Calcium-rich, refractile granules in Tetrahymena pyriformis and their possible role in the intracellular ion-regulation

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Abstract
Tetrahymena pyriformis GL is known to accumulate small, refractile granules during unfavourable growth conditions. Furthermore, electron microprobe analysis of granules in situ has shown them to be calcium-rich, with calcium bound in an apatite-like material. The present study deals with the identification of these granules in the electron microscope. During granule formation Tetrahymena contains several structures with increased electron density, e.g. mitochondria, lipid droplets, and autophagic-like vacuoles; however, the most conspicuous feature is a membrane-bound structure with amorphous contents of varying electron density. These structures are believed to represent the calcium-rich granules. Their limiting membrane resembles the cell membrane in structure and polarization, i.e. one side of the membrane bilayer is thick and electron-dense. The electron-dense layer of the granule membrane faces the lumen of the granule (high content of calcium) and that of the cell membrane faces the external medium (high concentration of calcium); in both cases the less electron-dense layer of the membrane borders the cytoplasm (low concentration of calcium). This polarization is believed to be connected with the functions of the membrane. The cells also contain small tubular structures with a membrane similar in structure to that of the granules and with an amorphous content similar to that of the granules. These tubular structures are often found close to or in direct connexion with the granules and their possible involvement in granulogenesis is proposed. The calcium-rich granules may play a role in the regulation of the intracellular concentration of calcium. Their formation correlates with a decrease in the general metabolism of the cells, i.e. conditions during which the intracellular concentration of calcium is likely to increase. A model is proposed for the possible pathways of calcium in Tetrahymena during normal and unfavourable growth conditions. In addition, the possible involvement of other cytoplasmic inclusions, e.g. mitochondria, in this regulation is discussed.