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Change of Rat Embryos from a Ventrally Concave U‐Shape to a Ventrally Convex C‐Shape

In an early stage of development in murine embryos, axial rotation occurs and the body axis changes from a ventrally concave U‐shape to a ventrally convex C‐shape. In this study, axial rotation in Sprague‐Dawley rat embryos occurred in about 5 h in vitro (from 27 h to 32 h in cultures of head‐fold stage embryos). In sagittal sections, the somites in the mid‐region of the body changed from a trapezoidal shape with a short dorsal side and long ventral side to the reverse trapezoidal shape with a long dorsal side and a short ventral side. The dorsal part of these somites acquired the ability to react with actin‐specific antibody and developed into dermatome. On treatment with 0.1 μg/ml cytochalasin D during this 5 h period, embryos became ventrally concave with two lordosis bends. The somites in the bends had a short dorsal side, which did not show any evidence of dermatome or intense immunocytochemical staining. These results suggest that the increase in length of the dorsal side of the somites is a cause of the axial rotation and that the organization of actin filaments plays an important role in the conformational change of the somites.