The shape of synaptic ribbons in the rat pineal gland.

Under the transmission electron microscope, synaptic ribbons (SRs) of the mammalian pineal gland appear as rod-like organelles. Their three-dimensional structure is not precisely known. In the present study, pineal SRs were investigated using serial sections obtained from rats killed at noon and midnight. The shape of the SRs was reconstructed based on SR profile length and the number of sections in which the profiles were contained. The results obtained show that SRs are basically flat plate-like structures with polymorphic lateral edges. Reconstructions of SRs revealed that they had average dimensions of 300x150x35 nm and were 19.3% larger at night than at day; the difference in SR size points to perhaps major differences in synaptic function between day and night.