Early development of the pineal photoreceptors prior to the retinal differentiation in the embryonic rainbow trout, Oncorhynchus mykiss (Teleostei).

The development of the pineal and retinal photoreceptors in the embryonic rainbow trout, Oncorhynchus mykiss was investigated by means of light and electron microscopy. The pineal photoreceptors endowed with photoreceptive outer segments and signal-transmitting synapses appeared first 15 days after fertilization, and well developed by 21 days. In contrast, the retinal photoreceptors appeared as late as 27 days after fertilization (1 day before hatching). This temporal disparity in photoreceptor development has been compared with the photoperiodic formation of otolith increments, which is initiated during an early developmental stage lacking retinal photoreception. It has been suggested that the early development of pineal photoreceptors may be involved in light-dark recognition, establishment of the diel and circadian rhythms, and entrainment to the environmental photoperiod in the embryonic rainbow trout.