Influence of photoperiod on dense-core vesicles and synaptic ribbons of pinealocytes of the djungarian hamster (Phodopus sungorus).

The ultrastructure of the superficial part of the pineal gland of Phodopus sungorus was examined. Dense-core vesicles and synaptic ribbons in pinealocytes of animals kept under different photoperiods were counted, revealing marked differences. Pinealocytes of short-day animals compared with long-day animals exhibit an increase of dense-core vesicles coincident with a decrease of synaptic ribbons. It is assumed that the corresponding numerical changes of these organelles are of functional significance in relation to pineal secretory activity.