The lacunar glial zone at the periphery of Aplysia giant neuron: Volume of extracellular space and total calcium content of gliagrana

Abstract

The relative volume of perineuronal extracellular space, the number of gliagrana and their total calcium content have been measured in Aplysia punctata and A. californica, at the periphery of giant neurons R2 and LPI. After chemical fixation, the extracellular space amounts to 26% of the periganglionic glial zone, but this increases to 36% after quick freezing and freeze-substitution. The glial cytoplasm contains gliagrana, membrane-bound granules approximately 0.3 μm in diameter. The number of gliagrana per μm 2 of section, defined as “abundance”, was counted in electron micrographs of chemically fixed tissues. The abundance of gliagrana appears to be directly proportional to the volume of the extracellular space when the values are averaged per individual Aplysia. The total calcium concentration of the gliagrana is measured by X-ray microanalysis on sections of ganglia processed by rapid freezing and freeze-substitution in the presence of oxalic acid: it was found to be very high. An individual granule may contain 100 mM Ca in A. californica and 50 mM in A. punctata but in both species the calcium concentration varies along a wide range as if there were different functional states of the granules with respect to this concentration. The total calcium stored in the specific granules of the glial zone was estimated. It was calculated that should the glial calcium store be entirely diluted in the extracellular space of the glial zone, it would raise the calcium concentration of this space by approximately 1 mM (0.1–2.7 mM). These findings are discussed with regard to the hypothesis of glial cells regulating the perineuronal calcium concentration.