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Influence of grafting a smaller target muscle on the magnitude of naturally occurring trochlear motor neuron death
G S Sohal1, S D Stoney, L R Campbell
1Department of Anatomy, Medical College of Georgia, Augusta 30912.
Abstract:
About half of the motor neurons produced by some neural centers die during the course of normal development. It is thought that the size of the target muscle determines the number of surviving motor neurons. Previously, we tested the role of target size in limiting the number of survivors by forcing neurons to innervate a larger target (Sohal et al., '86). Results did not support the size-matching hypothesis because quail trochlear motor neurons innervating duck superior oblique muscle were not rescued. We have now performed the opposite experiment, i.e., forcing neurons to innervate a smaller target. By substituting the embryonic forebrain region of the duck with the same region of the quail before cell death begins, chimera embryos were produced that had a smaller quail superior oblique muscle successfully innervated by the trochlear motor neurons of the duck. The number of surviving trochlear motor neurons in chimeras was significantly higher than in the normal quail but less than in the normal duck. The smaller target resulted in some additional loss of neurons, suggesting that the target size may regulate neuron survival to a limited extent. Failure to achieve neuron loss corresponding to the reduction in target size suggests that there must be other factors that regulate neuron numbers during development.
Insights
Target muscle size influences motor neuron survival during development. Reducing target size in experiments led to some neuron loss, but other factors also regulate motor neuron numbers.
Area of Science:
- Developmental neuroscience
- Neurobiology
- Cell biology
Background:
- Motor neuron development involves significant cell death, with target size hypothesized to regulate survival.
- Previous experiments altering target size did not fully support the size-matching hypothesis for neuron survival.
Purpose of the Study:
- To investigate the role of target size in regulating motor neuron survival by forcing innervation of a smaller target.
- To determine if reducing target size can induce greater motor neuron loss.
Main Methods:
- Creation of quail-duck chimera embryos by transplanting embryonic forebrain regions.
- Innervation of a smaller quail superior oblique muscle by duck trochlear motor neurons in chimera embryos.
- Quantification of surviving trochlear motor neurons in chimera embryos compared to normal controls.
Main Results:
- The number of surviving motor neurons in chimeras was intermediate between normal quail and normal ducks.
- Innervating a smaller target resulted in some additional motor neuron loss, but not proportional to the size reduction.
- These findings suggest target size regulates neuron survival to a limited extent.
Conclusions:
- Target size is one factor, but not the sole determinant, of motor neuron survival during development.
- Other unidentified factors play a significant role in regulating the final number of motor neurons.