Sensory receptor differentiation and axonal pathfinding in the cercus of the grasshopper embryo.

An immunological probe selective for insect neurons (L.Y. Jan and Y.N. Jan, 1982, Proc. Nat. Acad. Sci. USA 79, 2700-2704) was used to characterize the genesis of sensory neurons and the formation of the peripheral nerves in the grasshopper cercus. During embryogenesis the cercal ectoderm produces a characteristic set of sensory neurons in a precise spatiotemporal order. The first neurons migrate from the epidermis into the lumen and send out axons toward the CNS along the epidermal wall. These luminal neurons arise in four distinct groups, each of which establishes a separate branch of the cercal nerve, with the axons of the three distal groups converging onto the cell bodies of the more proximal neurons and thus seeming to use them as an intermediate target on route to the CNS. Epidermal neurons, whose cell bodies remain within the epithelium, begin to appear at a later stage. These cells come to innervate external sensory hairs, and in general their axons grow to the CNS along the preexisting nerves. Each sensory hair possesses two nonneuronal cells--the trichogen (shaft-forming) and tormogen (socket-forming) cells--which also stain with the antibody and begin to display immunoreactivity at the same time as the allied sensory neuron. The trichogen and tormogen cells do not form the hair shaft and socket until much later, with outgrowth occurring in an order quite distinct from that in which the receptors undertook their initial, biochemical differentiation. Thus, these two aspects of trichogen/tormogen differentiation appear to be under separate developmental control.