2 Neuroscience2.10 Neurodevelopment and neuroplasticity
2.10.1 Development and localisation of cerebral functions throughout the lifespan from foetal stages onwards
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Written by Resident Doctor Committee · Published
Neural tube and neural crest
Neurulation begins in week 3, when notochord signals turn dorsal ectoderm into neuroectoderm, which folds into the neural tube between days 18 and 28.
The tube's lumen becomes the ventricles and central canal, and failed closure causes anencephaly rostrally or spina bifida caudally.
Neural crest cells arise from the dorsal-most edge of the folding neural plate and closing neural tube, so they are neuroectoderm derived from ectoderm, not a separate germ layer.
The neural crest forms most of the peripheral nervous system: dorsal root and autonomic ganglia, Schwann cells, adrenal medulla and melanocytes.
Embryonic brain vesicles
Three primary vesicles form in week 4. By week 5 the prosencephalon (forebrain, the most rostral) and the rhombencephalon (hindbrain) have each divided in two, giving five secondary vesicles.
Primary vesicle | Secondary vesicle | Adult derivatives | Cavity |
|---|---|---|---|
Prosencephalon | Telencephalon | The cerebrum: all cerebral cortex including the hippocampus, the amygdala, basal ganglia and olfactory bulbs | Lateral ventricles |
Prosencephalon | Diencephalon | Thalamus, hypothalamus including the mammillary bodies, subthalamus, epithalamus and pineal, and their outgrowths | Third ventricle |
Mesencephalon | Stays whole as the mesencephalon | All of the midbrain: colliculi, red nucleus, substantia nigra, cerebral peduncles, tissue around the aqueduct | Cerebral aqueduct |
Rhombencephalon | Metencephalon | Pons including the locus coeruleus, and the cerebellum from the rhombic lips | Upper fourth ventricle |
Rhombencephalon | Myelencephalon | Medulla oblongata, including the area postrema at the obex | Lower fourth ventricle |
The posterior pituitary grows down from the floor of the diencephalon as the infundibulum, so it is neural tissue.
The retina and optic nerve come from the optic vesicles, which grow out of the diencephalon, not the telencephalon, so both belong to the central nervous system.
Building the cortex
Radial glia act as progenitors and as scaffolds, and excitatory neurones migrate along them, so deep layers VI and V form before superficial layers.
Migration runs mainly from weeks 12 to 24, and its disruption causes lissencephaly, heterotopia or polymicrogyria.
Gyrification is the folding of the cortical surface into gyri and sulci, which greatly enlarges cortical surface area during fetal life.
Maturation timetable
Brain regions mature in a fixed order, and folding finishes far earlier than synapse formation, pruning, myelination and cortical thinning.
Process | Timing |
|---|---|
Gyrification | Sulci appear about week 16; largely complete before birth, with the last neonatal folding finished by about age 2 |
Synaptogenesis | Synapses are overproduced after birth, peaking in the first two postnatal years |
Myelination | Begins prenatally; the optic nerve, a central tract, is myelinated around birth |
Grey matter thinning and myelination of dorsolateral prefrontal association cortex | Last to finish, in the early to mid twenties |
Myelination runs caudal to rostral and posterior to anterior, with projection and sensory fibres before association fibres, so frontal association regions finish last.
Programmed cell death removes whole neurones that fail to secure trophic support, whereas synaptic pruning removes single connections and the neurone survives.
Pruning keeps connections whose firing is correlated and weakens redundant input.
Critical and sensitive periods
A critical period is a window in which experience is required, after which equivalent development is largely impossible, as when an uncorrected congenital cataract permanently degrades acuity.
A sensitive period is a window in which experience has an outsized effect but later change remains possible, as in second-language phonology.
Adolescence and ageing
In adolescence cortical grey matter decreases while white matter increases, and reward systems can respond strongly before control networks mature.
Ageing involves synaptic and dendritic loss rather than widespread neuronal death, and processing speed declines more than established knowledge.