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Comparative methodological investigations on the cytochemical localization of calcium in brain and inner ear of cichlid fish

Four different methods for calcium precipitation are compared in the optic tectum and the inner ear of the cichlid fish, Oreochromis mossambicus. Several parameters are investigated concerning their influences on the reaction product. Three procedures (bichromate, fluoride, and oxalate‐pyroantimonate) produce fine‐grained deposits, often flocculent in the latter method. The fourth method (potassium‐pyroantimonate) generates predominantly coarse‐grained reaction product. The calcium content of the deposits is always proven with energy‐filtering transmission electron microscopy (EFTEM). In both tissues fine‐grained reaction product is found in endoplasmic reticulum and synaptic vesicles, and in addition in some mitochondria and at the cytoskeleton. The coarse‐grained deposits of the potassium‐pyroantimonate method have a more unspecific distribution. This is the only method which produces extracellular deposits in the inner ear, whereas in the optic tectum extracellular precipitates are always present except with the oxalate‐pyroantimonate procedure. Two factors have an influence on the reaction product: the duration of fixation and the type of resin. The prolongation of the fixation time up to 24 hours leads to an increase of the reaction product, which also becomes coarse‐grained. These observations are corroborated by quantification with image analysis. Furthermore the use of an epoxy resin compared to acrylic resins decreases the amount of reaction product produced. We show that the application of several methods is meaningful in order to understand the calcium properties of the investigated tissue, but it is necessary to optimize a certain method for a given tissue. © 1995 Wiley‐Liss, Inc.