The morphology of neurosecretory neurones in the pond snail,Lymnaea stagnalis, by the injection of Procion Yellow and horseradish peroxidase

Abstract

The morphology of neurosecretory neurones, the Dark Green Cells, Yellow Cells, Yellow-green Cells, Light Green Cells, Caudodorsal Cells and Canopy Cells, in the central nervous system of the snail,Lymnaea stagnalis, was investigated by the intracellular injection of Procion Yellow and, for the Yellow Cells only, of horseradish peroxidase. The cerebral ganglia neurosecretory cells (Light Green Cells, Caudodorsal Cells and Canopy Cells) had discrete neurohaemal organs and their axons projected exclusively to nerves and connectives close to the central nervous system. The Light Green Cells had single, undividing axons, which projected exclusively to the ipsilateral median lip nerve. Hormone release is thought to take place principally from the lateral edges of axons, at various points along their lengths, within the median lip nerve. The Caudodorsal Cells projected to the cerebral commissure, where their axons often branched before terminating at the edge of the neuropil. The degree of axonal branching and the location of the Caudodorsal Cell terminals varied widely in different cells. Axon terminals penetrated the perineurium and travelled for several hundred micrometres within the connective tissue sheath of the cerebral commissure. Again, release of neurosecretory material at various points along their lengths seems likely. The Canopy Cells (a pair of individually identifiable giant cells) had a single axon, which projected to the contralateral cerebral ganglion via the cerebral commissure. Axons of left and right Canopy Cells were closely apposed in the cerebral commissure and this is the likely site of the electrotonic junction known to connect them. Neurohaemal organs for the Caudodorsal Cells are the ipsilateral lateral lobe, cerebral commissure and contralateral median lip nerve. Neurosecretory neurones whose cell bodies were located in the pleural, parietal and visceral ganglia (Yellow Cells, Yellow-green Cells and Dark Green Cells) had extensive non-localized neurohaemal areas in the connective tissue sheath surrounding the central ganglia as well as peripheral nerve projections. The Yellow Cells had one or two axons, which, in neurones located in the visceral and right parietal ganglia, projected extraganglionically to the central sheath or to the intestinal and internal right parietal nerves. These nerve projections are appropriate for the innervation of the kidney, the peripheral target organ of the Yellow Cells. Yellow Cells, located in the pleural ganglia, only had axonal projections to the central sheath. Yellow Cells and Yellow-green Cells had well developed dendritic branching terminating in the central neuropil. Yellow-green Cells project mainly to the anal and external right parietal nerves. Pleural ganglia Dark Green Cells had a few terminals located beneath the perineurium of the pleural ganglia but most of their axonal projections were to peripheral nerves. All Dark Green Cells projected to the ipsilateral pedal ganglion and then to pedal nerves. In addition, some pleural Dark Green Cells had further projections to the internal and external right parietal nerves and median lip nerve of the cerebral ganglion. The widespread distribution of Dark Green Cell axons was consistent with their supposed role in regulating ion and water transport across the skin of the foot and mantle. The electrotonic junctions known to connect Dark Green Cells whose cell bodies are close together on the pleural ganglion surface are located in the pleural ganglion, pleuro-pedal connective and pedal ganglion.