Morphology of synaptic vesicles in axo-dendritic and axo-somatic collateral terminals of two feline spinocervical tract cells stained intracellularly with horseradish peroxidase.

Synaptic vesicles in initial collateral terminals of two feline spinocervical tract cells have been investigated after intracellular staining with horseradish peroxidase. A total of 5325 vesicles in 52 axodendritic and 26 axo-somatic terminals were analysed after aldehyde-osmium fixation. The greatest length and longest perpendicular width of each vesicle were measured, and the ratio and geometric mean of the diameters (gm-diameters) were calculated. The vesicles were divided into classes with round, elliptical and flat organelles. The variations in vesicle length, gm-diameter, and diameter ratio were statistically analysed by means of one- and two-way analyses of variance and t-tests. The diameter ratio, the length of round vesicles, and the gm-diameters of round and elliptical vesicles differed significantly between the cells. The length and gm-diameter of the elliptical vesicles differed significantly between the groups of axo-dendritic and axo-somatic terminals of each cell. Round vesicles were significantly longer in the axo-somatic than in the axo-dendritic terminals of each cell, and the gm-diameter showed this relation for both round and elliptical vesicles. It is assumed that a one-size vesicle could not account for all the measured profiles. The variability of synaptic vesicles within and between the functionally similar cells emphasize the difficulties in using the morphology of synaptic vesicles for a discrimination of axon terminals of different origin.