Fig. 1.

Smooth endoplasmic reticulum forms a network in the dendrite. Top, SER in a single section appears as thin cisternae with wavy membranes (arrows) surrounding a mitochondrion (∗). A mushroom spine (m) and a thin spine (t) originate from the dendrite on this section. Bottom, Three-dimensional reconstruction of the SER demonstrates that the cisternae form a network with larger flat compartments (arrowheads) connected by thin extensions (arrows). Most of the network lies in the periphery of the dendrite, surrounding the mitochondria (not shown in reconstruction). Seventeen spines originate from this segment of dendrite. SER is found in only three spines (∗), two of which are large mushroom spines with spine apparatuses. Scale bar, 1.0 μm.