work at full speed, axons need to be insulated with myelin, a fatty sheath produced by oligodendrocytes. Myelination does not happen all at once. Sensory and motor areas myelinate first, typically within the first year of life, while frontal and temporal regions follow a much more drawn-out timeline.
16PubMed Central. Cortical maturation and myelination in healthy toddlers and young childrenCompared to our closest relatives, this schedule is strikingly slow. In chimpanzees, myelinated axon density in the cortex climbs steadily until around the time of sexual maturity and then plateaus. In humans, childhood myelination is slower and the process extends well beyond late adolescence.
17PubMed Central. Prolonged myelination in human neocortical evolutionEven within a single cortical region, myelination is not uniform. Deeper cortical layers mature earlier, while the superficial layers continue to myelinate on a more protracted schedule. This layer-by-layer staggering means the prefrontal cortex, already the last region to myelinate, finishes its outermost layers last of all. That prolonged developmental window is thought to be part of what gives the human prefrontal cortex its extended period of plasticity and learning.
18BioRxiv. Heterochronous laminar myelination in the human prefrontal cortexCritical Periods and the Brakes on Plasticity
Early in postnatal life, certain brain circuits pass through critical periods, windows during which experience shapes their wiring with unusual power. The classic example is vision: if one eye receives degraded input during the critical period, the cortex permanently reallocates territory to the other eye. Similar critical periods exist in the auditory cortex, the hippocampus, and the amygdala.
What closes these windows? One of the key molecular brakes is a structure called a perineuronal net, a mesh of extracellular matrix proteins that condenses around the cell bodies and proximal dendrites of certain neurons, physically encasing their synapses. When researchers dissolve perineuronal nets in adult animals, critical-period-level plasticity returns, suggesting the nets act as a physical lock on circuit architecture once the sensitive period ends.
19PubMed Central. An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity 20Neuron. Dynamic Regulation of Perineuronal Nets in the Auditory Cortex Is Critical for Auditory Fear Learning and Consolidation
Spontaneous Activity Before Sensation
You might assume the brain is quiet before a baby starts seeing, hearing, and touching the outside world, but it is not. Waves of spontaneous neuronal activity sweep through the fetal brain well before sensory organs are functional. These self-generated signals are not noise. They play instructional roles, guiding neurogenesis and helping assemble the earliest functional networks. Recent work has identified the temporal and insular cortices as central hubs that propagate these early waves.
21PubMed Central. Structured Spontaneity: Building Circuits in the Human Prenatal BrainBlood Supply Keeps Pace With Demand
Neurons are metabolically expensive. As each cortical layer begins to mature, the local capillary network expands alongside it. The first intrinsic capillary plexus of the cortical gray matter appears to form right when the deepest pyramidal neurons begin sprouting dendrites and spines, a timing that suggests functional maturation and blood supply development are tightly coupled. This lockstep process repeats as successively higher layers mature, and continues into postnatal life.
22Frontiers in Neuroanatomy. The human brain intracerebral microvascular system: development and structureThe blood-brain barrier itself depends on coordinated signaling among endothelial cells, pericytes, and astrocytes. Pericytes, cells embedded in capillary walls, fine-tune this barrier and help regulate blood flow to meet the moment-to-moment energy demands of nearby neurons.
23PubMed Central. The CNS microvascular pericyte: pericyte-astrocyte crosstalk in the regulation of tissue survivalHow the Environment Gets Written Into Developing Circuits
Genes provide the blueprint, but environmental conditions during pregnancy and early childhood can alter how those genes are read. The primary mechanism is epigenetic modification, particularly changes to DNA methylation patterns that dial gene activity up or down without altering the underlying genetic code. Prenatal stress, toxin exposure, and nutritional deficits have all been linked to lasting epigenetic changes in the brain, including in genes that regulate stress hormones and neurotrophic support.
24PubMed Central. The Epigenetic Link between Prenatal Adverse Environments and Neurodevelopmental DisordersIn humans, prenatal stress has been associated with altered DNA methylation patterns linked to children’s emotional regulation, neurobehavioral outcomes, and stress reactivity.
25PubMed Central. Epigenetics of prenatal stress in humans: the current research landscapeRodent experiments have demonstrated more directly that experiences during sensitive developmental windows produce stable methylation changes in specific genes that persist across the lifespan and drive measurable behavioral differences. These findings suggest that early environmental conditions do not just transiently affect the brain. They leave a lasting molecular signature on it.
26PubMed Central. Annual Research Review: Epigenetic mechanisms and environmental shaping of the brain during sensitive periods of developmentWhen the Building Process Goes Wrong
Because brain development involves so many sequential, interdependent steps, it is vulnerable at many points. Schizophrenia and autism spectrum disorder, despite their very different clinical presentations, both involve disrupted synapse formation and function, particularly at glutamatergic (excitatory) synapses. Human genetic studies and cellular-level analyses have converged on this shared vulnerability.
27PubMed Central. Modeling synaptogenesis in schizophrenia and autism using human iPSC derived neuronsA powerful new approach for studying these disorders uses brain organoids, miniature three-dimensional structures grown from human stem cells that recapitulate many aspects of early brain development. By combining organoid technology with CRISPR gene editing and single-cell analysis, researchers have been able to systematically disrupt dozens of high-risk autism genes and observe which cell types are most affected. Intermediate progenitor cells, ventral progenitors, and upper-layer excitatory neurons appear especially vulnerable to disruption of these genes.
28Nature. Single-cell brain organoid screening identifies developmental defects in autism 29PubMed. Single-cell genomic analysis of human cerebral organoids
The Asymmetric Brain
The left and right hemispheres of the human brain are not mirror images. Language processing, spatial reasoning, and emotional processing are lateralized, and this asymmetry appears to have deep genetic roots. A large-scale genomic study identified over 200 genes associated with brain asymmetry and found that these genes cluster in functions related to axon growth, synaptic membrane composition, and microtubule structure. Microtubules are the internal scaffolding of axons and dendrites, so their involvement hints that left-right differences are at least partly built into the cytoskeletal machinery of developing neurons themselves.
30PubMed Central. Evolution of Human Brain Left–Right Asymmetry: Old Genes with New FunctionsHuman-Specific Tweaks to an Ancient Program
Most of the molecular toolkit for building a brain is shared across mammals. What makes the human brain distinctive is often not the invention of entirely new genes but the duplication or repurposing of existing ones. One example involves a gene called HYDIN, which in other apes has a single copy and plays a role in cilia and sperm motility. Humans carry an additional duplicate, HYDIN2, which is more broadly expressed in the brain, suggesting it has been co-opted for a new role in neural development.
31Cell Press. Neuronal Development: How the Human Brain Is BuiltThis pattern of gene duplication with functional divergence, combined with the expanded outer radial glia population and the unusually drawn-out myelination schedule described earlier, suggests that the human brain’s remarkable capacity is less about radical new inventions and more about pushing ancient programs further, slower, and with more stem cells than other species manage.