Fine structure of the contractile vacuole pore in Paramecium.

Abstract

SYNOPSISThe pore through which a Paramecium contractile vacuole communicates with the external environment is a 1.2 μm long and 1 μm diameter cylindrical orifice in the pellicle. During diastole, the vacuole:pore junction is closed by a substantial diaphragm which parts to the side at systole. The diaphragm is composed of inner and outer membranes continuous with the vacuole and pore membranes, respectively, and an intervening cytoplasmic layer containing filaments and irregular membranous tubules and vesicles. Microtubules, organized into 2 sets, are an important component of the pore apparatus. One set of ∼ 16 microtubules forms an annulus around the pore. These microtubules are organized into a right‐handed helix with a pitch of 0.5‐0.6 μm, and thus complete slightly more than 2 turns in their course from the level of the diaphragm to the pore outer lip. They appear to be embedded in a layer of dense material immediately adjacent to the pore membrane. The other set consists of 5 or more bands of 10–20 microtubules which radiate in a slight left‐handed helix from an insertion at the pore out over the vacuole surface to the ampullae.