FIGURE 1.

CA3 region of Hippocampus in a slice culture incubated for 2 weeks in vitro (DIV). The tissue was high-pressure frozen in the absence of chemical fixatives, subjected to freeze substitution, osmicated, and finally embedded in Epon (see Studer et al., 2014, for details on the method). Note that all tissue components of stratum lucidum are well preserved after the incubation period and subsequent freezing procedure. As known from many studies in perfusion-fixed material, the stratum lucidum mainly contains the thin unmyelinated axons of the mossy fibers (MF) and their giant boutons (mossy fiber bouton, MFB). The postsynaptic elements, the proximal dendrites (D) of CA3 pyramidal cells, are located in between the bundles of mossy fibers and give rise to large complex spines (S) for the contact with MFBs. Scale bar: 1 μm.