The circadian clocks are cell-autonomous intrinsic oscillators existing throughout the body to coordinate intracellular and intercellular functions of each organ or tissue. The circadian clock oscillation gradually emerges during mid-to-late gestation in the mammalian developmental process. Recently, it has been revealed that the in vitro differentiation of mouse ES cells recapitulates the circadian clock development. Moreover, reprogramming of the cells results in the redisappearance of the clock, indicating that circadian clocks are tightly coupled with cellular differentiation. Interestingly, before the circadian clock develops, the embryo is governed under ultradian rhythms driven by the segmentation clock. This short review explores these observations, discussing the significance of the emergence of circadian clock oscillation during the mammalian developmental process.

生物钟是存在于全身的细胞自主内在振荡器，用于协调每个器官或组织的细胞内和细胞间功能。哺乳动物发育过程中，在妊娠中后期逐渐出现生物钟振荡。最近，研究表明小鼠 ES 细胞的体外分化再现了生物钟的发育。此外，细胞的重新编程导致生物钟重新消失，表明生物钟与细胞分化紧密相关。有趣的是，在生物钟发育之前，胚胎受到分段时钟驱动的超昼夜节律的控制。这篇简短的评论探讨了这些观察结果，讨论了哺乳动物发育过程中生物钟振荡出现的重要性。