Image-EELS for in situ estimation of the phosphorus content of RNP granules

Electron energy-loss spectroscopic imaging is performed on ultrathin sections, in order to approach the in-situ distribution of individual elements in the cellular constituents. In a previous ultrastructural study of salivary gland cells of Chironomus thummi and Ch. tentans by means of electron spectroscopic imaging and cytochemistry, we have described a new type of phosphorus-rich small granular ribonucleoprotein (RNP) component in the nucleoplasm of these cells; an attempt is made in the present work to estimate the phosphorus content of this component by the above mentioned method, using ribosomes as internal standard. Results show that these RNP granules contain, on average 3600 phosphorus atoms per granule. Another nuclear constituent also studied, the Balbiani ring granule, contains, on average, 43 000 phosphorus atoms per granule.