Axon Guidance, Wiring, and Neurodevelopment
Building Circuits During Development
Developmental Logic
Neural circuits are assembled by transcriptional programs that specify progenitor competence, guidance cue receptors, adhesion molecules, and synaptic organizers. Axons and dendrites interpret attractive and repulsive cues, then prune and refine connections through activity-dependent competition.
Canonical Guidance Systems
Why are guidance cues described as context-dependent?
The response depends on receptor composition, co-receptors, and intracellular signaling state.
Correct answer: Because the same cue can elicit different responses depending on receptor and intracellular state
What is one mechanism by which developing circuits are refined after initial wiring?
Weak or inappropriate connections are eliminated to improve circuit specificity.
Correct answer: Activity-dependent synaptic pruning.
Developmental vs Adult Plasticity
Developmental wiring
- Establishes initial connections
- Highly dependent on guidance cues and cell migration
Adult plasticity
- Modifies existing connections
- More dependent on experience, activity, and local signaling
Frontier Topic
A major frontier is understanding how developmental programs are reactivated or disrupted in neurodevelopmental disorders, and how early molecular perturbations propagate into lifelong circuit changes.
Open Problems
We still lack a complete molecular code for synapse specificity in complex vertebrate brains. Another challenge is connecting rare developmental mutations to cell-state-specific and circuit-specific phenotypes across developmental time.
Dissecting Developmental Mechanisms
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Step 1: Identify the developmental stage and cell population.
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Step 2: Map expression of guidance receptors and adhesion molecules.
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Step 3: Perturb candidate cues or receptors.
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Step 4: Quantify migration, branching, and synapse formation.
Which family is most associated with contact-mediated positional signaling?
Eph receptors and ephrins mediate contact-dependent signaling that helps pattern tissues and circuits.
Correct answer: Ephrins