Figure 1.
Complexity of the hippocampal mossy fiber system. A, Electron microscopic image of two adjacent MFBs (MFB1, MFB2) taken from a P28 rat. Both MFBs outlined in yellow terminate on different dendritic segments (de1, blue contour; de2, orange contour), but preferentially on spiny excrescences (se1, black contour; se2, orange contour). AZs are given in red, PAs are given in magenta, mitochondria are given as white contours, and individual synaptic vesicles are represented by green lines. B, Dense and multiple innervation of individual MFBs of different shape and size (given in various colors) on a postsynaptic target dendrite (blue) in an adult rat. Two glial fingers (gf 1 and gf 2) that were located between individual MFBs are depicted in white. Scale bars: A, B, 1 μm. C, Here, the MF axon (black asterisk) gave rise to an end terminal located on a dendritic segment (de) in a P28 rat. Note the large pool of synaptic vesicles throughout the entire terminal and the mitochondria (white asterisks). The spiny excrescences (se) of the postsynaptic target dendrite were almost covered by the nerve terminal, and only here AZs at the postsynaptic apposition zone were observed. Scale bar, 1 μm. D, En passant MFB (P28) where the nerve terminal is invaginated by several axonal profiles (white asterisks) of unknown origin without establishing axoaxonic contacts. de, Postsynaptic dendrite; se, spiny excrescences. Scale bar, 1 μm. E, Shape and distribution of PAs (marked by black asterisks) at the apposition zone of an MFB and its postsynaptic target dendrite (de) in a P28 rat. Note the dense row of mitochondria near the apposition zone. Scale bar, 0.25 μm. Inset, Individual PA at high magnification. Note the typical symmetric arrangement of the electron-dense material.