Digestive system membranes: freeze-fracture evidence for differentiation and flow in Paramecium.

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Abstract
Freeze-fractured membranes of digestive vacuoles of randomly feeding P. caudatum exhibit dramatic differences in intramembrane particle (IMP) number and distribution on E- and P-fracture faces. By pulse-feeding latex spheres to cells, these differences were related to the age of the digestive vacuoles. The membranes undergo a specific sequence of changes during the digestive cycle. Young digestive vacuoles (DV-l; .ltoreq. 6 min), nascent vacuoles still connected to the cytopharynx, and discoidal vesicles, from which vacuole membrane is derived, all have a highly particulate E face and a less particulate P face. As early as 3 min after feeding, a 2nd category of digestive vacuoles (DV-II) can be recognized which are considerably smaller in diameter and lack particles on their E face. The endocytic removal of DV-I membrane material associated with the formation of DV-II vacuoles probably involves a concomitant and selective removal of E-face particles, as essentially no changes are seen in the density of P-face particles on the 2 types of vacuoles. Beginning at 10 min the 1st DV-III vacuoles are encountered. These are larger than the DV-II vacuoles and possess very prominent E-face particles, which resemble those on the E face of the numerous lysosomes bordering the digestive vacuoles. DV-III vacuoles exhibit a substantial increase in P-face particles. These membrane changes closely parallel, and are probably correlated with, the physiological events occuring within the vacuole lumen: concentration of food, killing of prey and digestion. Calculations of the amount of membrane removed from DV-I to form DV-II and of the increase in membrane surface area during the transition from DV-II to DV-III indicate that as much as 90% of the initial phagosome (DV-I) membrane can be removed before digestion begins. The enlargement of DV-II must be caused by fusion with adjacent lysosomes which also contribute the new populations of IMPs to the DV-III membrane. The appearance of numerous endocytic structures on older DV-III vacuoles suggests that membrane is retrieved from DV-III before defecation.