Mechanisms of nutritive endocytosis. III. A freeze-fracture study of phagocytosis by digestive cells of Chlorohydra

Freeze-fracturc was used to compare the ultrastructure of plasma with phagosomal membranes of digestive cells of green hydra. Changes in both the pattern and density of intramembrane particles (IMP) were evident during the transition from plasma to phagosomal membrane. Small particle-free regions and associated aggregates of three to eight IMP were observed in presumptive adherent and enveloping plasma membranes, as well as in fully formed phagosomal membranes. Larger particle-free regions were observed as rims around the tips of enveloping membranes which had nearly completed enclosure of particles. The density of IMP in newly formed phagosomal membranes was 1.5-fold greater than that in the parent plasma membrane from which they derived, and was greater also than in older phagosomal membranes.