Printed in Great Britain © Company of Biologist] Limited-T979 MEMBRANE RECYCLING AT THE CYTOPROCT OF TETRAHYMENA

Exocytosis and membrane recycling at the cytoproct (cell anus) of Tetrahymena pyriformis were studied using thin-section electron microscopy. Single cells were fixed at specific times relative to the elimination of the vacuole's contents — before elimination, at elimination, 3-5 s and 10-15 8 following elimination. The closed cytoproct is distinguished from other pellicular regions by a single membrane at the cell surface which is circumscribed by an electron-opaque flange that links or welds the plasma membrane to the underlying alveolar margins. Microtubules originating in the flange pass inward where they lie over, and possibly guide, the approaching food vacuoles to the cytoproct. Food vacuoles near the cytoproct are also accompanied by coats of microfilaments. These microfilamenta appear to be active in the channelling and endocytosis of food vacuole membrane. Upon cytoproct opening the plasma membrane and food vacuole membrane fuse. Elimination seems to be essentially passive and is accomplished by re-engulf-ment of the old food vacuole membrane which is constantly associated with microfilaments. Re-engulfment of all the food vacuole membrane requires 10-15 s and results in a closed cytoproct. The membrane remnants embedded in microfilaments form a cluster under the closed cyto-proct. At the periphery of this cluster remnants take the shape of 70 — 130-nm spherical vesicles or o-2-/im-long flattened vesicles