Magnesium and Calcium in Striated Muscle as Revealed by the Electron Microscope. ∗

Abstract

Several years ago McMillen and Scott 1 described a simple magnetic electron microscope designed for use in localizing certain inorganic salts in biological tissues. After some modifications of this apparatus, mostly in the direction of increasing the magnification, 2 and much experimentation it has been possible for us to localize magnesium and calcium in tissues. Striated muscle is an appropriate tissue for the purpose since it has been shown by other methods that the salt distribution is orderly and precise. 3 , 4

The nearest approach to chemical conditions obtaining in the living-is had by examination of tissues prepared for study by the Altmann-Gersh frozen dehydration method. Sections of tissues fixed by a modification of this technic were attached to a barium and strontium coated cathode and placed in the electron microscope. After evacuation of the instrument the cathode was heated slowly and the section vacuum-distilled until only the inorganic salts remained. The temperature of the cathode was raised further until active electron emission occurred. The electron source was imaged upon the fluorescent screen with appropriate magnetic lenses. Photographs were then made of the luminescent image.

After many tests with pure salts of magnesium and calcium it was determined that the electron source, responsible for the image, was these elements. Examination of such preparations enabled us to localize with some accuracy magnesium and calcium in striated muscle from several mammalian species. Due to the preliminary freezing process muscle is thrown into many strong contractions. In every case we could localize magnesium and calcium in these contraction bands. Little or none of these elements was present in the remainder of the sarcoplasm. It is also of some interest that there was no magnesium or calcium detected in the “tissue spaces” of Fenn. 5