Axonal Growth Cones & Neural Circuit Formation | Neurobiology

The final composition of cortical circuits is dynamically regulated by neural activity during critical postnatal periods. Increasing pyramidal cell activity between postnatal days 5-10 results in more surviving interneurons, while decreasing activity produces fewer interneurons. This demonstrates that neural activity serves as a survival signal for inhibitory neurons. The molecular machinery underlying this process includes caspases for cell death execution and Bax/Bak proteins for apoptosis regulation. These findings reveal that cortical circuit refinement is not predetermined but actively shaped by experience-dependent activity patterns during development.

Brain development involves critical periods or 'windows of development' during which the brain is particularly sensitive to environmental input. During these periods, the brain requires specific stimulation to develop properly. If the brain does not receive appropriate input during these critical periods, the affected systems may degenerate and cannot be recovered later. Brain development also involves synaptic pruning, where the brain initially produces an excessive number of synapses and then eliminates those that are not functional. This process refines neural circuits based on experience and activity. The brain produces an excessive number of synapses early in development and then prunes away those that are not functional, retaining only those that are used and strengthened by experience.

Synaptic pruning is the natural process of eliminating excess neuronal synapses during brain maturation, where the brain initially develops twice as many connections as adults and then removes approximately half through an activity-dependent mechanism following the 'use it or lose it' rule; this refinement process, primarily occurring during adolescence and continuing into young adulthood, helps sculpt efficient neural circuits, and impaired pruning has been linked to neurological disorders such as schizophrenia (associated with excessive pruning) and autism spectrum disorders/epilepsy (associated with insufficient pruning).

Critical periods are developmental windows when experience shapes brain circuits optimally. During these periods, unused synapses are pruned away while used ones are stabilized, creating efficient adult neural circuits. Children have more synapses at birth than adults, and this pruning creates stable circuits. However, the adult brain retains some latent plasticity that can be recruited under appropriate conditions. The key insight is that understanding molecular mechanisms controlling pruning could potentially allow scientists to reopen critical periods in adults, enabling brain repair after injury or treating diseases like Alzheimer's by restoring lost synapses for new memory formation.

This section presents two fundamental principles of neural development: Neural Darwinism and Hebbian learning. Neural Darwinism proposes that unused synapses are eliminated during development, with the principle 'use it or lose it' governing neural circuit refinement. Hebbian learning states that neurons that fire together become connected, explaining how the brain develops the ability to perceive objects as whole entities. The lecture explores the remarkable plasticity of neural networks during early development, explaining that different brain regions undergo synaptic overproduction and pruning at different times. The frontal neocortex, particularly enlarged in humans, undergoes synaptic development later than other regions. The speaker describes experiments demonstrating that neural functional organization can be dramatically altered during critical periods, allowing the brain to adapt its organization based on developmental experience.

Neural regeneration—the regrowth or repair of nerve tissues including neurons, axons, myelin, and synapses—occurs naturally in the peripheral nervous system but is severely limited in the central nervous system; in the PNS, injured nerves trigger macrophage-mediated debris clearance, allowing axonal sprouts to regenerate at rates of 2-5 mm/day through Schwann cell-guided pathways, while the CNS forms a hostile environment with inhibitory myelin-associated proteins (like Nogo-A and MAG), reactive astrocytes, and glial scars that prevent axon regrowth across injury sites.

This section explains why the peripheral nervous system can regenerate while the central nervous system cannot. Wallerian degeneration involves axonal breakdown and myelin fragmentation that macrophages must clear. The PNS regenerates because Schwann cells guide axon regrowth and form regeneration tubes. The CNS lacks Schwann cells and instead has astrocytes that form scar tissue. Astrocytes secrete factors that inhibit axon regrowth, creating a hostile environment for regeneration. This fundamental difference explains why CNS injuries often result in permanent neurological deficits.

Peripheral and central nervous systems differ fundamentally in regenerative capacity. Peripheral nerves can regenerate because Schwann cells clear debris, produce growth factors, and guide axon regrowth. Following surgical repair, Schwann cells help bridge gaps and facilitate functional recovery. In contrast, the central nervous system lacks this capacity because oligodendrocytes depend on continuous axonal signals to survive; without intact axons, supporting cells die, preventing regeneration. This explains why spinal cord injuries typically result in permanent paralysis despite similar initial injury severity.

Axons in the peripheral nervous system (PNS) can regenerate after injury, while axons in the central nervous system (CNS) generally cannot. This difference is due to the presence of Schwann cells in the PNS that support regeneration, and the inhibitory environment in the CNS. Regeneration in the PNS occurs at approximately 1 mm per day.

