The uterus sustains stable biological clock during pregnancy.

Maternal circadian information has been reported to play an important role in fetal physiology and development. Hormones and nutrition have been mainly investigated as circadian cues from mother to fetus. However, the influences of circadian properties of the pregnant reproductive organs on fetuses have not been fully investigated. To gain an insight on the circadian functions of the reproductive organs, we examined molecular clocks in the pregnant rat uterus and placenta. By using a Period1-luciferase (Per1-luc) rat, whose tissues express luciferase corresponding to activation of Period1, a “key clock gene”, we examined the uterus clock during non-pregnancy, on embryonic day 12 (E12), and on E22 (the end of pregnancy) in a light-dark (LD) cycle and constant darkness (DD). By in situ hybridization we further explored Per1 mRNA rhythms in the placenta on E12 and E22. The uterus in vitro showed clear circadian Per1-luc rhythms both in and out of pregnancy, having peaks at around the time corresponding to dusk in LD. Likewise, in DD, the uterus in vitro had the same Per1-luc rhythms. The decidua in LD showed circadian Per1 mRNA rhythms, peaking during night 6 h after dusk, while the decidua in DD showed the same Per1 mRNA rhythms only on E22. In contrast, the labyrinth showed no circadian Per1 mRNA rhythms in LD or DD during pregnancy. These results suggest that the uterus and decidua, a maternally-originated tissue of the placenta, but not the labyrinth, a fetus-originated tissue of the placenta, can provide the fetus with circadian information.