Introduction to Behavioral Neuroscience · Neurodevelopment
Growth and Development of the Early Brain
On this page 9 sections
In 30 seconds
By the end of the third to fourth week after conception, neurulation has closed the neural tube — the hollow cylinder that will become the entire central nervous system (see Topic 1). The story of this topic is how that simple tube becomes the folded, regionally specialized organ we recognize as the brain. Early development proceeds as an orderly cascade: the tube's front end balloons into vesicles that prefigure the major brain regions; huge numbers of neurons are produced near the central cavity; they migrate outward to their final positions; and each cell differentiates, extending axons and dendrites that lay down the first wiring.
Keep a central theme in mind: brain development is a chain of dependent events, and a disruption at one step produces a different outcome than at another. Knowing which step failed lets you predict the consequence — how clinicians reason about early brain disorders.
Why this matters
The early brain's developmental steps explain many real-world observations:
- Congenital conditions: Structural malformations — microcephaly, lissencephaly ("smooth brain"), heterotopia (misplaced neurons) — trace back to failures of specific developmental steps, so recognizing the step narrows possible causes.
- Prenatal health: Factors that disturb early development — infections, toxins, nutritional deficiencies — are most dangerous while the affected step is underway; timing matters.
- A foundation for everything else: Every later topic — synapses, plasticity, sensory systems, learning — assumes a correctly built brain.
- Exam logic: Questions about brain development almost always test sequence — which event precedes which and which adult structure comes from which vesicle.
The college version
Core Concepts
From three vesicles to five: the regional blueprint
Early in the fourth week of development (commonly taught timeline — verify against your text), the tube's anterior end expands into three primary vesicles:
- Prosencephalon (forebrain)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain)
Each Primary vesicle Early swelling of the neural tube that prefigures a major brain region Full entry → then subdivides to form five secondary vesicles, which map directly onto adult structures:
| Secondary vesicle | Derived from | Major adult derivatives |
|---|---|---|
| Telencephalon | Prosencephalon | Cerebral hemispheres: cortex, basal ganglia, hippocampus, amygdala, olfactory bulbs |
| Diencephalon | Prosencephalon | Thalamus, hypothalamus, epithalamus (including the pineal gland) |
| Mesencephalon | Mesencephalon | Midbrain: superior and inferior colliculi (tectum), tegmentum, substantia nigra |
| Metencephalon | Rhombencephalon | Pons and cerebellum |
| Myelencephalon | Rhombencephalon | Medulla oblongata |
The tube also bends at the cephalic flexure so the brain fits into the developing head. Notice what the table teaches: the forebrain gives rise to the enormous cerebral hemispheres — which is why the human brain's most distinctive features come from its most anterior region.
Cell proliferation: the neuron factory
Neurons are born near the tube's central cavity in the Ventricular zone Cell layer lining the tube's cavity where progenitors divide Full entry →. Progenitors divide rapidly — early divisions symmetric (more progenitors), later divisions asymmetric (one progenitor plus one neuron). Neurogenesis proceeds in waves.
Two facts matter most:
- Most human neurogenesis is prenatal. The vast majority of neurons are generated before birth; commonly cited estimates put the adult brain at roughly 86 billion neurons (treat as an estimate to verify).
- Adult neurogenesis is limited. Reliable adult production is confined to a few regions — classically the hippocampal dentate gyrus and the olfactory bulb — at far lower rates than during development, and the details remain an active research area.
Neuron migration: moving to the right address
After their final division, most neurons migrate outward to the cortical plate along elongated radial glial cells, guided by reelin released by Cajal-Retzius cells at the brain's surface, which signals neurons when to stop.
The cortex builds inside-out: deep layers (VI, V) form first, and each successive wave migrates past earlier neurons to settle above them, so superficial layers (IV, II) form last — a classic exam fact. When migration fails (e.g., disrupted reelin signaling), neurons land in the wrong layers, producing lissencephaly (smooth cortex) or heterotopias (misplaced neuron clusters).
Differentiation and axon outgrowth: cells find their roles
Once positioned, neurons differentiate: sprouting dendrites, extending an axon, and expressing the channels and receptors that let them communicate. The axon tip, the Growth cone Motile tip of a growing axon that senses guidance cues Full entry →, steers toward attractive and away from repulsive cues using guidance molecules (netrins, semaphorins, ephrins, slits — detailed in Topic 3). The earliest long-distance tracts, such as the corpus callosum, are laid down prenatally.
As the cortex grows it folds into gyri (ridges) and sulci (grooves) — Gyrification Formation of gyri and sulci, the brain's folds Full entry → — packing more surface area into the skull; its timing marks normal maturation.
