Ultrastructure and histochemistry of neurosecretory cells and neurohaemal areas in the pond snail Lymnaea stagnalis (L.).

Abstract

In the central ganglia of Lymnaea stagnalis neurosecretory cell groups have previously been identified by means of chrome-haematoxylin or paraldehyde-fuchsin stains. In the present study seven types have been distinguished within the class of Gomori-positive cells on the basis of different staining reactions with the alcian blue/alcian yellow technique. Five types are located in the cerebral ganglia and in the lateral lobes, whereas two cell types occur in the ganglia of the visceral ring. No neurosecretory cells have been observed in the buccal and pedal ganglia. The staining technique used proved to be superior to the classic neurosecretory stains, because with this method the secretory substances can easily be distinguished from nonsecretory Gomori-positive tissue constituents. One of the two Gomori-negative neurosecretory cell types of the cerebral ganglia react positively with the alcian blue/alcian yellow technique. In addition, two Gomori-negative neurosecretory cell types, which had not been described before, were identified in the visceral ring. The ultrastructure of the four neurosecretory cell types in the visceral ring is described. The electron microscope revealed that each of the histochemically distinguished secretory substances consists of elementary granules which differ in size and appearance from each other and from the neurosecretory elementary granules which have been described by other authors in the cerebral ganglia and in the lateral lobes. The neurohaemal areas of the neurosecretory cells in the visceral ring are very extensive and include the peripheral parts of the nuchal nerves and of the connectives and nerves of the ganglia of the ring. The perineurium and the adjacent parts of the connective tissue which surround the ganglia, the connectives and the nerves are regarded as additional neurohaemal zones, because in these regions many tiny nerves occur, which consist mainly of neurosecretory axons ending non-synaptically near parts of the vascular system. In the perineurium surrounding the cerebral ganglia and their neurohaemal area a similar network of neurosecretory fibres was observed. Indications of release of the secretory material were regularly observed. Release apparently takes place by exocytosis. A circadian rhythmicity was observed in the release activity of some of the neurosecretory cell types.