Abstract
The origin and morphogenesis of pioneer neurons were examined in the developing metathoracic leg of cultured grasshopper embryos. The first two pioneer neurons to differentiate in the limb-bud were studied using continuous observation, time-lapse microphotography, and intracellular dye-fills. A single mother cell located within the ectodermal epithelium at the limb-bud tip divided symmetrically to yield the first pair of sister pioneer neurons. These neurons migrated out of the ectoderm in a morphologically undifferentiated state and initiated axonogenesis at a characteristic location between the ectoderm and the inner mesodermal epithelium of the limb-bud. Following a period of initially undirected filopodial sprouting, the axon pair grew proximally along a highly sterotyped path between the limb-bud ectoderm and mesoderm. The path taken by the pioneer axons did not follow any gross anatomical features of the adjacent limb-bud epithelia. The pioneer neurons described in this paper ultimately extend from the proximal end of the tibia into the metathoracic ganglion, and they probably give rise to nerve branch 5bl of the metathoracic leg.
Published Version
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