Wiring, sculpting, and early myelination
The early brain is built with generous overproduction, then sculpted:
- Apoptosis Programmed cell death Full entry → (programmed cell death): many regions generate more neurons than the final brain keeps — commonly taught figures suggest roughly half are eliminated. Neurons compete for trophic factors (survival signals such as NGF and BDNF); useful, active connections win the support they need.
- Myelination Glial insulation of axons Full entry → begins prenatally (earliest in spinal cord and brainstem) but continues for decades — one reason brain maturation is so drawn out.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Primary vesicles | Secondary vesicles | Three vs. five; secondary vesicles are subdivisions of primary ones |
| Proliferation | Migration | Making new neurons vs. moving existing neurons to their destination |
| Inside-out layering | Outside-in layering | Cortex forms deep-to-superficial; "superficial layers first" is a classic test trap |
| Prosencephalon (forebrain) | Telencephalon | The forebrain splits into telencephalon + diencephalon; they are not synonyms |
| "Neurogenesis continues everywhere in adults" | Limited-region adult neurogenesis | Only a few regions (classically hippocampus, olfactory bulb) show reliable adult neurogenesis |
| Gyri | Sulci | Ridges/folds (gyri) vs. grooves (sulci) |
| Failure of neurulation | Failure of migration | Tube closure (Topic 1) vs. neuron positioning (this topic) — different steps, different defects |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The early brain starts as a simple straw-like tube. Its front end puffs up into three balloons, then five — like a city map dividing into neighborhoods — and each neighborhood becomes a different part of the brain. Billions of nerve cells are made near the center and crawl outward to their addresses. If a cell moves to the wrong address, the whole city works differently.
Worked example
Follow a single cortical neuron from "birth" to "career." It is produced by an asymmetric division in the ventricular zone around mid-gestation, then climbs a radial glial scaffold past earlier-born neurons in the deep layers. Guided by reelin, it stops in what will become layer V of the visual cortex. There it differentiates: dendrites grow toward synaptic inputs, and its axon extends — growth cone leading — through the internal capsule toward the thalamus. It competes for trophic support, survives, and later (Topic 3) its synapses are strengthened or pruned by the child's visual experience.
Now change one step: migration stalls, so the neuron never reaches the cortical plate and is stranded in the white matter — a heterotopia. One altered step illustrates the chapter's core lesson: each process depends on the one before it, and the location and timing of a failure determine the consequence.
Key takeaways
- Three primary vesicles → five secondary vesicles; know the adult fate of each (telencephalon and diencephalon come from the forebrain).
- Neurons are born in the ventricular zone and migrate outward along radial glia.
- The cortex builds inside-out: deep layers first, superficial layers last.
- Most human neurogenesis is prenatal; adult neurogenesis is restricted to limited regions (classically hippocampus and olfactory bulb).
- Growth cones steer axons using molecular guidance cues.
- Apoptosis and pruning sculpt the brain; trophic factors keep useful neurons alive.
- Gyrification and myelination extend past birth; myelination continues for decades.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
List the three primary and five secondary vesicles.
Show answer
Primary: prosencephalon, mesencephalon, rhombencephalon. Secondary: telencephalon, diencephalon (from forebrain), mesencephalon, metencephalon, myelencephalon (last two from hindbrain).
Where are neurons born, and what structure do most of them use to migrate to the cortex?
Show answer
In the ventricular zone lining the tube's cavity; they migrate outward along radial glial scaffolds.
In what order do cortical layers form?
Show answer
Inside-out: deep layers (VI, V) first; each new wave passes earlier neurons, so superficial layers (IV, II) form last.
Which adult structures derive from the telencephalon, and which from the metencephalon?
Show answer
Telencephalon → cerebral hemispheres (cortex, basal ganglia, hippocampus, amygdala); metencephalon → pons and cerebellum.
Name two processes that reduce neuron or connection numbers during development, and why does the brain overproduce?
Show answer
Apoptosis (programmed cell death) and later synapse pruning remove excess cells/connections. Overproduction lets activity and competition select the most useful circuits — "build more, keep the best."
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Primary vesicle
- Early swelling of the neural tube that prefigures a major brain region
- Secondary vesicle
- Subdivision of a primary vesicle (five total)
- Ventricular zone
- Cell layer lining the tube's cavity where progenitors divide
- Radial glia
- Scaffold cells that guide migrating neurons outward
- Inside-out layering
- Deep cortical layers form before superficial ones
- Growth cone
- Motile tip of a growing axon that senses guidance cues
- Apoptosis
- Programmed cell death
- Gyrification
- Formation of gyri and sulci, the brain's folds
- Myelination
- Glial insulation of axons
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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