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The neurotrophic hypothesis: where does it stand?
1School of Biological and Medical Sciences, University of St. Andrews, Fife, UK.
Summary
Target fields regulate neuron numbers in the developing nervous system. Neurotrophic factors, like nerve growth factor (NGF), are crucial for neuron survival, though their roles are complex and varied.
Area of Science:
- Neuroscience
- Developmental Biology
Background:
- Neuronal survival in the developing peripheral nervous system is regulated by target fields.
- The neurotrophic hypothesis, initially based on nerve growth factor (NGF), posits that target-derived neurotrophic factors are supplied in limiting amounts to match neuron numbers with target field size.
- Recent findings indicate that neurotrophic factor action is more complex, involving multiple factors and varied modes of action.
Purpose of the Study:
- To explore the mechanisms by which target fields regulate neuronal survival and innervation density.
- To review the evolution and current understanding of the neurotrophic hypothesis in the context of peripheral nervous system development.
Main Methods:
- Review of existing literature on neurotrophic factors and neuronal development.
- Analysis of experimental evidence demonstrating the influence of target field size on neuronal survival.
- Examination of studies investigating the roles of various neurotrophins and their modes of action (e.g., autocrine signaling).
Main Results:
- Target field size significantly impacts the number of surviving neurons, supporting the concept of target-derived regulation.
- Multiple neurotrophic factors, acting concurrently or sequentially, regulate the survival of certain neuronal populations.
- Some sensory neurons exhibit dependence on neurotrophins before significant cell death, suggesting non-limiting supply at early stages, and may utilize autocrine signaling.
Conclusions:
- The neurotrophic hypothesis, despite complexities, remains the primary framework for understanding how target fields control innervation density in the developing peripheral nervous system.
- Neurotrophin action is multifaceted, extending beyond the classical model to include pre-emptive and autocrine signaling pathways.
- Further research is needed to fully elucidate the intricate interplay of neurotrophic factors in regulating neuronal development and survival.