Direct illumination or stimulation with light of the lateral eyes induced no electrical activity in the central part of the pineal organ in the pigeon. In the pineal stalk, however, a train of impulses was observed that failed to respond to changes of illumination. The position of the tip of the recording electrode (iron deposit), was histologically clearly determined to be in the pineal parenchyma. A distinct nerve bundle could be traced down the pineal stalk. Adrenergic nerve fibers showing a green fluorescence accompanied pineal capsular vessels into pineal follicular tissue. Using the method of Falck-Hillarp, the pineal parenchyma showed high concentrations of yellow fluorescent material in the apical or basal parts of the pinealocytes. Only a part of the fluorescent material could be dispersed with reserpine. In the electron microscope, no typical photoreceptor cells were identified in the pineal organ. Only large bulbous cilia, and circular lamellar or tubular complexes, were observed; they sometimes contained a dense granular material. Occasionally, the lamellar complexes had a direct relation to cilia of club-shaped appearance. Signs of secretory activity were more definitive in the pigeon pineal organ than sensory structures. One type of pinealocytes contained many 800-1,200 Å diameter granular vesicles concentrated in the apical or basal (apposed to blood vessels) cell parts. At the occasionally lobulated basal end-feet, a disappearance of the content of the granular vesicles indicated a release mechanism. This secretory activity was discussed in relation to the 5-hydroxyindole content of the pigeon pineal organ (Quay) and the general problem of melatonin synthesis; also, the possibility was considered that the secretory activity may be stimulated by sympathetic nerve fibers. © 1969 Springer-Verlag.
