The ultrastructural features of the adrenergic nerve fibers in the pineal glands of the rat, ground squirrel and chinchilla are described. Frequencydistribution histograms of diameters of granulated and non‐granulated vesicles in the adrenergic nerve endings demonstrate that the pineal nerve endings in the chinchilla contain a considerable number of large granulated and non‐granulated vesicles, in contrast to those in the rat and ground squirrel. Synaptic ribbons seen in the pinealocytes of the ground squirrel were often localized near that plasma membrane which lay in close proximity to the axolemma of adrenergic nerve fibers. This observation may indicate that the synaptic ribbons are involved in the functional interconnection between pinealocytes and adrenergic nerve fibers. Localized dilations of the adrenergic nerve fibers were commonly observed in the pineal glands of all species examined. In addition to a variety of axonal constituents, various forms of inclusion bodies were tightly packed within these axonal dilations. The accumulation of the inclusion bodies may represent degenerative changes which occur in the pineal adrenergic nerve fibers in relation to the functional activity of the pineal gland.
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