Ultrastructural studies on the medulla of rabbit pineal gland in the diurnal rhythm.

Presence of numerous capillary vessels was detected, featuring thin unfenestrated endothelium with low number of plasmalemmal and pinocytic vesicles. Visual appraisal demonstrated no differences in number of cellular transport exponents between stages at which maximum and minimum melatonin levels were noted in the serum. Pineal parenchyma consisted of two types of pinealocytes: clear and dark ones. In hour of the peak gland activity the number of dark pinealocytes was doubled. No evident differences were observed between the two time points in the number of clear pinealocytes and of glial cells.