Critical period of rat development when sidedness of asymmetric body structures is determined

We recently reported that rat embryos cultured from the presomite stage in a medium containing the alpha-1 adrenergic agonist, phenylephrine, have a high incidence of situs inversus. In the present study, we have determined more precisely the critical period of development when situs inversus is induced. Rat embryos were harvested at 8 AM on Day 9 of gestation (plug day = Day 0), and divided into different stages of development, namely, early, mid, and late primitive streak stages and early, mid, and late neural plate stages. They then were cultured in rotating bottles to which phenylephrine, 0.5 mM, was added for various durations. After 49 hr of culture, embryos were examined for general morphology including sidedness of the bulboventricular loop, tail, and chorioallantoic placenta. Phenylephrine increased the incidence of situs inversus above control when administered throughout culture from either the early neural plate stage or before, and when administered for 4 hr or more from the early neural plate stage. This increase was significant even at the mid and late primitive streak stages when the control incidence was high. Our results suggest that sidedness of asymmetric body structures is determined during the early neural plate stage. This period is well before the 6-8-somite stage when morphological signs of body asymmetry first appear.