MEMBRANE BEHAVIOR OF EXOCYTIC VESICLES .1. ULTRASTRUCTURE OF PARAMECIUM TRICHOCYSTS IN FREEZE-FRACTURE PREPARATIONS

The spindle trichocysts of P. caudatum were examined by the freeze-fracture technique. The A-face of the greater part of the trichocyst membrane possesses randomly distributed 10-nm diameter particles present in a frequency of about 800 particles/.mu.m2. The B-face shows the same number of corresponding depressions. The membrane surrounding the distal 1/3 of the trichocyst tip is encased within a tubular collar. This external specialization of the trichocyst membrane is complemented in the same area of the membrane by an intramembranous differentiation; i.e., the A-face and the B-face are, respectively, free of particles and depressions. The particles with a circular profile which were in this area during trichocyst development, are concentrated within the trichocyst membrane at its distal tip. Rows of oval particles that appear on the A-face of the unfixed-frozen trichocyst membrane after collar formation show that this existing membrane has undergone a process of differentiation.