Figure 2.
The number of the docked vesicles correlates linearly with the size of the active zone. (a) An electron microscopic image of a synaptic contact between a CA3 pyramidal cell spine (s) and axon terminal (t) in the str. oriens taken from a 20 nm thick electron microscopic section. The postsynaptic density (PSD) is clearly visible and arrows demarcate the active zone. One of the vesicles (*) is closely apposed to the presynaptic membrane. (b) Higher magnification view of the active zone region showing that a gap between the presynaptic- and vesicle membranes can not be resolved, the distance between the middle of the 2 membranes is <5 nm. Vesicles with such a small distance from the active zone plasma membrane are considered to be docked. (c) 3D reconstructions of active zones (light blue) from serial 20 nm thick electron microscopic sections (number of sections used for reconstruction ranged 11 – 22). White spheres correspond to docked vesicles. If a vesicle appeared on consecutive sections in the same position, was considered to be cut in half and was counted as a single vesicle. (d) Active zone area shows a tight, positive correlation with the number of the docked vesicles (n = 15 active zones in n = 1 animal). Scale bars represent: 50 nm (a-b), 200 nm (c)