The abdominal ganglion neurones responsible for the secretion of the peptide hormone bursicon from the transverse nerves of the tobacco hornworm Manduca sexta have been localized. Each unfused abdominal ganglion produced bursicon during adult development. The hormone was then transported to the next posterior transverse nerve for subsequent release following adult emergence. Of the two efferent pathways that connect ganglia to posterior transverse nerves, only one had ultrastructural features consistent with a neurosecretory function. All axons in this pathway contained numerous, elementary, dense-cored granules with diameters ranging from 100 to 300 nm.
In pharate adults, cobalt back-filling of this neurosecretory pathway revealed two groups of cell bodies in abdominal ganglia: three dorso-laterally and eight at the mid-line. Hormonal activity was present in lateral regions of the ganglion but not in mid-line regions. In the dorso-lateral region, four blueish cell bodies could be seen in the living tissue. The two combined clusters of four cells from both sides of the ganglion contained more than 60% of the bursicon activity present in the entire ganglion. More careful dissection of the cluster detected the presence of activity in cell pairs and individual members.
Individual dye-fills of these blue cells demonstrated that three of these were the three lateral neurones that were back-filled from the transverse nerve. The fourth blue cell may be identical to a neurone whose axon travels in the descending, contralateral connective and exits the nervous system via the next posterior ganglion. On the basis of dendritic geometry, each of the neurones in the bursicon cell cluster could be identified as a unique, individual neurone.