Formation, storage, and release of neurosecretory material studied by quantitative electron microscopy in the fresh water snail Lymnaea stagnalis (L.).

Abstract

The process of neurosecretion in the fresh water snail Lymnaea stagnalis was analysed quantitatively at the ultrastructural level. Special attention was paid to the phloxinophilic neurones in the cerebral ganglia, the Caudo-Dorsal Cells (CDC). The rates of synthesis, storage and release of neurosecretory elementary granules were studied in the CDC of animals fixed at time intervals of 4 hours, during a 24 hour period. The processes concerned show a diurnal rhythmicity. In the cell bodies synthetic activity, as determined by counting the number of active golgi zones per surface unit, is high during the night and low during the day. The reverse was found for the amount of secretory material (number of elementary granules per surface unit) stored in the cytoplasm. From these results it is concluded that also the rate of transport of the elementary granules through the axons fluctuates rhythmically. It has its maximum during the night and its minimum during the day. This conclusion was substantiated by the observation that the highest number of granules in the axons is found during the night. The axon terminals of the CDC pass through four different stages during the 24 hour period. In axons being in the accumulation stage, which are found from the morning until the evening, the number of elementary granules increases rapidly. The neurohaemal zone increases in thickness during this process. In axons in the release stage, which predominate during the evening and the night, the contents of the elementary granules are extruded from the axon terminals, apparently by exocytosis. The rate of release of the secretory material into the body fluid, which is very low during the day, shows a rapid increase a few hours before sunset, and is probably correlated with the light intensity. A marked decrease of the amount of secretory material is noted in the neurohaemal area during the release stage. A daily turnover of nearly all secretory material accumulated in the axon terminals is suggested by the observations. Axons being in the reconstruction stage are mainly found during the night. They are distinguished by the presence of vesicular and tubular structures. These probably originate by fusion of microvesicles, which are considered to arise from the membranes of the elementary granules after exocytosis. After disappearance of the vesicles and tubules the axons enter the empty stage. This is frequently found in the night and in the early morning hours. In the morning the axon terminals enter the accumulation stage again. Furthermore, accumulation and release phenomena were quantified in the axon terminals of 5 other neurosecretory cell types present in the ganglia. A diurnal pattern rather similar to that of the CDC was found in the Light Green Cells (the Medio- and Latero-Dorsal Cells in the cerebral ganglia). No rhythmicity was found in the neurohaemal areas of the four types of secretory neurones located in the ganglia of the visceral ring. A constant level of storage and release was established during the 24 hour period. The relationship between synthesis, storage and release of the secretory material of the CDC is discussed.