The effects of different methods of fixation on central nervous system synaptic pinocytotic vesicles.

Synaptic pinocytotic vesicles (invaginating from the surface membrane) and coated vesicles inside rat mossy fiber endings were counted after the use of different kinds of fixatives. Significantly greater numbers of pinocytotic vesicles and coated pinocytotic vesicles per unit length of membrane were found when osmium was used as the first fixative. A high positive correlation was found between these values and the number of coated vesicles per unit area of mossy fiber ending profiles. The results emphasize the need for caution when considering the theory that in vivo synaptic vesicle recycling involves a coated vesicle invagination of the surface membrane followed by internalisation and loss of coat of the vesicle.