Circadian expression of clock genes clock and Cry1 in the embryonic chicken pineal gland.

Clock and Cry1 expression were examined in the pineal gland of chicken embryos incubated under constant darkness from embryonic day (ED) 0. From ED13, Clock and Cry1 mRNA levels showed episodic alterations. After ED17, circadian pattern of clock gene expression was seen both in vivo and in vitro. Our results support the idea that rhythmic environmental factors are not necessary for the generation of circadian patterns of clock gene expression during development.