Cyclic renewal of whole pineal photoreceptor outer segments.

In the light-sensitive epiphysis cerebri of adult frogs (Rana esculenta) adapted in summer to an artificial long day (L:D = 17:7; L = 90 lx at 2,300 degrees K; controlled temperature of 8 degrees C) outer segments of cone-like pineal photoreceptor cells exhibit a cyclic and complete renewal. Shortly after 'lights off' inner segments protruding into the pineal lumen bear exclusively bulbous cilia of the sensory type. Considering these results, concepts dealing with different types of sensory pinealocytes have to be re-evaluated.