The development of glio-interstitial tissue in Mytilus retractor muscle depends on Na+-Ca2+ antagonism

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

The abundance of Aplysia gliagrana depends on Ca2+ and/or Na+ concentrations in sea water

The glial cells surrounding the identified giant nerve cell bodies R2 or LP1 of Aplysia punctata were studied by quantitative electron microscopy. They contain specific, electron-dense but non-osmiophilic membrane-bound granules, approximately 0.3 microns in diameter, called gliagrana. Similar glial granules are more often found in marine than in freshwater molluscs, possibly because they represent a calcium store used to compensate excess Na+ in the extracellular milieu of marine species and to regulate perineuronal calcium concentration. In agreement with this hypothesis, the abundance of gliagrana (= number of glial granules per microns 2) is found to be higher in animals adapted to low Ca2+ artificial sea water than in animals kept in high Ca2+ (or low Na+) conditions. This finding is not observed after 1 week but after 2 weeks of adaptation.

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

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 LP1. 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 micron in diameter. The number of gliagrana per micron 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.

Quantitative X-ray microanalysis of calcium with the Camebax-TEM system in frozen, freeze-substituted and resin-embedded tissue sections. Application to molluscan glio-interstitial granules

The relevance of the continuum method for a quantitative X-ray microanalysis of epon embedded tissue sections in the particular conditions offered by the Camebax-TEM system was tested and an improved model of specimen holder is proposed. The absolute calcium concentration [Ca] of membrane-bound intracellular glio-interstitial granules was determined by X-ray microanalysis in transmission electron microscopy of Mytilus retractor muscle. The Ca peak and background values were measured by the wavelength-dispersive spectrometer of the Camebax; the mass thickness of the section was recorded simultaneously with an added energy-dispersive detector. The tissue was frozen at approximately equal to 77 K in a mixture of liquid propane and butane, freeze-substituted in the presence of oxalic acid and embedded in epoxy resin. The calcium concentration of glio-interstitial granules can be as high as 180 mmol.kg-1 of epoxy-embedded tissue, with an average of 40 mmol.kg-1. The sampling of the data through repeated experiments is discussed and it is proposed that the cell would be the main level of variation. The Ca content of glio-interstitial granules is significantly lower in the tissues of animals submitted to high-potassium artificial sea-water for 10 min. This finding was predicted by the hypothesis that glio-interstitial tissue is a regulator of calcium concentration in extracellular spaces.