The ultrastructure of the rabbit pineal gland after sympathectomy, parasympathectomy, continuous illumination, and continuous darkness.

The ultrastructure of the rabbit pineal gland was investigated after sympathectomy (extirpation of decentralization of the superior cervical ganglia), parasympathectomy, continuous illumination and continuous darkness. The similarity of the ultrastructural changes in the light pinealocytes occuring after sympathectomy and after continuous illumination was striking. It is supposed that these changes have a common cause, viz. the lack of free noradrenaline, the pinealotropic neurotransmitter. The smooth endoplasmic reticulum present in the terminals of the offshoots of the light pinealocytes is possibly involved in pineal indoleamine synthesis.