• chick;
  • Clock;
  • Bmal1;
  • circadian;
  • salt and pepper


Background: The circadian clock is a well-described temporal organizer in adult organisms. Despite the particularly evident need for temporal control during embryo development, the effect of environmental cues is still greatly neglected. Few studies have reported circadian clock gene expression in early embryonic stages. However, nothing is known about circadian clock gene expression and function in the first stages of avian embryogenesis. Results/Conclusions: In this work, the presence and spatial distribution of core circadian clock Bmal1 and Clock transcripts were thoroughly characterized during the first 50 hr of chick development using reverse transcriptase-polymerase chain reaction (RT-PCR), single and double whole-mount in situ hybridization and subsequent cross-section histology analysis. RT-PCR detected both Bmal1 and Clock transcripts since the egg is laid and until the embryo reaches the 22-somite stage. Whole-mount in situ hybridization showed that Bmal1 and Clock are expressed in the Hensen's node and primitive streak at early gastrula stage. Later, both mRNAs are present in the developing nervous system, optic vesicle, notochord, foregut, and somites. Clock was further identified in the developing heart. Noticeably, Bmal1 and Clock are expressed with a “salt and pepper” pattern, suggesting the existence of nonentrained oscillatory transcription which could play a nondependent dark/light function during chick embryo development. Developmental Dynamics 241:1365–1373, 2012. © 2012 Wiley Periodicals, Inc.