The Fine Structure of the Contractile Vacuole in Ameba

The contractile vacuole is an organelle found in most freshwater Protozoa. In ameba, as studied with the light microscope, it is usually described as spherical, with a rather thick limiting membrane and surrounded by a layer of mitochondria (Fig. 1). The contractile vacuole undergoes a continuous series of cyclic changes. I t enlarges gradually (diastole), collecting water until it reaches a critical size, and then suddenly contracts (systole), expelling its contents to the outside of the cell, whereupon a new diastole begins. Although the precise function of the contractile vacuole is not known, it is believed to be primarily an osmoregulatory organelle (7). The limiting membrane of the contractile vacuole of Amoeba proteus was estimated by Mast (11) to be as much as 0.5 micron thick--much thicker than the membrane of any other organelle. The cytoplasm surrounding the vacuole wall in Pelomyxa carolinensis has been described as being in a gel state (17), having a higher optical density (1), and extending for a depth of about 2 microns, with a layer of mitochondria embedded in this gel. With the higher resolving power offered by the electron microscope, it seemed likely that the so called thick limiting membrane and its surrounding gelated cytoplasm would show evidence of specialized structures not seen with the light microscope. Three species of fresh water amebas were studied, Amoeba proteus, Pelomyza carolinensis (Chaos chaos), and Hartmannella rysodes. P. carolinensis has numerous contractile vacuoles, whereas A. proteus and H. rysodes have only one.