Neural Development 2: Neuronal Connections

Neuroblasts migrate along radial glia to the outer cortex (radial migration).
Preplate separates into marginal zone and subplate; cortical plate forms in-between.
Neuroblasts move into the subventricular zone, then use radial glia to migrate radially.
Cortical plate forms between marginal zone (Cajal-Retzius cells) and subplate neurons.
Neurons migrate to form the cortex in an inside-out manner (newer cells further out).
Tangential migration from ganglionic eminence contributes interneurons to specific layers.
Cajal-Retzius cells secrete reelin, signaling neuroblasts to stop migration; mutations cause abnormal layering.

Neurons connect by extending neurites with growth cones.
Growth cones are guided to targets, similar to cell migration.
Environmental cues can be attractive or repulsive, acting at long or short ranges.
Intracellular responses affect microfilaments, causing polymerization/depolymerization of actin.
Growth cones use cell adhesion; pioneer axons guide others.
Retinal ganglion neurons use Eph receptors and Ephrin ligands for retinotopic mapping.
Some axons cross the midline and avoid re-crossing, involving Slit and Robo.
Comm prevents Robo expression until midline crossing, where netrin-frazzled signaling downregulates Comm, allowing Robo expression and Slit repulsion.

Synaptogenesis involves contact, stabilization, adhesion, signaling, and maturation.
Forming a neuromuscular junction requires signaling between cells to develop specializations.
At neuromuscular junctions, multiple axons converge, but many are eliminated; the surviving axon branches and is sheathed by Schwann cells.

More than half of neurons die during normal development, regulated by target tissue innervation.
Motor neurons that fail to make synapses undergo apoptosis; mutations in Fz3 can cause stalled migration and apoptosis.
Target tissue produces survival factors (neurotrophins) like NGF, which are endocytosed and retrogradely transported to the cell body.
Neurotrophins also play a role in axon guidance and synapse formation.
Different neuronal populations respond to different neurotrophins (e.g., NGF, BDNF).

Neurogenesis is followed by gliogenesis, with changing expression patterns.
The central nervous system consists of grey matter (neuronal cell bodies) and white matter (axon tracts).
Peripheral nervous system includes sensory, motor, and interneurons.
Ganglia are aggregations of neuronal cell bodies with satellite cells.
Peripheral nerves consist of axons (sensory and motor) with myelinating Schwann cells.