Development of the trochlear nerve: Effects of early removal of periphery

Development of the trochlear nerve after early removal of the superior oblique muscle was studied in white Peking duck embryos. Unilateral optic vesicles together with the surrounding mesoderm were removed on Day 4 of incubation. Axon counts of the normal nerve indicate that the trochlear fibers sprout profusely shortly after contacting the periphery. Subsequently, large numbers of fibers are lost from the developing nerve. Most of the fiber loss appears to be via retraction. A significant number of trochlear neurons are also eliminated during this period. The direction of cell death may be from soma to axon. In the axon initial signs of degeneration appear in the axoplasm which are later followed by myelin deterioration. The peripherally deprived trochlear nerves grow properly to the region of their target organ. Nerve sprouting is delayed and occurs at a significantly reduced rate. Our observations indicate that nerve sprouting is triggered by the periphery. Some sort of interaction with the periphery appears to be necessary for myelination. Electrical stimulation on Day 26 of incubation indicates that the peripherally deprived trochlear fibers do not make inappropriate connections.