Day-night differences in the number and structure of synaptic ribbons in chick pineal

The objective of this study was to evaluate the effect of lighting (day-night) changes on pinealocyte synaptic ribbon shape (conformation) and numbers. Three-dimensional reconstruction analysis of pinealocyte basal processes revealed that 30% (6/20) of all ribbons from dark-adapted animals were either curved or split. Synaptic ribbons from light-adapted animals did not show this variant morphology; all were linear structures. An analysis of each section from each series, containing curved or split ribbons, revealed that 44% (16/36) of all ribbon profiles would yield inflated counts if used in random morphometric sampling protocols. Therefore, split and curved (variant) ribbon morphologies could result in an overestimation of synaptic ribbon populations of approximately 13% (0.44 X 30%) of dark-adapted samples. In spite of this potential sampling error, the fourfold increase in the number of synaptic ribbons observed during the dark phase of a light-dark cycle remains highly significant (P less than 0.0001).