Structural transformation of the phagosomal membrane in Tetrahymena cells endocytosing latex beads

A model system with a high phagosomal membrane turnover has been developed: During a 45-min period Tetrahymena cells endocytoze 186 latex beads (diameter: 2.02 mum) per average cell; 166 of these beads are then exocytozed in the course of the following 145 min. During the endocytotic phase an average cell is approximated to fabricate 1200 mum2 phagosomal membrane. Freeze-etch electronmicroscopy reveals that both fracture faces of the nascent phagosomal membrane are associated with the typical 85 A-particles in approximately equal numbers. Mature phagosomal membranes, however, show an unequal particle distribution. Smooth areas, smooth areas bordered with a fracture rim, and particle-associated depressions up to a diameter of 130 nm can be observed especially on fracture faces of mature phagosomes in the endocytotic phase. These are discussed with respect to membrane fusion.