Origin and morphogenesis of sensory neurons in an insect antenna

Each antennal flagellum of the moth, Manduca sexta, contains about 2.5 × 10⁵ primary sensory neurons. The neurons are components of small sensory organs (sensilla) and send axons through antennal nerves to the brain. The neurons, sensilla, and nerves differentiate as the antenna develops, during the 18 days of metamorphosis from pupa to adult. Neurons arise from divisions of epidermal cells between 25 and 60 hr after pupal ecdysis and elaborate axons and dendrites soon thereafter. Neurons have the bipolar form, ciliated dendrite, and glial sheath characteristic of the adult within a few days of their birth. The axons grow along small pupal nerves to form the adult antennal nerves, and the dendrites grow beyond the apical margin of the epidermis, where they are enveloped by a growing process of the sensilla's trichogen cell. Cuticle secreted by the trichogen cell forms the seta or sensory hair of the sensillum. Later, the neuronal somata migrate from the basal to the apical margin of the epidermis. Finally, the cytoplasm withdraws from the seta, leaving the dendrites imprisoned in a cylinder of cuticle. All of the neurons in the flagellum differentiate nearly synchronously, facilitating correlation of morphogenetic results presented here with biochemical and electrophysiological analyses of the developing neurons.