Myelinated neurons have very limited regeneration capacity. For successful peripheral nerve regeneration, two critical conditions must be met: the neuron's cell body must remain intact, and Schwann cells must remain functional. In contrast, the central nervous system (brain and spinal cord) has very limited or no ability to repair damaged neurons, even when the cell body remains intact. This fundamental difference explains why peripheral nerve injuries can heal while brain and spinal cord injuries are typically permanent.

Horizontal gaze palsy with progressive scoliosis results from axon guidance gene mutations. Robo3 mutation causes classic phenotype with concave pons from midline cleft and absent facial colliculi. DCC mutation causes similar phenotype with corpus callosum agenesis (type 2). Pontocerebellar cap dysplasia shows transversely organized fibers in abnormal position, causing hypotonia, cranial nerve deficits, and possible cardiac/skeletal abnormalities. Dandy-Walker malformation (hypoplastic vermis communicating with fourth ventricle, enlarged posterior fossa) is a phenotype, not specific diagnosis, associated with multiple genetic conditions. Recent evidence suggests vascular insults to rhombic lip before 14th week may cause Dandy-Walker malformation, indicating it may be a phenotype rather than specific diagnosis.

Corpus Callosum disorders result from disrupted development during weeks 12-16 of gestation when the interhemispheric fissure closes like a zipper. Glial cells migrate forward to create a bridge allowing axons to cross midline. Disorders are classified as complete agenesis (entire structure absent), partial agenesis (abnormally thin connections), or hypoplasia (underdeveloped but present). Absence causes characteristic ventricular changes called colpocephaly with enlarged posterior horns. Advanced imaging reveals axons rerouting anterior-posterior instead of crossing midline, creating 'Bundes of Bres.' This represents a malformation where fibers exist but follow incorrect pathways. The condition frequently co-occurs with neuronal migration disorders like double cortex syndrome.

Corpus callosum agenesis results from failure of callosal development, characterized by absent corpus callosum, parallel ventricular configuration, and prominent periventricular white matter tracts. Holoprosencephaly results from incomplete forebrain cleavage, classified as alobar (complete failure), semilobar (partial cleavage), or lobar (mild anterior fusion). Septo-optic dysplasia involves septum pellucidum agenesis, optic nerve hypoplasia, and pituitary abnormalities. Neuronal migration disorders result from abnormal migration during the critical period (6 weeks to 5 months gestation). Lissencephaly features smooth cortical surface, thickened cortex (4-5mm), and abnormal sulcal pattern. Schizencephaly involves clefts in the cerebral hemisphere, classified as open-lip or closed-lip.

The Corpus callosum is the largest fiber track in the brain, connecting the two cerebral hemispheres, and is only present in placental mammals. Agenesis of the Corpus callosum (ACC) occurs in over 300 congenital syndromes, with symptoms ranging from high-functioning individuals with social interaction problems to severe cases like Aardi syndrome. Midline guidance mechanisms are highly conserved from flies to humans, involving growth cones with receptors for guidance cues (Slit, Semaphorin, Netrin) expressed by glial cells regulated by Nuclear Factor one transcription factors. In individuals with complete ACC, the septum pellucidum shows failure of midline remodeling, preventing collosal axons from crossing the midline. Early in mouse brain development, the brain begins as a single prosencephalon that grows out to form two telencephalic hemispheres with a large fissure between them, requiring a major remodeling event for axon crossing.

This comprehensive section covers the molecular and cellular mechanisms governing corpus callosum development and the spectrum of axonal guidance disorders. The neurexin-1 receptor (NRP-1) plays a crucial role in regulating axonal topography, controlling the formation of specific somatotopic columns in somatosensory areas. Experimental manipulation through in utero electroporation reveals that NRP-1 controls somatotopic connectivity—knockdown delays refinement and reduces branching, while overexpression reduces actual branching without affecting proportions. The excitatory-inhibitory balance during early postnatal development critically influences corpus callosum formation, with diazepam administration demonstrating that disrupting this balance alters both callosal and intramural networks. Axonal guidance disorders represent a heterogeneous group of conditions arising from abnormalities in axonal growth cone signaling, ligand-receptor interactions, and cytoskeletal proteins. Key genetic syndromes include L1 syndrome (hydrocephalus, abnormal corpus callosum, brainstem/cerebellar hypoplasia), Joubert syndrome (midbrain-hindbrain malformations with molar tooth sign), and tubulinopathies (abnormal cortical development). Prenatal diagnosis requires understanding brain development timelines, with key findings including persistence of brainstem kinking, ventricular stenosis, abnormal fourth ventricle shape, and absence of corpus callosum primordium after 14 weeks gestation.