Membrane events in the acrosomal reaction of Limulus sperm. Membrane fusion, filament-membrane particle attachment, and the source and formation of new membrane surface.

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Abstract
The membranes of Limulus (horseshoe crab) sperm were examined before and during the acrosomal reaction by using the technique of freeze-fracturing and thin sectioning. Stages in the fusion of the acrosomal vacuole with the cell surface are examined. Fusion takes place in a particle-free zone which is surrounded by a circlet of particles on the P face of the plasma membrane and an underlying circlet of particles on the P face of the acrosomal vacuole membrane. These circlets of particles are present before induction. Up to 9 focal points of fusion occur within the particle-free zone. A system of fine filaments, each 30 .ANG. in diameter, which lies between the acrosomal vacuole and the plasma membrane is described. These filaments change their orientation as the vacuole opens, a process that takes place in < 50 ms. Membrane particles seen on the P face of the acrosomal vacuole membrane change their orientation at the same time and in the same way as do the filaments, thus indicating that the membrane particles and filaments are probably connected. The source and the point of fusion of new membrane needed to cover the acrosomal process are examined. This new membrane is almost certainly derived from the outer nuclear envelope and appears to insert into the plasma membrane in a particle-free area adjacent to an area rich in particles. The latter is the region where the particles are probably connected to the cytoplasmic filaments. The relevance of these observations in relation to the process of fertilization of sperm is discussed.