Topographical relationships of synaptic ribbons in the pineal system of the vole (Microtus agrestis).

In the pineal system of the vole (Microtus agrestis) both the superficial and the deep pineal exhibit a high percentage of synaptic ribbons lying in intimate contact with the cell membrane of the pinealocytes. At the sites of contact, densities resembling the presynaptic dense projections of synapses are arranged between the ribbons and the cell membrane. Opposite the sites of contact various elements were found. The quantitative estimation revealed that in the superficial pineal about 40% and in the deep pineal about 60% of the membrane-contacting ribbons are located opposite glial cells: in both organ parts about 18% of the membrane-contacting ribbons were found opposite adjacent pinealocytes. The location of ribbons at the perivascular space was almost exclusively found in the superficial pineal, while the cerebrospinal fluid-contacting area in the deep pineal exhibited ribbons in intimate contact with the lumen of the third ventricle. The heterogeneity of the topographical relationships would seem to indicate a diffuse functional effect of the synaptic ribbons in the mammalian pineal gland.