Pioneer neurones in an insect embryo

INSECT sense organs are produced by small groups of specialised epidermal cells1. The receptor neurones differentiate at the surface, so developing sensory axons grow inwards from the epidermis to the central nervous system (CNS). How do they find their way? During larval life the axons of newly differentiated sense cells combine with those of neighbouring receptors and thus are guided to the nearest branch of a peripheral nerve which carries them to the CNS2. At metamorphosis the axons of adult sensory neurones reach their central destination by growing along persistent larval nerves which are associated with the developing imaginal disks3–5. Thus with pathways to the ganglia already established, growth along existing nerves ensures the delivery of each generation of sensory axons to within a few hundred micrometres of their central targets. Just how the connection between the surface and the CNS is first established, whether by an outgrowth of nerves from the centre or by pioneering axons which grow inwards from the surface has never been shown, although some descriptions of embryonic development imply that the first axons to enter the developing appendages are growing outwards from the CNS6,7. Presumably these early centrifugal axons would provide a route for the later differentiating sensory fibres to follow in growth to the centre. Here, however, I report observations on the embryonic nervous system of Locusta migratoria which show that the first pathways between the epidermis and the central ganglia are formed by axons which grow inwards from peripheral neurones which differentiate early in embryonic life.