Anatomical experiments are proposed to study developmental processes which serve to establish, refine and stabilize the initial innervation pattern of the chick wing. The results of these experiments will provide additional basic information about the cellular mechanisms involved in establishing and maintaining connections between neurons and their targets. This information may be important for understanding how development processes may be disturbed as a result of disease, injury or genetic defects. Three sets of experiments are proposed. In the first, the major emphasis will be on the period between muscle nerve formation and the post-hatching elimination of polyneuronal innervation. The general objective is to determine how projection patterns of single neurons are initially formed and subsequently modified. Preliminary experiments have shown extensive axonal branching, including some collateral having projections to two different nerves. Experiments to determine what neuron types form two- nerve collaterals, and the mechanism of their subsequent loss are proposed. In other experiments, the pre-terminal branching patterns of individual motor axons will also be characterized to determine the spatial extent of single fiber projections, and to determine when the preterminal branching pattern becomes fixed. The methods involve labeling specific neuronal populations with different dyes and the reconstructing projection patterns using light microscopy. These experiments should provide information about the refinement of initial projection patterns, and about the processes involved in the establishment of the fine-grained topographic projections of motoneurons onto muscles. Another set of proposed experiments is to study the ability of a neurotrophic factor to prevent motoneuron death in chick embryos having one muscleless wing. The treatment regimens needed to rescue target deprived motoneurons will be determined. A related study will determine whether cell death is permanently blocked by factor administration, or whether cell death is delayed. A possible relationship between axon branching and neuron survival will be studied by characterizing the pre-terminal branching patterns in muscleless and factor-treated muscleless limbs. The third set of experiments will explore the role of Schwann cells in limb innervation. Chick/quail chimeras will be used to specific ally label neural crest-derived Schwann cells along the peripheral nerve pathways. The distribution of Schwann cells along the peripheral nerves, will be studied at different developmental stages in an attempt to confirm our previous finding that Schwann cells cluster at muscle nerve branching points early during peripheral nerve formation. Experiments to determine whether peripheral nerve formation is delayed or prevented in the absence of Schwann cells are also proposed.
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