Direct in utero perception of light by the mammalian fetus.

The primary neural pathway for entrainment of circadian rhythms in rodents is the retinohypothalamic tract, which conveys lighting information from the retina to a biological clock in the hypothalamic suprachiasmatic nuclei (SCN). In a precocious rodent species, the spiny mouse, the retinohypothalamic tract is present and functioning within the SCN on the day of birth, as assessed by HRP histochemistry and 14C-labeled 2-deoxyglucose autoradiography, respectively. Furthermore, direct perception of environmental lighting was observed in fetal spiny mice late in gestation. Direct, retina-mediated fetal light perception appears to be a less potent entraining agent than maternal cues communicating time-of-day information to the fetal spiny mouse. Nevertheless, direct fetal light perception may reinforce maternal entraining signals during the prenatal period and therefore be of physiological significance for entrainment of circadian rhythmicity in the fetus.