Osmotically Induced Changes in the Activity of Neurosecretory Cells Located in the Pleural Ganglia of the Fresh Water Snail Lymnaea Stagnalis (L.), Studied By Quantitative Electron Microscopy

Abstract

The neurosecretory cells present in the pleural ganglia of the fresh water snail Lymnaea stagnalis, the Dark Green Cells, were studied to establish whether these neurones are engaged in osmoregulation. Groups of 5 snails were exposed to conditions which induce changes in the diuretic activity (de-ionized water or a 0.1 M sodium chloride solution) for 1, 8, and 15 days, respectively. Animals kept in tap water were used as controls. The cell bodies as well as the axon terminals of the Dark Green Cells were studied quantitatively at the ultrastructural level. To estimate the rate of secretory activity of the neurones changes in the relative extent of the granular endoplasmic reticulum (determined by lineal analysis) and in the number of active Golgi zones were used as parameters. Lineal analysis was also used to determine the fractional volume of the mitochondria and of the cytosomes. Variations in the amount of secretory material in the cell bodies were determined by counting the number of elementary granules per surface unit. Accumulation and release of the secretory material in cross sections of the main neurohaemal area (the right pleuro-parietal connective) were determined by counting the total number of granule-containing axon profiles, and the number of these axons showing release phenomena, respectively. Exposure to de-ionized water, presumably leading to enhanced diuresis, showed a statistically significant increase (a duplication in 15 days) of the extent of the granular endoplasmic reticulum and of the number of active Golgi zones per surface unit (2000 μ 2 ). Moreover, a twofold increase of the volume of the cytoplasm per cell body was noted. In the axon terminals located in the right parieto-pleural connective an enhanced release of secretory material was observed. These trends, which were statistically significant, point to an activation of the Dark Green Cells when the animals are exposed to de-ionized water. During exposure of the snails to 0.1 M sodium chloride, a solution, which is hypertonic to the blood and known to reduce the rate of urine formation, the secretory activity of the DGC was markedly reduced. The observed decrease of the extent of the granular endoplasmic reticulum and of the number of active Golgi zones per surface area was statistically significant. At day 8 the occurrence of cellular atrophy was indicated by the presence of large numbers of newly formed lysosomes of autophagous nature. After two weeks the cytoplasmic volume per cell body was reduced to at least half the control value. In the axon terminals release of secretory material was hardly observed. The secretory material in the axon terminals, rapidly accumulated during the first hours of exposure, gradually diminished during the experiment which was partly due to release, and partly to lysosomal breakdown. The changes observed in both experiments were reversible: the quantitative data obtained from snails kept in tap water for one week after a stay of 15 days in the experimental solutions was within the control range. The results are in line with the hypothesis that the Dark Green Cells synthesize a substance which acts in diuresis. Additional physiological evidence is, however, needed to define more accurately the function of these neurones in the maintenance of the water balance.