Axenic Paramecium caudatum. II. Changes in fine structure with culture age

The fine structure of axenically grown Paramecium caudatum undergoes characteristic changes which reflect the cell's nutritional and metabolic conditions. Cells in early log phase of growth contain large lipid accumulations and extensive foci of rough ER bordered by Golgi stacks, indicating an active metabolism. Lipid bodies and ER foci are reduced during mid log phase. By days 5 and 6 (late log phase) some rough ER and a few lipid bodies are found in the cortex while the endoplasm is filled with free ribosomes and small vesicles. Conjugating cells, observed at the onset of stationary phase (day 6), show an accumulation of secondary lysosomes concomitant with a lack of digestive vacuoles. Lipofuscin-like granules and autophagosomes appear on day 7 and increase in number throughout the stationary and death phases. Immature autophagosomes are smaller and are bounded by a "thin" membrane lacking a glycocalyx. Mature autophagosomes (autophagolysosomes) containing partially digested mitochondria have a "thicker" membrane with a glycocalyx, a characteristic of lysosomes. Fusion of lysosomes with autophagosomes is verified by the presence of the natural lysosomal marker, the paracrystalline sheets, in the autophagolysosomes. The residue remaining from the lipid fields on days 5 to 20 includes highly convoluted membrane-like patches with long non-membrane-bound cylinders. Concomitant with the decrease in the number of digestive vacuoles, lysosomes, autophagosomes and lipofuscin-like granules proliferate from day 7 and become the striking feature of death-phase cells. With increased culture age the cisternae of Golgi stacks become dilated and decrease in diameter, and the number of ribosomes on the ER at the cis-face of these stacks is reduced. Mitochondria sometimes increase in density, and some have multi-lamellar whorls extending from their outer membrane.