Smooth endoplasmic reticulum in fish Mauthner cells at different functional states.

The ultrastructure of Mauthner cells of goldfish fry and adult xenotoca in intact state and after prolonged natural stimulation has been studied qualitatively and quantitatively. Additionally, Mauthner cells of intact adult goldfish and adult rotan Percottus glehni were investigated. In all adult fish the dendroplasm of the two major dendrites was shown to contain a regular network of smooth endoplasmic reticulum, with cisterns and tubules arranged transversally to the dendrite stem. In the Mauthner cells of intact goldfish fry, the reticulum was not clearly expressed, the transversal cisterns occurred occasionally. After stimulation, however, it became more developed probably due to proliferation of additional transversal cisterns. The periodicity of transversally oriented cisterns in the dendrites of Mauthner cells in each fish species studied was nearly the same. However, the number of transversal cisterns per unit of dendrite length, and the total length of cisterns and tubules per unit of cross-section area varied both within and among the species. These parameters increased after stimulation. It is suggested that the proliferation of the transversal cisterns in the endoplasmic reticulum and the extent of their development depend on the functional state of the afferent synapses and the plasticity of the smooth reticulum reflects the involvement of postsynaptic mechanisms in regulation of Mauthner cell stability presumably via the regulation of calcium homeostasis under varying conditions of functioning.